Illinoia

Illinoia Wilson

This page updated: December 2025.

This is one of the groups I study extensively in the field, and puzzle over in the lab.  It is almost entirely North American and is likely very closely related to Macrosiphum; in fact the two genera might be better thought of as a mish-mash of similar species haphazardly assigned to genera based on a possibly trivial character, i.e. clavate siphunculi.  But as you’ll see from careful study, some species placed in Macrosiphum have clavate siphunculi as well.  So what gives?  Hard to say.  Probably extensive field work and fresh collections for DNA analysis would be required to clarify the genus-level classification.  Meanwhile, there are many cool and easily recognized species of Illinoia to enjoy in the field.

The 2017 collecting season was very productive for me in terms of Illinoia.  During winter of 2018 I therefore spent some time sorting many of the unidentified Illinoia from many hosts.  One main accomplishment was deciding what to consider as Illinoia thalictri MacGillivray.  I knew I had at least two species of the genus from Thalictrum, but had not taken the time to decide which should be considered I. thalictri.  See below for a photo and an account of this species.  One highlight of my collection work is listed below, and the re-arrangements and species name assignments were reflected in the slide collection database when the 2018 version was released. A paper published January 2021 reveals the identity of most Illinoia living on Holodiscus to be I. gracilicornis (MacGillivray). I worked on Illinoia classification again in fall of 2020 and decided to broaden what I identify as I. corylina to include most specimens living on Corylus, Aguilegia, and Thalictrum (the latter two genera both in Ranunculaceae). These specimens are obviously different from another species, possibly I. macgillivrayae, that also lives on Corylus (see below for more information).

The year 2021 was our first living in western Colorado. It was overall a light year for collecting Illinoia, with the exception of samples from various Asteraceae. My first collecting goal here was finding Illinoia masoni, which is known to feed on native Helianthus (the sunflower genus). To my surprise, finding this species was easy — it lives on native Helianthus and apparently other Asteraceae herbs all over our local mountains. I had wondered whether this species would be morphologically distinguishable from others of the genus living on Asteraceae, such as I. goldamaryae, I. grindeliae, and I. subviride. During December of 2021 I carefully sorted through the specimens I had identified as all of these 4 species. Making a long story short, I was able to make most of the specimens fit into 4 groups, and was able to make a case for species name assignments for each group based on old literature. The collection database released in 2022 will therefore reflect some changes in determinations. Going into this sorting process I understood that 3 of these 4 species had a fairly tight host range. I thought I. grindeliae fed on Grindelia, I. masoni on Helianthus, I. subviride on Aster/Erigeron, and I. goldamaryae was thought to be oligophagous on several Asteraceae genera. Now, it seems clear to me that: I. grindeliae feeds on various desert-inhabiting Asteraceae including Grindelia, Heterotheca, and a couple others I couldn’t identify; I. masoni seems to feed on several genera in wet montane forests, including Helianthus, Helianthella, and Erigeron; so far I. subviride does in fact seem to feed only on Aster/Erigeron. This messy sorting job then made me wonder whether these Asteraceae-feeding species can be distinguished from the I. macgillirayae/spiraeae group? A quick glance indicates that much study will be needed to understand Illinoia species delimitation, host ranges, etc.! No characters screamed out to me saying that none of the Asteraceae-feeders are in fact part of that shrub-feeding group. What a taxonomic mess it would be if we learned that most Illinoia species are oligophagous or polyphagous! I think I need to set aside some time in retirement to do host transfers among all sorts of plants using local Illinoia.

For a complete report on my Asteraceae-feeding Illinoia, please see the essay under the “Taxonomy Stories” tab above.

Since I work so much with Illinoia and especially its relationship and interactions with Macrosiphum and Uroleucon, I thought I’d make a similar listing effort as I did on the Macrosiphum page, i.e., discuss all the species in my collection, all those recorded in North America but not in my collection (click here to jump to that list), and those species I have that seem to be undescribed (click here to jump to that list). According to Aphid Species File there are 41 recognized species of Illinoia, 40 of them being North American, and 31 of them are represented in my collection.

Species I’ve identified in my collection (click on the name to jump to that species):

Illinoia alni (Mason)

This species was described by P.W. Mason in 1925 based on 3 apterous viviparae collected by Edith Patch in Orono, Maine from Alnus incana on 7 August 1916. He also had 2 other specimens from Orono on Alnus incana, 1904 and 1909. As with almost all descriptions from that era, Mason provided almost no useful information for recognizing the species today. MacGillivray (1958) provided much more detail and was forced to note major mistakes made by Mason, such as the count of lateral setae on the cauda: Mason recorded 2 pairs, MacGillivray counted 3 pairs on the same specimens. She also considered some specimens collected in Maine during August 1956 by D. Hille Ris Lambers from Kalmia (Ericaceae) and Thalictrum (Ranunculaceae) to be this species, including oviparae on Kalmia. She considered that these specimens could have fallen from nearby Alnus plants. Given the apparent host alternation of Illinoia corylina between a Betulaceae and Ranunculaceae (see below), her grouping of these specimens under one name may have been correct.

I was excited to take a collecting trip to Northeastern U.S.A. back in 2015, and I. alni was one of my collecting targets. I was lucky enough to find two specimens on an Alnus in Coos County, New Hampshire on 25 July 2015. These were cryptic on expanding young leaves. This find on Alnus in mid-summer might be evidence against heteroecy, but more field work in the area will be necessary to understand the host plant biology and life cycle of this species.

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Illinoia azaleae (Mason)

Gosh golly, here is another old species description (Mason, 1925), this time based on a single specimen collected in Florida on an ornamental azalea in February, 1924. Mason provided little useful information. Much as in the case of I. alni, MacGillivray (1958) tried to piece together a better story around this species. It has spread around much of the world and is believed to be anholocyclic in Europe. It is easy to recognize in Europe because it is one of the common Ericaceae-feeding Illinoia to be introduced there. Supposedly it is native to eastern North America.

I have a few samples filed under the name I. azaleae, one each from Oregon, California, and Quebec. They sort of key out to I. azaleae in MacGillivray (1958), but I have many samples of Illinoia from Ericaceae across western North America that morphologically may be species such as I. spiraeae, I. pepperi, I. azaleae, etc. Illinoia species seem especially prone to variable morphology and species that are very similar in appearance when all variation is considered. More collecting in this group reduces my confidence in any species identification since, as more variation is found, subtle distinctions that seemed previously to separate species are washed away.

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Illinoia borealis (Mason)

Wow, 3 in a row from Mason’s 1925 paper, this one yet again described with approximately zero useful information, based on a single specimen, this time collected by Edith Patch in Maine from a Gaultheria (Ericaceae). We can basically ignore what Mason said, and turn to MacGillivray (1958). She included specimens from several Ericaceae species and Myrica (Myricaceae) collected in northeastern North America, mostly Maine. She discussed the fact that D. Hille Ris Lambers thought this name might be a synonym of I. azaleae. She disagreed, pointing out the difference in setal numbers on first tarsal segments, with I. borealis having 3 and I. azaleae having 5. This and a couple other characters she pointed toward are, in my experience, not as useful and consistent as was thought back then. So, for me, it is not clear whether this is a recognizable species or not.

I have one sample of 2 apterous viviparae identified as I. borealis from Quebec in early August collected on a Vaccinium.

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Illinoia brevitarsus (Gillette &Palmer)

This species was described by Gillette and Palmer (1933) with subsequent information in Palmer (1952). They had several samples from throughout the growing season, collected near Fort Collins, Colorado, showing a monoecious life cycle with alate males. It is slightly more distinctive than the previously discussed species that feed on Ericaceae — its siphunculi are unusually long and the setae on the head are not extremely short. Oddly, the species name refers to short tarsi, but this species has very typical tarsi for this genus.

Since moving to Colorado in 2021 I’ve been able to collect this species a few times on its host, Clematis ligusticifolia, in the desert canyons just west of our home. I also have one sample from central New Mexico. I have not thoroughly studied the morphology of this species in comparison to similar Illinoia, but I should soon tackle such work to confirm or deny its distinctiveness from all the Illinoia on Asteraceae in the same desert habitats.

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Illinoia ceanothi (Bartholomew)

This is an unusual species of Illinoia, with some unusual morphological features, such as the almost annular imbrications on the antennae, the almost lack of reticulations on the siphunculi, and the spinulose imbrications on those siphunculi.  Also, it is an unusually fast running aphid when disturbed on its Ceanothus host plants.  It is monoecious holocyclic, feeding on at least a couple species of Ceanothus (e.g. C. velutinum and C. cordulatus).  I have tapped on hundreds and hundreds of Ceanothus bushes over the years, and am satisfied that this aphid is quite rare in most places I’ve traveled.  I have samples from Washington, Oregon, Idaho, and California.

Illinoia ceanothi fundatrix from southern Oregon near Lakeview, living on C. velutinum.

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Illinoia corylina (Davidson)

During fall of 2020 I re-evaluated the Illinoia in my collection, particularly the specimens categorized as I. spiraeae/macgillivrayae. I also looked at my material identified as I. corylina collected from both Corylus and Aquilegia. Fresh eyes and mind brought about by 2 years of not diving into these specimens allowed me to recognize that most of my material from Corylus, Aquilegia, and Thalictrum share important features and seem to belong to a single species. These features include relatively numerous sensoria on antennal segment III, and much longer dorsal and antennal hairs. Some of the specimens I have seen developing on Corylus in the spring are a different species, such as I. macgillivrayae (see below), and of course much material from Thalictrum is I. thalictri.  The host association of I. corylina with Aquilegia was suggested by other specialists many years ago.  After examining my many specimens from all three hosts, I see that this early speculation was correct and I am stretching the idea to include Thalictrum, another Ranunculaceae.

I have identified this species from Washington, Oregon, California, Montana, Idaho, Utah, and British Columbia.

Illinoia corylina? feeding on Thalictrum in north-central Oregon.

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Illinoia (Amphorinophora) crystleae (Smith & Knowlton)

I have a lot of affection for this species, and have pursued it avidly in the field for over 25 years.  It lives on one of our native honeysuckles, Lonicera involucrata (a host whose berries were described as inedible and possibly poisonous, “but they are so disgusting that there is little chance of anyone eating enough to worry.”  Plants of the Rocky Mountain Region).   My gut feeling (forgive the pun) is that this species belongs to a Caprifoliaceae-associated lineage that includes Macrosiphum stanleyi, Macrosiphum raysmithi, Macrosiphum diervillae, Macrosiphum schimmelum, and a few others.  This is another research topic I invite others to pursue.  It’s a very interesting biogeographical puzzle.  I so far have this species from the ocean to rather high mountains; states/provinces include: British Columbia, Washington, Oregon, Idaho, Utah, and Colorado.

Illinoia (Amphorinophora) crystleae from the eastern slope of the Cascade Mountains in Washington.
Illinoia (Amphorinophora) crystleae from the eastern slope of the Cascade Mountains in Washington.

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Illinoia (Oestlundia) davidsoni (Mason)

Described in 1925 by Mason, this species is one of the most common in the mid-elevation forests that I have frequented over the past 40 years. One might reasonably ask, then, why I don’t have a photo of it? I have no justification for this oversight.

Mason’s description was based on 3 alate viviparae collected in California in 1911. MacGillivray (1958) subsequently analyzed many more samples from western North America. She concluded that the species is likely heteroecious, migrating between Rubus parviflorus (thimbleberry; Rosaceae) and various species of Arnica (Asteraceae). My field observations strongly support heteroecy, with this species living on Arnica in areas where R. parviflorus can be found, the colonies on Arnica usually including male nymphs, and the mass production of alate females and males at the end of the season. Interestingly, however, I have never seen an ovipara, and all the males I have seen were on Arnica. One reason others have questioned heteroecy is the fact that viviparae can be found on R. parviflorus all summer. Such is not uncommon, in my experience, with primary host plants that produce fresh growth all summer, as does R. parviflorus. For example, some of the many species that use Rosa as primary host can be found on both primary and secondary hosts throughout the summer, e.g., Macrosiphum euphorbiae, Chaetosiphon thomasi, and Macrosiphum rosae. So, I am confident in heteroecy despite the shortcomings in my samples, assuming we can agree that species with partial migration are still called heteroecious (might it be useful to have two terms for obligatory migration versus facultative migration?).

Another concern MacGillivray (1958) and others have expressed is whether there are 2 or more species lumped under this name, possibly one or more monoecious forms confused with a heteroecious species. This worry is spawned by both observations of host plant biology plus morphological variation. I have looked through all my slides (about 60, representing a few dozen samples), and observed morphological variations such as cauda large or small, tibiae uniformly brown to dark only at base and apex, siphunculi in apterae entirely dark to almost entirely pale, quite a range in density of secondary rhinaria on ANT III, length and spininess of setae on tarsi and R IV+V, presence/absence of the dark spot on the tip of the forewing, size and number of lateral tubercles, etc. While it may be that more than one species is involved, I could not develop a set of characters that could be reliably used for the separation. Additional field observations and laboratory analyses may yet reveal more than one species in this mix.

An additional complication I found in my work is that this aphid sometimes uses a Potentilla (possibly an Argentina if you accept the division of Potentilla along those lines). I have two samples collected from northeastern Oregon in dense colonies that were developing on a Potentilla with pinnately compound leaves. Finding this phenomenon twice in 2 different years gives me confidence it is at least a somewhat common habit of this species. It might be interesting to note that the alatae from these samples tended to have paler tibiae than most other samples.

I have samples from British Columbia, Washington, Oregon, California, Idaho, Montana, Utah, and Colorado.

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Illinoia goldamaryae (Knowlton)

As mentioned at the top of this Illinoia page, this is one of possibly several recognizable Illinoia sensu stricto species that live on Asteraceae.  I have identified this species from several different host genera, but I am currently wondering whether samples from plants other than Solidago may be other species.

The place I was most able to study it and collect all its life cycle stages was on Solidago on the shores of Moses Lake in central Washington.  I have fundatrices, oviparae, and alate males from Solidago.  My specimens are from: Washington, Oregon, Idaho, Montana, Utah, Colorado, and Wyoming.

Illinoia goldamaryae aptera from Solidago in central Washington.
Illinoia goldamaryae aptera from Solidago in central Washington.

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Illinoia gracilicornis (MacGillivray)

For many years I have been collecting unidentified Illinoia from Holodiscus all across the western U.S. and Canada.  A paper was published January 2021 that revealed the identity of almost all these Illinoia. I wrote the following about it in that paper.

“MacGillivray (1958) described this species based on two oviparae collected in western Colorado in September of 1956 with the host plant recorded as Cercocarpus montanus. As discussed above under Aphis cercocarpi, I suspect that all records of aphids on Cercocarpus in western USA represent misidentifications of Holodiscus. Evidence supporting this idea is the description of I. gracilicornis, in which MacGillivray (1958: 74) stated, “Though only oviparae of this species are known there is little doubt that it is undescribed. Among species with 3 hairs on the 1st tarsal joints it stands apart because the hairs on the VIIIth abd. tergite are unusually long in comparison to the thin swollen area of the siphunculi so that recognition is easy.” After many years of looking for this aphid on Cercocarpus, I realized during study of my specimens collected on Holodiscus that they shared with I. gracilicornis this feature of unusually long setae on ABD 8. Herbarium records from Colorado showed that both C. montanus and H. dumosus grow in the area of Carson’s Hole, Colorado, the type locality of I. gracilicornis. One of the cotype specimens of I. gracilicornis was therefore borrowed from the BMNH, easily confirming that samples from Holodiscus and MacGillivray’s species are morphologically indistinguishable and should be considered conspecific.

All evidence available as of this writing indicates that this species is monoecious holocyclic on Holodiscus. Especially convincing on this point are the apterous males and regular detection of apterae throughout the summer. As noted below, there is some uncertainty about the conspecificity of the material from H. dumosus versus from H. discolor. Additional collecting of Illinoia from H. discolor across the west may reveal an as–yet–undetected heteroecious life cycle of populations using that host plant. That said, samples collected in mid-July 2020 near Coeur d’Alene, Idaho strongly argue for a monoecious life cycle.

 As noted above, the determination of samples as I. gracilicornis was made based on the suspicion that the original collection of this species by Hottes and Hille Ris Lambers was based on a misidentification of the host plant as Cercocarpus rather than Holodiscus. Both plants have been seen growing in proximity, and in fact the collection site of most oviparae and males in Chaffee County, Colorado was home to a mixture of large Holodiscus and Cercocarpus shrubs on the same slope.

This species seems to live wherever H. dumosus occurs, from mid–elevation forests to higher elevation mountain–top rock outcrops. This means that its annual growing season varies greatly in length. A simple demonstration of this is the collecting dates of the available fundatrices from the Warner Mountains in Lake County, Oregon: 19 May on the slope below Abert Rim (an open rocky west–facing slope at about 1500 m elevation), 11 June in the sparse forest of Can Spring (a partially shaded northeast–facing stream drainage at about 1900 m elevation), and 2 July on the summit of Crane Mountain (an exposed rock outcrop on a mountain top, about 2500 m elevation). The Abert Rim location’s growing season in most years will be early May through mid–October, the Can Spring site slightly shorter, and the Crane Mountain site will usually be early July through early September, i.e., the life cycle will vary for this species from about 2.5 months to about 5 months, all within the same mountain range.

So far this aphid is known from southern British Columbia, Washington, Idaho, Oregon, Nevada, California, Colorado, and Arizona.”

Illinoia gracilicornis from Holodiscus.

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Illinoia grindeliae (Williams)

Williams (1911) described this species based on an undisclosed number of apterous and alate viviparae, providing notes mostly on color and general appearance in life, with measurements and setal counts limited to total length of body, wing, and siphunculus of a single measured alate vivipara. In other words, he provided little information useful in modern taxonomy. As with species discussed above, MacGillivray (1958) attempted to make sense of this species and provided much more information. She erected a new subspecies for specimens with longer processus terminalis and shorter R IV+V than Williams’ material. Alas, she measured only 5 and 6 apterae of the two subspecies, respectively, so our total published information on this species is still limited. A key feature MacGillivray noticed was the presence of secondary rhinaria on the ANT IV and sometimes ANT V of alatae. This is a useful feature, but as noted by Blackman and Eastop, “Alatae may have 1-2 secondary rhinaria on ANT IV and V.” The species also features a relatively short and thick siphunculus with little to no reticulation and a relatively short ANT VI.

As noted in the introduction to this genus above, I now consider I. grindeliae to use various desert-inhabiting Asteraceae (especially Heterotheca) plus I have identified it from the ocean coast of Oregon on a few unidentified Asteraceae, one of which was probably Grindelia and the other an Erigeron or similar. I have samples from Oregon, California, Idaho, Utah, and Colorado.

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Illinoia (Masonaphis) lambersi (MacGillivray)

This species was described by MacGillivray (1960) as a follow-up to her 1958 massive treatise on the genus. It is reasonably easy to recognize, being a sort of mix between species like I. azaleae and I. (Masonaphis) rhododendri. I see that Blackman and Eastop don’t consider it part of subgenus Masonaphis, and I can guess why – the second segments of hind tarsi are only sparsely spinulate, unlike typical members of the subgenus. It is known to live on Rhododendron but I have it from an ornamental Ericaceae with small urn-shaped blue flowers as well, indicating a broad taste for Ericaceae under the right conditions (the latter plants were on the Oregon coast, definitely an unusual habitat).

I have samples from Washington, Oregon, California, and Idaho, mostly in cultivated settings.

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Illinoia liriodendri (Monell)

This unusual species was described way back in 1879, probably a highly noticeable species back then just like today, building up large populations on its host, the massive tulip tree (Liriodendron tulipifera), native to eastern North America. Morphologically it is a typical Illinoia, usually having dark antennae in both alatae and apterae. MacGillivray (1958) did not cover this species, probably because it has been shuffled between Macrosiphum and Illinoia over the years and she may have considered it Macrosiphum.

I have only a few samples from Washington, Oregon, California, and Germany (it has been introduced and is widespread in Europe).

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Illinoia masoni (Knowlton)

This species was described by Knowlton (1928) from samples collected in Utah in June and July of 1925. The recorded host was Helianthus annuus, what he referred to as “young sunflowers.” The collection locality of St. George, Utah, indicates that the original samples were from a garden or on plants volunteering in or near this small desert town. This initially made me suspicious as to whether these original specimens are conspecific with more typical material that I and others find on native forest-inhabiting Helianthus and other Asteraceae at high elevations. Examining the writing and figures of Knowlton (1928) and MacGillivray (1958), plus my samples from various habitats in western U.S.A., I am reasonably convinced that there is a single somewhat recognizable species we can call I. masoni.

I have samples identified as this species from Washington, Oregon, Idaho, Colorado, Utah, and New Mexico. Host plants include Helianthus, Helianthella, Erigeron, Aster, and what was probably Heliomeris. I even have a sample recorded from Solidago that I could not identify as I. goldamaryae and that seemed to fit I. masoni.

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Illinoia (Oestlundia) maxima (Mason)

This is a beautiful aphid that can be found on Rubus parviflorus (thimbleberry) in the moist parts of northwestern North America. As noted by Blackman and Eastop, it has an abbreviated life cycle and is monoecious on this shrub. I have samples from British Columbia, Washington, Oregon, and California.

Illinoia maxima aptera in the coastal area of Oregon, near the Nehalem River.
Illinoia maxima aptera in the coastal area of Oregon, near the Nehalem River.
Illinoia maxima alate vivipara, also from near the Nehalem River, Oregon.
Illinoia maxima alate vivipara, also from near the Nehalem River, Oregon.

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Illinoia (Masonaphis) menziesiae Robinson

This is an interesting species that is an example of an ‘Illinoia‘ with little to no swelling in the siphunculi.  It is also unusual in that it causes discoloration of the leaves on which it feeds (host being Menziesia ferruginea, Ericaceae).  Once I learnt to recognize Menziesia from among all the other nondescript shrubs in northern Idaho, this aphid was easy to find.  Another interesting tidbit is that the specimens collected on Menziesia near the ocean coast in Oregon’s Tillamook County differ rather markedly in morphology from specimens collected east of the Cascade Range in Washington, Idaho, and Montana; the leaf discoloration symptom of infestation was the same.  Similarly, I have 2 samples from Oregon collected on Rhododendron albiflorum. I initially considered these a separate, undescribed species, but additional study in October 2025 forced me to conclude that these specimens may be I. (M.) menziesiae. Their morphology is within the range of what I’ve seen in I. (M.) menziesiae from Oregon’s habitats, so if I consider western and eastern Oregon samples on Menziesia to be the same species, I should also lump the material on R. albiflorum. This is especially true in light of the leaf discoloration I saw on R. albiflorum leaves – very similar to what this species causes on Menziesia. More study might determine that 2 or 3 species are involved, but I’ll not be able to do such work any time soon.

Illinoia menziesiae aptera from northern Idaho along the Pack River.
Illinoia menziesiae aptera from northern Idaho along the Pack River.

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Illinoia morrisoni (Swain)

Described by Swain (1918), this species is very unusual among Macrosiphini for feeding on conifers, usually Cupressaceae. Coincidentally, it is also fairly distinctive morphologically, with unusually long tarsi, low antennal tubercles, and a tendency to have small spinal tubercles on abdominal tergite VIII. It is known to be widespread in western North America, extending into Central and South America, and has been introduced into Europe.

My samples were all collected from Juniperus, most often a form (species?) that grows as an understory tree in dry mid-elevation mixed conifer forests of New Mexico and Colorado. Blackman and Eastop indicate that the life cycle of the species is unclear, pointing out that populations in California seem to be anholocyclic. I have sexuales (males alate) from south-central Colorado in late September, 2019.

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Illinoia pallida (Mason)

Mason (1925) described this species based on 3 apterous viviparae collected on Clintonia (Liliaceae) in Orono, Maine in August of 1918. As usual, this was far too few specimens to adequately characterize a species. MacGillivray (1958) also pointed out that he made some important errors in his description. She redescribed the apterous vivipara based on 10 specimens from Maine and New Brunswick. She pointed out its useful diagnostic features including short tarsi and long setae on front of the head. She noted that although Mason had claimed the species has pale siphunculi, her specimen from New Brunswick had “rather dark” siphunculi.

I have several apterae collected in August, 2008 in New York. I noted the host as “lily with all basal leaves,” which certainly points toward Clintonia. On this same trip I also collected a single alate vivipara on a Streptopus that seems to be I. pallida. These specimens conform well to MacGillivray’s notes, including having noticeably dusky siphunculi.

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Illinoia (Masonaphis) paqueti (MacGillivray)

This species is known from Quebec, Labrador, and Alaska, having been described by MacGillivray (1958) based on specimens of all morphs collected in late August in Quebec by D. Hille Ris Lambers. As she noted, this is a distinctive species: it is large overall, with long setose rostral IV+V, and long and very spinulose tarsi. It feeds on Rhododendron (=Ledum) groenlandicum.

I have several specimens collected in early August 2013 in Quebec. My host plant notes are “Kalmia or Ledum,” the uncertainty stemming from my poor knowledge of shrubby Ericaceae in eastern North America.

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Illinoia patriciae (Robinson)

One of my graduate school mentors, A.G. Robinson, described this species in 1969 based on 3 alate and 10 apterous viviparae collected from Tsuga heterophylla (western hemlock, a conifer in the Pinaceae) in Vancouver, British Columbia. As he noted, it resembles I. morrisoni but the tarsi are a bit shorter, the spinal tubercles more prominent, and the rostrum is shorter overall.

I have one oviparous female collected on 17 September 1991 from Tsuga heterophylla near the summit of Mary’s Peak in western Oregon. Its morphological features and distinctions from I. morrisoni fit Robinson’s notes very well.

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Illinoia phacelia (Essig)

Essig (1942) described this species based on many specimens (20 apterae, 9 alatae) collected by P.S. Bartholomew on 10 July 1931 in northwestern California. It was recorded as living on Phacelia sp. Essig’s description is good, adequately characterizing the unusual morphology of this species, which is characterized by very long and setose rostral IV+V, long dorsal setae, and siphunculi barely swollen with almost no reticulation. These features of the siphunculi call into question the relatedness of this species to other members of this genus, but Illinoia seems to be as good a place as any to park it until a better classification is developed.

I have collected I. phacelia twice on an unidentified species of Phacelia near the ocean coast of southern Oregon in May. This Phacelia species looked different to other Phacelia I had collected on up until then (and since, for that matter), suggesting to me that I. phacelia may be host specific to this unusual Phacelia, not being oligophagous across the genus Phacelia as I had previously guessed may be the case.

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Illinoia (Masonaphis) pinawae (Robinson)

Grant Robinson described this species in 1973 based on 27 apterous viviparae from a few locations in Ontario and Manitoba, the specimens living on Rhododendron (=Ledum) groenlandicum. [I am compelled to point out that this is the first of the species I’ve written about here in October 2025 that was described from a reasonable number of specimens and across some geography. By 1973 we were definitely beyond the time when it was reasonable to describe new aphid species based on a single sample and/or a single locality. Because of this kind of work by Robinson, the format I used for my species descriptions was modeled after his.] As noted by Robinson, this species is similar to I. (M.) paqueti but has shorter, less spinulose tarsi, a shorter (but still relatively long) R IV+V, fewer rhinaria on a.s. III, and is smaller overall.

I have one sample of this species from Quebec in early August, collected in the same locality as one of my samples of I. (M.) paqueti. I don’t remember if both species were on the same bushes, but I think they were.

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Illinoia (Masonaphis) rhododendri (Wilson) 

This is a fun species because its seasonal life cycle is so dependent on the phenology of its main host (Rhododendron macrophyllum). Blackman and Eastop’s website notes, “the life cycle is uncertain but may be abnormal, with fundatrices in late July, and possible early production of sexuales (MacGillivray 1958).” This species does indeed have an abbreviated life cycle, but when during the year it occurs depends on habitat of the host plant. Near the ocean, this aphid is active in April and May, with sexuales on the fruits in May and June. In the mountains, sexuales occur in September because the host plant is in fruit during that time. This aphid starts off the season in longitudinally rolled leaves and during the sexual stage moves to the developing fruits. I have material from Oregon and California.

Illinoia (Masonaphis) rhododendri collected in September in the Cascade Mountains of Oregon. This is an ovipara.

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Illinoia richardsi (MacGillivray)

This aphid is common and easy to find on its host Anaphalis margaritacea (Asteraceae).  It has an impressively large, apparently natural distribution that reflects the distribution of its host across North America.  I have specimens from Washington, Oregon, Idaho, Utah, Colorado, and Quebec.

Illinoia richardsi from the Eagle Cap Mountains of eastern Oregon.
Illinoia richardsi from the Eagle Cap Mountains of eastern Oregon.

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Illinoia (Oestlundia) rubicola (Oestlund)

This species was described way back in 1886 by Oestlund in Minnesota. It is known to feed on Rubus, mostly various raspberry-like species. Based on my field work and that of others, this species is widespread but uncommon in North America. There is no evidence of heteroecy, the aphids living on growing leaves of Rubus throughout the season. Incidentally, MacGillivray (1958) has a useful key for separating the 3 recognized species of the subgenus Oestlundia.

I have 3 samples of this species, one each from Quebec, New Hampshire, and Idaho.

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Illinoia spiraeae/macgillivrayae

In my collection I group specimens together that might be identified as either I. macgillivrayae or I. spiraeae (in fact, several of the species known from Ericaceae are also very difficult to recognize when looking at the full range of material at hand).  The reason for this is that, although I get the general trend in size of ultimate rostral segment and cauda that is used to separate these species, their distribution and broad host range almost completely overlap.  This means separating the species is troublesome at best, and may be misguided if all these specimens in fact represent only one species.  Illinoia in this general group are common in the mountains in Washington, Oregon, and Idaho.  Alates can be abundant and found colonizing a wide range of plants on which they may or may not be able to complete development.  The latter fact makes it hard to know the full host range of these Illinoia aphids, but here is a start on the list (update from winter of 2018, and another run at sorting this material was just as frustrating and resulted in what appears to be a continuous range of variation in features of the siphunculi, cauda, and rostrum):

Amaranthaceae: Polygonum

Betulaceae: Corylus

Caprifoliaceae: Symphoricarpos

Asteraceae: Hieracium, Senecio

Ericaceae: Arctostaphylos, Phyllodoce, Rhododendron, Vaccinium

Hypericaceae: Hypericum (an Illinoia is commonly found in the Pacific Northwest on invasive and ornamental Hypericum)

Orobanchaceae: Pedicularis

Rosaceae: Amelanchier, Crataegus, Geum, Holodiscus, Prunus, Sorbus, Spiraea

Salicaceae: Salix

I’m starting to lean toward the idea that most of these specimens belong to one variable polyphagous species that lives on woody and herbaceous plants in several families across the whole west.

Illinoia from Spiraea in the Blue Mountains of Oregon.
Illinoia from Spiraea in the Blue Mountains of Oregon.

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Illinoia spiraecola (Patch)

The great Edith Patch described this species in 1914 based on material she collected in Orono, Maine on “Spiraea salicifolia.” MacGillivray (1958) subsequently redescribed the species based on several samples from various places in Maine and eastern Canada, collected on Spiraea and Thalictrum (Ranunculaceae). She did not attempt to conclude a heteroecious life cycle, but such is likely based on these two host plants and would comport well with other Illinoia that seem to use Rananuculaceae as secondary host (e.g., I. alni, I. corylinum). I agree with MacGillivray that this is a distinct species from I. spiraeae.

I have two samples of this species collected in August in New Brunswick and Quebec.

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Illinoia subviride (MacDougall)

This is one of a few very interesting species noticed in British Columbia and described by Alice MacDougall in 1926. Her description seems to be based on very few specimens, possibly one each of apterous and alate viviparae, but one can’t be sure. She says her material was “first collected in Bootahnie [sic] Valley, June 27, 1925, and then at intervals until August 2 of the same year.” This implies multiple specimens, but she measured only 1 of each morph, so I suspect she used only 2 specimens for her description. And, as far as I can tell from much effort, MacDougall’s types have been lost. So, it is hard to know for sure what she was looking at. All that said, I have separated off some material of the several species that use Asteraceae as hosts in western North America and labeled them I. subviride. Characters I have used to group specimens under this name include a moderately long R IV+V with a decent number of setae, strongly spinulose ventral side of head, short tarsi, and relatively thick, almost cylindrical, and not strongly reticulated siphunculi.

I have material identified as I. subviride from 2 collections in southern Oregon living on what were probably Erigeron species in the forest at moderate elevation.

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Illinoia thalictri (MacGillivray)

During winter of 2018 I sorted through my many slides of material of Illinoia collected from Thalictrum all across the west.  I had intentionally been saving material from this host until I had samples from many locations and times of year, allowing better-informed decisions about which samples represent I. thalictri.  In the end, my samples from the Rocky Mountain region seem to group together based on a relatively short ultimate rostral segment, and seem to comport with the idea of I. thalictri laid out by MacGillivray.  Therefore, I added species names to quite a few slides.  In some regions closer to the Pacific Ocean, Illinoia from Thalictrum have a markedly longer ultimate rostral segment and pretty clearly a different species, possibly in the Illinoia macgillivrayae/spiraeae group mentioned above. I’ve now identified this species from Idaho, Wyoming, Utah, Colorado, New Mexico, and Arizona.

Illinoia thalictri from the Apache Kid Wilderness Area in New Mexico, October 2012.

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Illinoia wahnaga (Hottes)

F.C. Hottes described this species in 1952, 20 years after first finding it northeast of Grand Junction, Colorado. He noted that he delayed the description of the species for so long because he was waiting to find the alate vivipara, which did not happen until May of 1952. This aphid is known to feed on Maianthemum (formerly Smilacina, Asparagaceae), plus a report of living on Convallaria majalis in Winnipeg, Manitoba (Robinson, 1965), the latter plant growing in a garden (it is not native in North America). Hottes did some good work, and waiting to describe a species until one has adequate material is to be applauded.

This species is fairly recognizable, being very pale in life, having a relatively short R IV+V, fully imbricated antennal segments III-VI, and the usual presence of spinal tubercles on abdominal segment VIII.

I have collected this species on Maianthemum racemosa and Maianthemum stellata, with samples from Oregon, California, Arizona, Utah, Colorado, and New Mexico. I note that the specimens from M. racemosa tend to have markedly longer setae on the tarsi than specimens from M. stellata. Interestingly, I have found numerous alate viviparae, with most of my samples including alatae. I have a fundatrix from central Oregon and oviparae from Colorado, New Mexico, and Arizona. I have found this species only in very large patches of Maianthemum growing under the shade of trees.

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Recognized species of North American Illinoia not yet identified in my collection:

Illinoia andromedae (MacGillivray)

This species name was coined by MacGillivray (1958) for 2 apterous viviparae collected on Andromeda glaucophylla (Ericaceae) in Quebec in late July 1956. Alas, this small number of specimens prevented her from providing anywhere near enough information to recognize this species, at least in the context of today’s knowledge that some Illinoia species are not strictly host specific, but tend toward oligophagy on a plant family or within a habitat type. In her diagnostic notes MacGillivray mentions the only difference between these two species and her conception of I. azaleae to be 3-4 setae on first tarsal segments versus 5 setae in I. azaleae. As noted elsewhere, I think this character is of limited value when the many dozens of Illinoia samples in my collection are evaluated. Most specimens of all species vary in this character, and when looking at 2 or more specimens from a single collection one can almost always find a tarsus with 3 such setae when the species is known for 5 and vice versa. Long story short, I think the species I. andromedae is not recognizable. Perhaps many more samples from Quebec and nearby regions, all collected from Andromeda, would reveal adequate characters and biological features to justify a separate species.

Illinoia canadensis (MacGillivray)

Another of MacGillivray’s 1958 new species, this one was yet again based on only 2 specimens. They were found feeding on Myrica in New Brunswick in early September 1956. Although she does not provide figures, she claims that the very large flange on the siphunculi is diagnostic for the species. Perhaps she is right, but I will not be convinced until I see several samples from Myrica that we might identify as this species.

Illinoia finni (MacGillivray)

I feel like a broken record here (a metaphor potentially lost on some young people, I know), but here is yet another species MacGillivray (1958) described based on 2 specimens, this time 2 alate viviparae. The specimens were collected on something recorded as huckleberry (probably a Vaccinium, possibly a Gaylusaccia) in Ohio, 30 May 1925. Atypically for her work she provided a drawing — of the unusually cylindrical siphunculus of this species. This drawing makes me think of Catamergus and Macrosiphum, both genera perhaps should have been considered for her specimens. Blackman and Eastop suggested that the Illinoia they had seen from Vaccinium in western U.S.A. could be this species, but I disagree. I have looked through my dozens of samples collected on Vaccinium in the west, compared them to the key to species provided by Blackman and Eastop, and concluded that chaos reigns in this situation. I present more of my thoughts below in the undescribed species section.

Illinoia (Masonaphis) magna (Hille Ris Lambers)

Described by Hille Ris Lambers in 1974, this species feeds on an unidentified Asteraceae and is known from a few collections in British Columbia and Alberta. In 1965 Grant Robinson had published Macrosiphum olmsteadi, which was found living on Aster macrophyllus in Ontario. I have studied these descriptions and my many samples of specimens in this ballpark, concluding that there is almost no distinction between the two accounts other than the number of setae on first tarsal segments, with 3 recorded for M. olmsteadi and 5 for I. magna. I briefly mention all of this under my blurb about M. olmsteadi. I also have mentioned elsewhere that number of setae on first tarsal segments is not as stable and useful a character as was apparently once believed, leading me to consider the two names to be likely synonymous. I ended up using the name M. olmsteadi for my samples mostly because it is the older name and would be used were the two species declared synonyms. It is reasonable to argue both sides — for two species and for one. I am choosing to keep my specimens filed under M. olmsteadi for now.

Illinoia pepperi (MacGillivray)

This is a very interesting case of MacGillivray (1958) having lots of specimens from many collections to work with — J.O. Pepper provided her with numerous samples from his collection, mostly found in Pennsylvania and Maine. She had all morphs, including 2 fundatrices, 29 apterae, 18 alatae, and one each alate male and ovipara. The key distinguishing feature of this species was the dark siphunculi and appendages. She seemed to have trouble identifying other features that would consistently separate these specimens from other species such as I. pallida, I. spiraecola, or I. borealis. She also noticed that all these specimens varied as to number of setae on first tarsal segments, with variation among 3, 4, and 5. The latter is consistent with my observations of most Illinoia sensu stricto species of which I have studied numerous samples. This species name has become somewhat commonly associated with Illinoia aphids collected on cultivated blueberry. While in eastern North America this might be relatively straightforward, in the western states it is not so clear. Several years ago I participated in a research project on blueberry aphids in western Washington State. Many of the aphids were Illinoia. These tended to be fairly darkly pigmented, but rarely were the siphunculi entirely dark as documented by MacGillivray, and there were plenty of specimens that were mostly pale. In my opinion it is not clear what species name(s) to use for Illinoia collected from Vaccinium, cultivated blueberries, and some other Ericaceae as well. I plan to write more about this issue below under the undescribed species section.

Illinoia reticulata (Mason)

Mason (1925) described this species based on a single alate vivipara collected in Washington, D.C. on raspberry (Rubus). Thankfully, Louise Russell published a short note in 1961 attempting to clarify the generic placement and taxonomic features of this specimen. Her writing makes it clear that the specimen is indeed an Illinoia, but we lack adequate information to know whether this specimen represents a distinct species. She expressed strenuous doubt that the true host of this aphid was Rubus, having failed to collect anything similar from Rubus in the D.C. area after years of looking.

Illinoia (Masonaphis) rhokalaza (Tissot & Pepper)

From what I can tell this is a distinctive species that was well-documented by Tissot & Pepper (1944) and MacGillivray (1958). The original description was based on hundreds of specimens rather than the more typical condition we’ve seen so far in Illinoia of one or a few specimens. It is known from eastern U.S.A. on various Ericaceae.

Illinoia simpsoni (MacGillivray)

This species, described by MacGillivray (1958), has been of particular interest to me for decades because its documented hostplant, Linnaea borealis (Caprifoliaceae), lives in almost all the mountain habitats I frequent. I figured surely, after 35 years of watching for aphids on this plant, I’d find Illinoia simpsoni. But no. A part of me, therefore, worries that Linnaea is not the true host of the aphids described as I. simpsoni. On the other hand, I am aware of plenty of aphid species that seem to have restricted geographic ranges despite using plants with broad ranges. So this species’ range could be restricted to eastern North America, living on a plant with a Holarctic distribution. MacGillivray claims that this species is distinctive for its short ANT V compared to the base of ANT VI and for its markedly swollen siphunculus with a wide flange. Having not seen this species, I can’t comment on how distinctive it seems to me.

Illinoia wilhelminae (Hille Ris Lambers)

Hille Ris Lambers described this species in 1962 based on a single apterous vivipara collected on Alnus rugosa (nowadays known as Alnus incana rugosa) in Quebec, 25 June 1960. Due to its unusually long and setose R IV+V, Hille Ris Lambers considered that Alnus was likely not this species’ normal host since aphids with very setose R IV+V usually use hairy, fuzzy, or sticky host plants. He also noted that its siphunculi were not at all swollen, that the tarsi were very short with spinulose imbrications and only 3 setae on first segments, and that the head was smooth. This does indeed sound like a rather distinctive species, but without confirmed host plant information and with only 1 specimen, I am unconvinced of the value of the species name. If I were to describe as new every unusual specimen in my collection I’d be writing and publishing like mad for years.

Putative undescribed species in my collection:

As is my way, I try to collect many samples of unusual, possibly undescribed, aphids from many sites and times of year before publishing or commenting confidently that I have an undescribed species. Below are the Illinoia in my collection that I think might be undescribed species.

Unidentified species on Chamaebatiaria millefolium, fern bush: when I first found aphids on this plant in SE Idaho in 2015, I wrote Acyrthosiphon on the label due to the more or less cylindrical siphunculi without reticulation.  Since that time I have collected this apparently single species in a few places in California and Nevada.  In some samples the apterous viviparae have a couple rows of reticulation on the siphunculi, and the male and alate vivipara possess distinct reticulations and look like more typical Illinoia. In other respects the range of variation and the unusual features of the aphid suggest that a single species is involved.  One of the unusual features is the long siphunculi, slightly outwardly curved, with swelling near the apex, and a trend of 1-3 small setae near the base in the larger specimens.  These features of the siphunculi are similar to Acyrthosiphon macrosiphum Wilson. To my surprise this aphid has somehow found, or been transported with, an ornamental fern bush that we planted in our Colorado yard. This presents a fun opportunity to study an unusual aphid right here at home.

Unidentified species on Monardella in the mountains of New Mexico: During our almost-annual fall visit to New Mexico, I spend 2 or 3 days collecting aphids and have found many interested samples over the years. One I’ve pursued each time I visit the Magdalena Mountains is an Illinoia that lives on Monardella (Lamiaceae) near the summit of South Mountain. In 2022 I was able to get large numbers of specimens including the sexuales (males are alate). Evaluation of all my material indicates that this species is recognizable and probably undescribed.

Unidentified species feeding on Vaccinium in western U.S.A.: As noted above under I. pepperi, I have many samples of Illinoia collected from various Vaccinium species across western U.S.A. These represent well over 100 specimens and dozens of samples, both apterous and alate females with a couple sexuales mixed in here and there, collected in Washington, Oregon, California, Idaho, and Colorado. Frankly, these are not identifiable using the current key to species on Blackman and Eastop’s website. Let’s look at the relevant section of their key:

6. R IV+V 0.67-1.1 × HT II …..7
– R IV+V (1.1-) 1.2-1.8 × HT II …..8
7. SIPH dark, sometimes paler at extreme base, less than 10 × longer than maximum width of swollen part …..Illinoia pepperi
– SIPH pale and more than 10 × longer than width of swollen part …..Illinoia finni
8. First tarsal segments with 3 (rarely 4) hairs. R IV+V 1.4-1.8 × HT II. SIPH 1.7-2.3 × cauda …..Illinoia borealis
– First tarsal segments with 5 (rarely 4) hairs. R IV+V 1.1-1.4 × HT II. SIPH 2.2-2.5 × cauda …..Illinoia azaleae

The first of these couplets is fine, I suppose. The ranges don’t overlap horribly, and based on my study of the genus these ratios seem reasonable — they are basically asking whether a) R IV+V is shorter than or about equal to HT II, or b) R IV+V is distinctly longer than HT II. The second couplet is where everything goes awry. It is trying to separate two species, one of which, I. finni, is only known from alate viviparae (this is a key to apterous viviparae). So, it is guessing about the features of the apterae of I. finni. In addition, it is relying heavily on pigmentation, which is too variable to use in a key. If we take my Illinoia specimens found on Vaccinium, they almost all key to I. finni, but they have distinctly swollen siphunculi with very few rows of reticulation, unlike the 2 known specimens of I. finni, which have narrow, cylindrical siphunculi with several rows of reticulations. It is very unlikely that an Illinoia species would vary so much in these features. If we use our imagination about color before treatment with NaOH for clearing, we might declare a handful of my specimens to be relatively pale I. pepperi (relative compared to the original description).

Anyhow, the crux of the issue is whether we can assign a name to my scores of specimens found on Vaccinium in western U.S.A. I don’t think I. finni can be argued as reasonable based on the siphunculi shapes just mentioned. If we declare them to be I. pepperi, then we need to provide diagnostic features for the species completely ignoring pigmentation since many of my specimens are almost completely pale compared to the types from eastern U.S.A. This might be a reasonable course of action, but I would want to see more Illinoia specimens feeding on Vaccinium in many places across North America to evaluate pigment variation and search for additional characters useful in circumscribing the species across its entire range of ecology and geography.

We should probably also study the next couplet. Could all my specimens be one of these two species? Almost all my specimens from Vaccinium have 3, 4, and 5 setae on first tarsal segments, so that character is of no value here. Assuming that the R IV+V/HT II ratio is a useful character, some of them could fall on the low end of the R IV+V/HT II ratio for I. azaleae. Many of my samples, however, have this ratio less than 1, which would seem to far outside the accepted range for I. azaleae.

So, what to do? I don’t know. I have all these Ericaceae-feeding samples together with my many samples of the I. spiraeae/macgillivrayae complex I discussed above. One possible resolution is that my Vaccinium (and some other Ericaceae) specimens are all part of this complex, representing one or two oligophagous or polyphagous species. Another possibility that deserves serious attention at some point is whether Illinoia are mostly host-specific down to plant species, meaning, for example, that my material from Vaccinium scoparium (~half my samples) is a different species from my samples from Vaccinium uliginosum (~half my specimens). Much more study will be required to settle on a decision here. I may tackle such work eventually, but alas it may require borrowing material from other collections which is getting more difficult every year with postage rates and reliability going drastically up and down, respectively, and the shrinkage of staff at university and federal collections.