Interference patterns

One thing that I'm constantly aware of is the dramatic impact that we humans have on the wildlife that surrounds us. We cut (thrash!) hedges in summer, remove hedgerows (only to replace them with a nice fence), spray with insecticides, let our cats wander freely to bring home kills, scrape lichens off trees. And then we wonder where our wildlife is disappearing to. I like to think that I help a little by keeping some wild patches on my land, and I spend a lot of time in schools, spreading the benefits of wildlife and conservation to as many people as possible. But this year I have (albeit unwittingly) tilted the balance very much in favour of some of our local wildlife.

For many years, the Large White and Small White butterflies were not recorded in my 10k square. There are a few reasons for that, but one of the main causes is that people have stopped growing brassicas in their gardens, so the opportunity for the larvae to feed has been removed. A couple of years ago, I started a vegetable plot, but yields were bad due to very wet summers. I did notice a few Large and Small Whites, though. This year, the weather in July was superb, and the vegetable garden grew like crazy, with large leafy growth of cabbages, broccoli and cauliflower. I noticed a few eggs of Large White and watched a few of them eating some leaves at the edge of the patch, so I left them to it. September was a bit damp, so I didn't spend much time in the vegetable plot for a couple of weeks. Last week I went out to find that entire rows had been eaten back to the stalks, with dozens (maybe hundreds) of larvae on each skeletal plant, and in all stages of development from eggs to fully-grown. This is the time of year when many caterpillars seek somewhere sheltered to pupate, and the walls, windows and doors of my house are a local and acceptable place for just that purpose: this weekend, I found dozens of Large White caterpillars all round the house.

Larva of Large White butterfly ready to pupate

Having observed a large number of specimens, it seems that they choose a spot for pupation and then stay motionless for around three days before shedding their skin and making a chrysalis. I suppose this three day period is the time when the internal changes are taking place.

Successful larvae end up in a chrysalis which is usually fixed at the tail end, hanging in a head-down position:

Chrysalis of Small Tortoiseshell butterfly

I have these dangling from doors, windows, gutters, soffits and facings.

All is not rosy for some of the caterpillars, though. Large Whites have a parasitoid which is unique to them: the Braconid wasp Apanteles glomeratus. Most parasitoid wasps lay a single egg in the host's body, but Apanteles glomeratus is very small and lays many eggs inside the 1st instar caterpillar. While the caterpillar is growing, the parasitoid larvae eat the fat stores in the caterpillar, leaving all the working parts untouched. When the caterpillar has reach its final position but before it pupates, the parasitoid larvae burst out through the skin all at once:

Larvae of the Braconid wasp Apanteles glomeratus emerging from a Large White butterfly caterpillar

After a day or two, the skin blows away and we are left with a stack of pupae of the wasp:

Pupae of the wasp Apanteles glomeratus

I estimate that there are 50 - 60 pupae in that stack.

I did a rough count of pupae vs. chrysalis and I reckon 30% of caterpillars are parasitised in my sample.

Now all I have to do is keep watching those stacks to see if a hyperparasite turns up.

A completely unrelated caterpillar walked across my field of view as I was watching the Large White larvae. This is the caterpillar of the Bright-line Brown-eye moth:

Larva of Bright-line Brown-eye moth

It's interesting that the common names for moths seem to have been chosen in many different ways. I suppose early identifiers/classification experts were just as confused as we are today about which species were which, and clung to almost any clues they could get. I have identified at least:

• Habitat (e.g. Latticed Heath)
• Adult pattern (Broken-barred Carpet)
• Adult colours (Red/Green Carpet)
• Larval pattern (as in this case)
• Name of finder/identifier (Svensson's Copper Underwing)
• Food plant (Foxglove Pug)
• Whimsy (Cousin German) 
• Adult season (November Moth)
• Indecision (Uncertain)

And some I can only guess at (Vapourer).

Late update: of the most recent Large White caterpillars that I have observed to attempt pupation, 100% have been parasitised. That brings the overall total to around 80%, which is the usually-quoted percentage. A few things come to mind:

The Large White is almost continuously-brooded, since I have seen caterpillars of almost all sizes simultaneously. The parasitoid is known to be present from spring onwards, but it seems like my later caterpillars have been parasitised the most. It is also known that the parasitoid injects all of its eggs in a single injection, so it is clear that the parasitisation occurs very briefly. Perhaps the parasitoids come through in 'waves'.

I am now collecting parasitoid pupae to see how many of them have been parasitised.