The branches are bare of leaves and several of the bushes are completely defoliated. The culprits are not hard to spot: small speckled greyish sawfly caterpillars.
A few weeks ago I mentioned that I had seen my first berberis sawfly, Arge berberidis, fluttering around a bush of Berberis thunbergii. I made light of it since the bush is fairly thuggish and nothing else much eats it. My interest was purely academic. Having heard about this invader, which probably arrived about 10 years ago in imported horticultural material, this was the first time I had seen it in south-east London.
My interest in this bush was first roused a few years ago when I found a small picture-winged fly, Rhagoletis meigenii (pictured above), in London's Battersea Park in June 2000. At the time this pretty little insect was thought to feed on the native Berberis vulgaris, but was believed to be extinct in Britain, having been last recorded in 1897. I went back the next month and found it on the B. thunbergii bushes (the purple-leaved cultivar) in the ornamental gardens. Despite close attention to the many hedgerow bushes in East Dulwich and elsewhere, I've not seen it again. A colleague found it a couple of years later on the same garden shrub in Essex, but as far as I know this is the sum total of its known occurrences here. Curiously this was almost exactly the same time that the now widespread berberis sawfly arrived in Britain.
Unlike the sawfly, which feeds on the leaves, the grubs of the picture-wing fly develop in the small berberry fruits. Having seen bushes weighed down with berries in autumn, I've often wondered why the fly has not been more widely seen. A very similar species, Rhagoletis cerasi, is a major pest of cherries on the European mainland and I'll bet UK cherry growers are keen to know whether it is spreading.
I walked past the same bushes at the weekend and my observations now take on a different tone. The branches are bare of leaves and several of the bushes are completely defoliated. The culprits are not hard to spot: small speckled greyish sawfly caterpillars. If they keep on like this B. thunbergii is doomed.
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