How we learned to tell apart Eupeodes americanus and pomus

More than 400 observations of Eupeodes larvae have been shared on iNaturalist from around the United States and Canada. Based on their distribution, the vast majority could only belong to one of the two widespread Eastern species: Eupeodes americanus and E. pomus. A series of reared larvae recently enabled us to rediscover characters to separate larvae of these two species that were previously described (under synonyms) in Heiss (1938). Many thanks to @molanic @zdanko and @trinaroberts in particular for their help (let me know if I'm forgetting anyone else please).

@nsouc reared an E. americanus about a year ago (iNaturalist#52819675). Along with @zdanko, I reared several Eupeodes americanus in 2021 and two males successfully emerged as adults (iNaturalist#83068603, iNaturalist#96176999). @molanic found several Eupeodes larvae in June and shared them on iNaturalist, where we invited them to study these larvae further. @molanic ultimately reared 13 E. pomus and 15 E. americanus, of which 4 E. pomus and 3 E. americanus successfully emerged. Despite strong differences between the larvae of these species, adults do not noticeably differ except in male terminalia:

E. pomus male terminalia: https://www.inaturalist.org/photos/153537921
E. americanus male terminalia: https://www.inaturalist.org/photos/157406323
E. pomus female https://www.inaturalist.org/observations/93163802
E. americanus female https://www.inaturalist.org/observations/94974788

With the ability to confidently identify the larvae of these species, we now have a dataset of their respective occurrences based on iNaturalist and BugGuide data. You can see maps here:
https://www.inaturalist.org/taxa/625990-Eupeodes-pomus
https://www.inaturalist.org/taxa/460811-Eupeodes-americanus
https://bugguide.net/node/view/2045484/data
https://bugguide.net/node/view/33142/data

Eupeodes americanus larvae are grey to brown in color. Starting in the second instar, they possess a small round patch of small black setae on the side of each abdominal segment. These black setae persist into the puparium and are an easy way to separate larvae and pupae from E. pomus.

E. americanus feeding on Aphis nerii on milkweed in IL:
2nd instar https://www.inaturalist.org/observations/94630689
last instar https://www.inaturalist.org/observations/94632720
pupa https://www.inaturalist.org/observations/94632719

E. americanus feeding on aphids cf. Uroleucon on a yellow composite in MD
1st instar https://www.inaturalist.org/observations/95500012
2nd instar https://www.inaturalist.org/observations/95500005
Last instar https://www.inaturalist.org/observations/95500004
pupa https://www.inaturalist.org/observations/96177020

Eupeodes pomus larvae are yellow to green in color, with bright red and white markings dorsally. Last instars also lack the long bristles typical of E. americanus or larvae with similarly-colored patterning like Dioprosopa clavata or Paragus spp.

E. pomus feeding on Aphis nerii on milkweed in IL:
first instar https://www.inaturalist.org/observations/91681414
medium early instar https://www.inaturalist.org/observations/92015773
later early instar https://www.inaturalist.org/observations/80171846
last instar https://www.inaturalist.org/observations/91575907
fresh pupa https://www.inaturalist.org/observations/91575909
Mid pupa https://www.inaturalist.org/observations/91680956
Late pupa https://www.inaturalist.org/observations/92296961

This last set wasn’t reared but illustrates some of the color variation typical of E. pomus larvae on unusual species of host aphid:
E. pomus feeding on* Aphis craccivora* on redbud in IL:
Last instar https://www.inaturalist.org/observations/18787138
Middle instar https://www.inaturalist.org/observations/18787141

Many other larvae of these species have been photographed and shared on iNaturalist and BugGuide, representing many other aphid and ant hosts.

Heiss, 1938. A classification of the larvae and puparia of the Syrphidae of Illinois, exclusive of aquatic forms. University of Illinois Urbana-Champaign. https://archive.org/details/classificationof16heis/page/132/mode/2up

Posted by edanko edanko, October 25, 2021 22:16

Comments

Should be noted that we have multiple image vouchers of reared adults that have genitalia photos.

Posted by zdanko about 1 month ago (Flag)

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