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iridescent larva - Trirhabda bacharidis

iridescent larva - Trirhabda bacharidis
Sanibel, Lee County, Florida, USA
December 7, 2009

Images of this individual: tag all
iridescent larva - Trirhabda bacharidis iridescent larva - Trirhabda bacharidis

Moved from Trirhabda. Based on comment in this forum by Michael C. Thomas (24 May 2010) that there is only one Trirhabda species in Florida and it is T. bacharidis and it only feeds on Baccharis halimifolia.

Moved from ID Request.

Looks like Trirhabda larva. I
Looks like Trirhabda larva. It's a little Chrysomelid

Host plant?
Bo, do you know what plant it was on? (Leaf beetles are highly host-specific, so knowing the plant can help ID the insect.) If not, if you could provide a wider view of the leaf of plant (perhaps the uncropped original or related photo), I may be able to identify it. (I already have a suspicion for this one, but will wait for a wider photo to remove doubt.)

I did have a shot of the plant but I deleted it. No, I do not know the name. This is the only other pic left but I doubt it adds much.

Groundsel Tree
Thanks, the second photo does provide a glimpse of another leaf in the background. I believe it is Groundsel Tree (Baccharis), either B. halimifolia or B. glomeruliflora, both widespread across Florida. According to Native Trees of South Florida, the differences between them are that B. halimifolia ("Saltbush") occupies mainly coastal areas (and have long-stemmed flowers) while B. glomeruliflora occurs "in nearly every habitat in South Florida, although its favorite is wet ground with full sun" (and has short-stemmed flowers). I cannot see the flower stems, but knowing that Sanibel is located on the coast make me suspect B. halimifolia.

The other factor that makes me suspect B. halimifolia is that the oily-black larvae of Trirhabda bacharidis, a leaf beetle, are known to use it as a specific host plant. I recently assisted with identification of Baccharis halimifolia on two other postings:

so I have begun to recognize the larvae, the plant (typically chewed up), and their relationship from photos. (It may be that the two Baccharis species are so similar that the beetles occur on both, but that's just a speculation.) As with the other two, I will leave the final ID of the insect to a entomologist (I am merely a botanist). To facilitate that, I will cross-post this one with them.

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