Tuesday, 26 July 2016

More parasitoid behaviour

I have written many times about parasitoid wasps and their lifestyle. In summary, the female wasp finds her target host - usually a caterpillar or larva of a sawfly - and deposits an egg inside it. The egg remains dormant until the host larva has grown sufficiently large and then it hatches and the wasp larva eats the host larva. Sometimes the wasp waits until the host larva has pupated, but the outcome is the same: the host dies and the wasp larva pupates and emerges as an adult wasp at a later date.

There are many variations on the theme: sometimes the host larva is kept alive and it can move away after the wasp larva has emerged. This is thought to be a distraction strategy where the host larva acts as a decoy, attracting secondary parasitoids that would normally target the primary parasitoids using the host as a vector.

I recently found specimens of the aphid Eucallipterus tiliae on lime and spotted this odd structure under one specimen.

This aphid has been parasitised by the Braconid wasp Praon flavinode which targets mid-instar aphids. The aphid is glued in place, although it can still feed (the specimen above is now an adult) and grow. The braconid feeds inside the aphid and then drops into the 'podium' where it pupates.

So in addition to using the aphid as a food source, it's also using it as shelter for pupation.

To give some sense of scale, the 'podium' is about 2 mm. diameter at the base.

Here's a shot of some of the earlier instar aphids on a leaf:

Eucallipterus tiliae aphids on Tilia

No matter how much I find out about these wasps, I am constantly surprised by their range of habits and techniques.

Both species are new to my Species list, and Praon flavinode is a first Irish record.