This is the body of a cabbage white caterpillar, near the tip of its food plant - a hedge mustard, growing in the new perennial meadow area. The caterpillar has been attacked by a small parasitic Braconid wasp, almost certainly Apantales glomeratus. The female wasp lays up to 50 or so eggs in the live caterpillar, which as they hatch into larvae, proceed to eat the caterpillar alive, feeding first on its blood and fat bodies. This has minimal effect on the caterpillar initially, and it continues to grow and moult while the wasp larvae themselves continue feeding and growing inside it. But eventually they switch to consuming the internal organs of the caterpillar and when they become fully developed larvae, they emerge from the now empty husk of their host. Then they spin a silky yellowish cocoon around themselves and pupate within it until about a week later they hatch into adult wasps and begin another cycle of parasitism. The effects of the wasp can be to reduce Cabbage White populations by as much as 50 and even up to 90 per cent.
This wasp was deliberately imported into the USA in 1883 to help reduce the population of Cabbage White butterflies - one of the early examples of employing a parasitic insect as a means of biological control of a pest insect species.
A pity I don't seem to get these wasps operating on my allotment!
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