Cotesia parasitoid

I found these cocoons on my cabbage the other day and got quite excited. These cocoons were likely spun by a Pieris rape, Cabbage white moth caterpillar. However inside the cocoons are not the Cabbage white itself, inside are the emerged larvae of the Cotesia glomerata wasp. 



The parasitoid wasp,  Cotesia glomerata, lays its eggs inside a Cabbage white caterpillar while it is feeding on my cabbage. While the parasitoid eggs hatch and feed on the caterpillars blood the caterpillars saliva changes it's chemical profile. The saliva from an non-parasitised caterpillar when in contact with the plant host, my cabbage, causes the plant to emit volatiles which are recognised by other adult insect pests of the Brassica, which signal them towards the plant as a good place to lay their eggs. The saliva from a parasitised caterpillar does not invoke this reaction from the cabbage and no signal is released to other moths to lay eggs on the plant, which is good.

When the parasitoid larvae mature in the caterpillar, they erupt out of the caterpillar. The host catapiller is not killed by this mass exodus and starts weaving it's silk around the parasitoid larvae to protect them while they pupate into adult wasps. The Cabbage white caterpillar can often be found on top of the cocoons guarding them from predators until it dies of starvation.

So if you see these cocoons leave them and you will help protect your brassicas from pests.

Regards

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