Monday, February 16, 2015
In summer tiny picture-winged flies, similar to this one although not the same species, lay their eggs amongst the florets of knapweed Centaurea nigra, like the one in the picture above. The usual species that parasitises knapweed is Urophora jaceana - click here for a picture.
After the larvae hatch they crawl down into the base of the inflorescence and produce a hard woody gall, where they feed on the seeds and are well protected through the winter.
Parasitised seed heads tend to have thicker stems but the easy way to detect the presence of the developing larvae in winter is just to squeeze the knapweed seed heads between finger and thumb. If they are galled you can feel the hard lump inside.
Opening up the gall reveals the developing larvae down at the base. You can see how they've chewed their way down through the gall, growing fatter as they progress and finally resting in a chamber where they'll pupate and hatch as picture-winged flies in spring.
Knapweed seed heads are carried on stiff stems that often protrude through the snow in winter. Then they tend to be shredded by hungry birds - probably blue tits and great tits. It's hard to be sure whether the birds are after some of the smaller insect larvae that sometimes colonise these seed heads or whether they can winkle out the picture-winged fly larvae from inside their woody gall - I suspect that they can.
For a closer look at the larvae, click here.