Death to the Cabbage White Caterpillars!!

[Colin_M]

Ok, a bit dramatic, but judging by the previous thread, many of you are having a hard time with these this year. As well as all brassicas on our plot getting attacked (incluing things I don't normally net like Swedes), we've even got a few caterpillars climbing up the front of our house

However today, I noticed something else that hates these almost as much as we do. The sequence below is not for the squeamish. It looks as if the parasitic fly in the first shot below is finding sick & elderly caterpillars and laying its eggs nearby them.

In the very last picture, the grubs you can see chomping away were a writhing squirming mass. Now: how do we get these chaps into our gardens at the start of the Cabbage White infestation?!

[FelixLeiter]

What you're seeing there is an Ichneumon wasp. The caterpillars in the last image have newly emerged from the hapless caterpillar host, the internal organs of which they have been feasting on for most of the season. They are finishing of the carcass before pupating into the hairy cocoons you see in the first image. The adults emerge and go off to hibernate. Next year they will find other newly-emerged caterpillars to lay their eggs into. The grubs which hatch from these eggs eat away at non-essential parts of the caterpillar so that it continues to grow and chomp away at our Brassicas unhindered. Externally, there is no way of knowing that the caterpillar is a host to dozens of these larvae until they emerge. From the gardener's point of view, they do not significantly control numbers of caterpillars nor the extent of their wreckage. I can't help thinking that maybe they actually eat more, faster, in order to sustain their grizzly cargo. Eating for dozens, you could say.

[Colin_M]

Hi Felix, now that's interesting.

If you look at this article, it mentions several different approaches, including the one you mention (death from the inside).

I will keep an eye on this lot over the next few days. Either way, it sounds a pretty grizzly end - shame we can't make use of this out in the open.

[glallotments]

I think the first photo is of a new ichnemon wasp emerging from a cocoon.
When the grubs first emerge they are white and then they spin a yellow fluffy covering in which to pupate.

This year I gathered about half a dozen or so large white caterpillars as I wanted to photograph the life cycle for my website. Of the caterpillars gathered only one survived and produced a butterfly. All of the rest had been parasitized.

I think this must have some effect on white butterfly numbers. It may not kill the caterpillars before they have devastated the cabbges but at least it means less butterflies around to lay more eggs so less caterpillars. I know we have hundreds of them this year but maybe without the wasp activitiy we would have far more.

[Elaine]

Hi Felix. Slightly different but on the same topic.
I was sat in my garden a couple of weeks ago, when a bluebottle literally dropped out of the sky and landed at my feet on the patio. I watched with a degree of revulsion and fascination, as dozens of minute maggots crawled out of the bluebottle.
Any idea what could have parasitized this bluebottle please? Cheers.