In late summer little insects called picture-winged flies lay their eggs in the inflorescences of knapweed and ...
... when they hatch the larvae crawl down into the seed head, feeding on the seeds and producing a woody gall that persists through the winter. You can tell they are there just by squeezing the seed head, when you can feel the hard gall within.
Here is a seed head carefully cut open, to reveal the larvae in their woody chamber within...
.... including this one that looks well fed and ready to pupate.
I harvested some galled seed heads back in November and today the adult insects began to emerge.
The tiny flies have rather beautiful eyes but....
... their most striking feature is their wing patterns that give them their common name.
I think this species is Urophora jaceana - thanks to @SK53onOSM for correct ID
Update: Geoffrey Wilkinson (see comment below) informs me that this species is Chaetostomella cylindrica
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Our meadow has been grazed down this winter, but I'll be on the hunt for these little creature next year. I never knew they were there
ReplyDeleteVery pretty little flies. The adults are on thistle and knapweed flowers in late summer and you can find the galls in knapweed just by squeezing the seed heads in November. I think the larvae can destroy a lot of the seed and I think they have been tested as biological control agents in places like New Zealand, where knapweed has been introduced and has become invasive
ReplyDeleteThe photographed adult is Chaetostomella cylindrica: the scutellum has a black apical spot and a pair of black basal spots. U. jaceana lacks those spots and has solid black markings on the wings. Nice pictures of the larvae - they are certainly squeezed tightly into those cavities!
ReplyDeleteThanks Geoffrey - i have updated the post
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