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.

Picture-winged fly Urophora jaceana

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

Hardheads

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.

A butterfly bank: second visit

About a month ago I posted some pictures of a beautiful magnesian limestone embankment along an old disused railway line at Trimdon near Durham city. This weekend we went back for another look and the flora was even better.

The viper's bugloss, which was only rosettes of leaves on our last visit, had now produced tall spires of flowers.

At the moment, this is just about the most flowery spot I know, with ....

...... bladder campion .......

...growing here with viper's bugloss ....

..... intensely fragrant burnet rose ......

..... eyebright, a plant that's partially parasitic on the roots of other plants....

..... greater knapweed .........

....... rock-rose .....

... and common spotted orchids that were too many to count, with numerous variations: this is an albino form.

Twayblade orchids, that are well hidden amongst the background vegetation..........

... and drifts of viper's bugloss, which bees can't resist.

Wild strawberries ..........

....... and wild thyme.

Magnesian limestone flora at its best!

...... and now for the good news.....

If you let it, taking an interest in natural history could be very depressing because so much that is written about our flora and fauna in the press seems to be unrelenting bad news. The abysmal weather in the early part of this summer  produced also sorts of hyperbolic headlines of the disastrous/cataclysmic/catastrophic kind about its likely impact on birds/butterflies/insects and almost everything else in the natural world - and it's undoubtedly true that it has been a bad summer for all of these (although it's important to bear in mind that over last few thousand years there have been periods of far more adverse climatic conditions that our wildlife experienced and rebounded from: like investments, rates of growth in populations of living organisms can go down as well as up, all the time). The natural world isn't a natural history museum, it's a living, constantly evolving interplay between organisms.

Anyway, last weekend we happened to notice a positive effect of the wet early part of the summer, when we took a walk along the cliff tops between Dawdon and Hawthorn Dene on the Durham Coast Heritage Path. The summer wild flowers there seem to be flowering more spectacularly than I can ever remember. I have a theory as to why this might be so, and it's all down to the rain. 

The cliffs are porous magnesian limestone which is free-draining. In a 'normal' summer the surface soil dries out quickly and the plants suffer from moisture stress, so they flower quickly while they have the chance, when they are quite small. This year, with plenty of water, the well-watered vegetation is unusually lush and as a result the flowers are truly spectacular. So, without further ado, here are some examples....

Drifts of meadowsweet in the gullies on the cliffs and ....

.... forming a backdrop for devil's bit scabious which .....

...... includes some exceptionally fine examples ..... although this one looks a tad unusual - very short bracts under the inflorescence and it looks like it might be male-sterile, judging by the tiny stamens.

... along with field scabious .....

... flowering in profusion in the meadows at the end of Hawthorn Dene ...

... where there are some fine specimens of giant bellflower.

I suspect that alkaline soil might be the reason for the very blue hue of the tufted vetch on the cliffs but .... 

... this unusually pale betony plant (normal flower behind) is probably a genetic mutation.

Bloody crane'sbill always puts on a good show ....

.... and this year centaury has done well too, with some very floriferous plants ...

This is hemp agrimony .....a major constituent of the taller vegetation while .....

.... hoary plantain is blooming down amongst the grasses.

Plenty of knapweed Centaurea nigra, alongside the ....

.... greater knapweed Centaurea scabiosa.

Drifts of marjoram aka oregano (crush the leaves and sniff for a tantalising hint of Mediterranean cuisine!)....

Spear thistle already running to seed ....

.... likewise agrimony, producing its little bell-shaped hooked fruits ....

Yellow loosestrife ....

.... and some wonderfully robust specimens of yellow-wort, with its strange glaucous, stalkless leaves.

In dry summers yellow-wort, which is an annual, is often a stunted plant and only produces a few flowers - but this year some plants have responded to the rain with a wonderful display...

... and the North Sea provides a fine backdrop for the floral spectacle ....

..... and for haymaking ....