Tuesday, 22 October 2013

Update to Interference Patterns (1st October)

A couple of weeks ago, I posted about the braconid wasp Apanteles glomeratus, which is parasitic on the Large White butterfly. At that time I mentioned that I had taken a sample batch of the parasite cocoons, and yesterday I noticed that the petri dish was full of minute wasps. I popped one under the microscope:

The braconid wasp Apanteles glomeratus - adult
I appear to have both males and females, although the females are more numerous, as you might expect (female Ichneumonids are usually self-fertile). Both measure 3mm long, excluding antennae. There are 32 in the sample. 

So we have a few interesting numbers resulting from this sample:

  • Pupation period of the wasp is around 21 days.
  • One fully-grown larva of the Large White can support 32 wasp larvae.
  • A few of the wasp cocoons have not (yet) hatched. Perhaps these are parasitised and will emerge later.

I was a bit surprised that they emerged so quickly, since there are very few Large White larvae around at this time of year, so I presume they must overwinter a hibernators, ready to restart the parasitization cycle when the first batch of Large White larvae hatch next year. 

Documentation of these observations seems quite scarce, and I have communicated the sightings to the appropriate people.

Wednesday, 16 October 2013

Bioblitz at Cultra

The final bioblitz of the year was held in the grounds of the Ulster Transport Museum in Cultra. It's quite late in the year, but plants and fungi were still around in good numbers. The area is largely parkland, with mature woodland patches and fringes. Many of the trees and plants are clearly introduced, but there are good stands of Oak, Birch, Beech and other species that support various fungi. I was mainly recording leafminers and fungi, but I added a few interesting plants, too, including a single specimen of Epipactis helleborine which is quite scarce. One new plant for me was the introduced Pheasant Berry, which was heavily infested with mines. The mines seemed familiar, and when I got back to the office and on to the internet, I found that it shares the same miners as Honeysuckle.

One of the first fungi I found was Cortinarius hemitrichus:

Cortinarius hemitrichus
I don't usually try to identify Cortinarius to species, but there are only a few grey ones, and the description of the stipe: 'scurvy white below the cortina (ring), but plain white above it' fits very well.

New to my Species Index.

Another new species for me is the very distinctive Chroogomphus rutilus:

Chroogomphus rutilus
I found this quite early on, so I had a chance to make a spore print during the day. The spores are dark purple, almost black and these, along with the deeply decurrent gills, quickly led to the identification. These were rather numerous under conifers. Quite a handsome beast.

New to my Species Index.

One very distinctive species was found in a number of places, especially under Birch: its normal associate:

Amanita muscaria - Fly Agaric
I found this batch growing in a lawn under a large Fir, hundreds of yards away from any Birch specimen, and I have occasionally found A. muscaria under other conifers with no sign of Birch anywhere nearby. I can only assume that there's some old remnant of a birch root still in place under the ground many years after the tree has died or been removed.

One of the great things about bioblitzes is that you have the opportunity to meet people who specialise in different areas, so you're always finding out something new. One of the recorders was targeting spiders and harvestmen and he showed me this specimen of the harvestman Paroligolophus agrestis:
The harvestman Paroligolophus agrestis
That shot quite neatly shows the difference between harvestmen and spiders: harvestmen have a body formed from a single oval, without a waist.

I think I feel a book on spiders and harvestmen coming on.....

Tuesday, 1 October 2013

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.