Knotweed Leaf Beetle

2023 October 22

I found this one on the sidewalk while walking across campus on July 17, 2023, so I popped it in a little bottle that I always carry with me so I could bring it home for pictures.

You would think that one would be able to immediately identify a bright-green beetle with an orange pronotum and a grotesquely swollen abdomen, but it turned out to be harder than I thought.

It didn’t show up on BugGuide with the obvious search terms “orange green beetle”. Oh, it did turn up a bunch of orange and green beetles, but they were obviously not the right ones. They all had the wrong head/antenna shape, and frequently also the wrong head color.

So I ended up doing a blanket search of the whole Internet. And, after going through a couple of pages, I suddenly spotted some photos that were an exact match. Unfortunately, they were on one of those “stock image” sites, and the photographer only identified them as “orange and green beetles”. Well, heck. I knew that.

So then, I took that stock image, and had Google do a direct image match search. And this lead me to an image that was used on the Wikipedia page for Gastrophysa polygoni. It doesn’t say much (about 4 lines of actual text, which may set some sort of record for a Wikipedia page that isn’t tagged as being a “stub” article). But, it did give me a name!

Armed with the actual species name, I was now able to find it on Bugguide. This is very clearly a female Knotweed Leaf Beetle, Gastrophysa polygoni. I know it is a female because in this species the mature females become massively distended with eggs. The males have the same coloration, but their abdomens fit neatly under their wing covers. The genus name “Gastrophysa” basically means “enormous gut”, and “polygoni” refers to the family of plants that they like to eat, the Polygonaceae, or knotweeds.

These are not native beetles in North America, they were accidentally imported from Eurasia at some point. Of course, a number of the plants that they eat are also imports. And some, like Japanese Knotweed, are noxious invasive weeds to boot. So if these beetles want to eat those plants, I am sure they are welcome to them.

Incidentally, the number of beetles that are partly iridescent green and partly orange makes me think that this is not a coincidence. They are probably another mimicry complex, where at least some of the green-and-orange beetles are deeply unpleasant to eat.

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