After > 4 yrs of this thread laying dormant, I was just finishing writing a (typically long) comment at the bottom of the post...and when I look up to go to move your image, lo and behold, you'd just done it! :-)

There's a perfect word for this phenomenon...but I can't remember it. Something like "synchronicity", but better. Oh well...the "main" thing is, we got your Trirhabda to species! :-)

I would say (:

But I think you may have triggered it by moving some T. on Brittle bush to geminata

Now I just need to find out the plant.

en jij bedankt voor al de mooie bladhaantjes (but let´s stick to english)
Hope we can figure it out! There are several people here with considerable knowledge on western Trirhabda species and hostplants by now.

was a new word for me: I like it! As for the plant: Brickelia, Brickel bush or tassel flower seems right. I need to get back into botany - I'd be much more at home in Holland.

I hope Aaron Schusteff, Hartmut Wisch or Boris Beuche will see this and look up their references for species with Brickelia as host

...with several suggestions as to species on the other page: B. rusbyi, B. californica, and I'll add in B. grandiflora.

I just happened upon this post yesterday, and (as Rob hoped for :-) I've checked my references for Trirhabda. Indeed, according to the info in the article:

"Phylogeny and host-plant association in the leaf beetle genus Trirhabda LeConte (Coleoptera: Chrysomelidae)" by Zuzana Swigo and Karl M. Kjera
in the journal MOLECULAR PHYLOGENETICS AND EVOLUTION, vol 32 (2004) pp 258--274

...there is one species, Trirhabda schwarzi, which occurs in Arizona and is listed as using the brickel bush Brickellia californica (which also occurs in Arizona) as a host plant. However, the table on pg 271 of that article indicates that T. schwarzi show have a metallic green, blue, or bronze elytra with stripes...which seems not to fit your photo. It sure looks like a Trirhabda, but I wonder if it may be some other similar looking related genera? I don't really know, Rob is much more knowledgeable about beetles than I am! However, as I recall, there is a key to the subfamily Galerucinae in:

Blake, D.H. 1931. Revision of the species of beetles of the genus Trirhabda north of Mexico. Proc. U.S. Nat. Mus. 70:1-36.

So Margarethe, if you can find a specimen of your beetle and are feeling very gung ho, you may be able to track down that reference at the University of Arizona library and have a go at keying it out :-) Also, if you want a chance to brush up on your botany, see my comment attached to the other photo in your linked pair here.

thanks for looking into it Aaron. I don´t want to exclude the species you mention directly. There are several species of Trirhabda with variable colourpattern with both striped and all dark specimens among the variation. Maybe T. schwarzi is one of them.
I guess the key won´t give a definitive answer as Doris Blake has described the species herselve in 1951

...and also the accumulation of more references and knowledge (hopefully even a bit of wisdom :-)

Since our earlier thread of comments in 2007 (above) both the 1931 treatment by Blake, and the nearly equivalent 1965 key of Wilcox(1) have become freely available online. Margarethe's beetle here seems to go reasonably well to something Blake called T. nigrohumeralis...referring to the two small dark spots on the "shoulders" (i.e. anterior corners of the elytra). If you like, you can go through the key characters online, starting in the middle of pg. 9 of Blake. The couplet sequence is:

1c, 29b, 30b, 31a......nigrohumeralis (pg. 30).

And here's a link to the description, on pg.30 of Blake. The description fits Margarethe's beetle fairly well. At least there are no blaring visible inconsistencies...except that the spots on M's beetle look "medium-size" rather that "small" to me, but spot size can be (annoyingly:-) variable. There's even a line drawing, Fig. 20 here.

Note, in particular, the AZ locales mentioned and the listing of the host plant as Brickellia sp., with reference to "E. A. Schwarz".

The 1965 key of Wilcox is structurally almost identical to Blake's 1931 key...except in includes the taxa described in the interim. Taking Margarethe's beetle through that key, with virtually the same key break choices, leads to T. schwarzi. And Wilcox mentions (on pg.25) that Blake's 1931 nigrohumeralis is a synonym of her 1951 schwarzi.

Either way, it appears there is still, to this day, just one species of Trirhabda known to have Brickellia as a host, according to Swigoňová & Kjer(2)...and that's T. schwarzi. And from the figure and description in Blake (1931), and other expert determinations presently on BugGuide, it appears the entry in the table on pg.371 of Swigoňová & Kjer(2), indicating T. schwarzi has "metallic green, blue, or bronze elytra with stripes" is either in error, or inconstant. So Rob was correct not to exclude T. schwarzi :-)

elytra-color with age. Never saw a metallic one, but I raised T. geminata and got all kinds of variants, and then they all got darker for weeks, not just during a usual teneral phase...

The ones from Brittle bush, now T. geminata, consistently have much smaller shoulder spots than this guy

I didn't know they started out light and got darker...especially over such a long time span. Good to be made aware of that.