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Unidentified Trirhabda leaf beetles sorted by host plant

by John Pearson

To facilitate species-level identications of insect images in the undifferentiated Trirhabda gallery, I identified the apparent host plant captured in the photos (or confimed previous identifications, when present) and grouped them within broad categories defined based by maturity (larvae versus adults) and geography (eastern North America versus western North America). Two caveats became apparent during this compilation. The sedentary larvae are likely to be restricted to the host plant, but are discouragingly similar in appearance; coupled with the fact that multiple species may utilize the same host plant, larvae remain difficult to differentiate. Adults display more traits, but being more mobile are likely to show up in some photographs on incidental "hosts". Nonetheless, I thought it would be useful to organize the images in this way in hopes that some additional identifications could be made. Just the act of focusing new attention on these images, many of which are several years old, might result in some new IDs by bringing to bear any new perspectives that may have been gained since the photos were first examined. I thank the submitters who endured my numerous requests for additional photos of the host plant!

The following discussion is organized into four sections: Eastern Larvae, Western Larvae, Eastern Adults, and Western Adults. Within each section, images are organized into host plant groups. Notes on tentative insect identification from previous commenters are also provided.

EASTERN LARVAE - Two groups of plants were evident here: Groundsel-tree (Baccharis halimifolia) and Goldenrod (Solidago).

On Groundsel-tree (Baccharis halimifolia). Given this host plant, it is tempting to identify the larvae as Trirhabda bacharidis, but uncertainty whether this is the only Trirhabda species to utilize this host plant has inhibited this conclusion by commenters to date. If anyone can resolve this uncertainty, please comment accordingly!

On goldenrod (Solidago).
Host plant ID uncertain, but probably goldenrod.
Several of the images in the above two rows of photos appear to show Tall Goldenrod (Solidago altissima), but this species is part of a confusing complex with two other closely related goldenrods: Canada Goldenrod (S. canadensis) and Late Goldenrod (S. gigantea), making exact identification difficult from fragmentary images. John Watts noted that there are several species of Trirhabda that feed on goldenrod, but among the ones suggested by commenters for these images are T. virgata and T. canadensis.

WESTERN LARVAE - Four groups of plants were evident in this collection of images: rabbitbrush (presently two genera but both once united as Chrysothamnus), coyotebrush (Baccharis, specifically B. pilularis), brittlebush (Encelia farinosa), and sagebrush (Artemisia, specifically A. california).

On Rabbitbrush (Ericameria nauseosa, Chrysothamnus viscidiflorus). One suggestion from a commenter was T. nitidicollis, but John Watts noted that several species of Trirhabda occur on rabbitbrush.

On Coyotebrush (Baccharis pilularis). Trirhabda flavolimbata was suggested by a commenter for this image.

On Brittlebush (Encelia farinosa).

On California Sagebrush (Artemisia californica). John Watts noted that Trirhabda nitidicollis, T. pilosa, T. sericotrachyla, T. confusa, and T. luteocincta are potentially hosting on this plant.

EASTERN ADULTS - Goldenrod (Solidago) dominates the host plant images in this section (further sorted into verified, probable, and speculative identifications).

On goldenrod. Commenters suggested Trirhabda adela, T. borealis, T. canadensis, T. convergens, and T. virgata for this and following groups.

Host plant ID uncertain, but probably goldenrod. (2 rows)

Not on goldenrod, but goldenrod possibly nearby.

WESTERN ADULTS - Five plant groups are evident in this section: goldenrod, rabbitbrush (possibly confounded with coyotebrush due to proximity), brittlebush, sagebrush, and brickellbush.

On goldenrod (Solidago gigantea, S. canadensis). Trirhabda borealis, T. convergens, and T. virgata were suggested by various commenters.

On rabbitbrush.
On rabbitbrush, but near coyotebrush? Commenters variously suggested Trirhabda flavolimbata and T. confusa.

On brittlebush. Trirhabda geminata was suggested for two of the four images here; in fact, the fourth one has already been moved to the species page as a result of this discussion.

On sagebrush (Artemisia, possibly A. nova).

On Brickellbush (Brickellia rusbyi).

Thanks for your efforts here, John!
I've been studying up on Trirhabda and will be working on IDing more posts, and moving them to species pages, as time permits.

I love this genus :-)

Moved this one today:

Eastern adult on sunflower

UPDATE: Now identified as probable T. adela.

One of my own
Eastern larva on goldenrod

Three more
I found these three images in the family (Chrysomelidae) gallery; commenters suggested they might be Trirhabda, but verification of the genus is needed. If they are indeed Trirhabda, they would fit into my above summary in the following categories:

Eastern Larvae on goldenrod
Western Larvae on rabbitbrush
Western Larvae on brittlebush

There is
only one Trirhabda species is Florida and it is T. bacharidis and it only feeds on Baccharis halimifolia.

Based on your comment, I moved the two Florida images to the Trirhabda bachardis guidepage.

Yes, fantastic work, John
Just a note, here near Chicago we also find these larva on Helianthus grosseserratus if that helps clear up any of your plant ID work. Will try to remember to get some images for you this year.

fantastic -- thanks!
i shot a notice to Ed Riley, with hopes this will help him ID the beetles.

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