The hidden life in knapweed seed heads

Recently I wrote a piece for the Guardian Country Diary about the insects that live and breed in knapweed seed heads. During the summer bees, butterflies and many other insects visit the flowers for pollen and nectar, but some flies and small wasps also visit to lay eggs in the seed heads.

One of the common names for knapweed is hardheads, and if you squeeze the seed heads you find that some are harder than others, as though they have a small nut inside. These are galled by a tiny tephritid fly called Chaetostomella cylindrica , whose larvae feed on the seeds as they grow

This is a seed head cut open, in March, to reveal the barrel-shaped white larvae, inside their gall. By this stage most of the seeds have been eaten. Goldfinches visit knapweed to eat the seeds in autumn, so some of these larvae may be eaten by them too, though it seems unlikely that the birds can open such a tough, woody gall.

But the tephritids have other enemies.

                   This is the adult, rather beautiful fly, Chaetostomella cylindrica, with iridescent eyes and smokey bands across its wings.

It's easy to collect these, from seed heads enclosed in a jam jar in late winter. The hole in the lid allows the flies, which naturally climb, to enter the plastic collecting tube on top, where they can be removed. They climb more reddily if the lower jar is enclosed in a dark paper sleeve, which encourages them to climb towards the light.

There are some surprises amongst the emerging insects.

These are chalcid wasps, which are parasites on the tephritid fly larvae. So far, two species have appeared - this, one, with the brown abdominal bands .... 

....... and this exquisite little metallic green species. I think this one is a male ......

..... and this one, with the elongated, pointed abdomen, is the female. All have spent the winter feeding on their tephritid larval hosts during the winter.


Many other species in the daisy family, like thistles and burdock, have large seed heads that persist through winter and host a variety of small insects.