Wednesday, 16 July 2014

Background research

Weather hasn't been too good recently, so I have been working away researching all manner of creatures from spiders to beetles, to parasitoids.

A colleague in Galway found a parasitised larva of the 4-spotted Footman moth a couple of weeks ago. Here is her original image:

The larvae of the parasitoid have consumed non-essential parts of the moth larva and have emerged and pupated en-masse. The original larva is still alive, and will move away from the batch of parasitoid cocoons. There are a number of strategies that parasitoids use to avoid being parasitised themselves, and allowing the original host to temporarily survive and walk away will lessen the chances of the parasitoid cocoons being detected by the hyperparasitoid, since the hyperparasitoid will use the original larva as a vector and be distracted by it.

I received the cocoons last week and yesterday two adult Braconid wasps emerged. The remaining 22 cocoons are still intact.

I used the key to Braconidae to get the specimens to genus (Apanteles), and then used a 1973 paper from the Bulletin of Entomology to refine the search (there are over 250 species of Apanteles on the UK list). The most striking photographic evidence for identification are the wing-veins. 

This is my photograph of the diagnostic part of the wing veins of the Galway specimen:

And this is the reference image of the wing veins (rotated to be in a similar position to the sample above):

The similarity is striking, and leads us to Apanteles octonarius, which is known to be a parasitoid of the 4-spotted Footman moth. The adult parasitoid wasp is 2.5 mm long.

There is one previous record of Apanteles octonarius on the Irish list, and that was from Mayo in 1911. I am now seeking validation of this identification, since this group is new to me (apart from Apanteles glomeratus, which parasitises the Large White butterfly, and which I showed last year.)

Another bit of research centred around the Tachinid Eriothrix rufomaculata. This Tachinid can be readily identified by the two red marks on the top of the abdomen, (and which give it its specific name):

I predicted last week that this fly would shortly appear, since I had noticed a large number of other migrant species, including the Red Admiral butterfly, the hoverfly Scaeva pyrastri and a number of migrant moths. I am certain that this fly is migrant to this country, since I can predict its arrival by monitoring the arrival of other species.

That photograph was taken using the new 70D, which continues to amaze me daily.


Gill said...

Fascinating stuff - have you got a picture of the little wasp(s)? That's an awful lot of cocoons for one caterpillar - which I'm guessing isn't all that large since footman moths aren't. Presumably it (the footman larva) eats like crazy to feed the wasps?

What's the little insect with iridescent wings - flying ant? That camera is giving a good depth of field. Did you also use it for the wing veins, or is that a microscope shot?

Keep up the (very) good work.

stuart dunlop said...

Gill: the resolution of the camera (20 Megapixels) means that I don't have to get so close, so the DOF is greater. The little insect with iridescent wings is a Braconid wasp. A close-up would show detailed differences, but to me it 'just is' from that distance. I suppose wing-shape is a good clue, thinking about it: flying ants have longer, slimmer, relatively less-veined wings. (But they're very closely related).

Apanteles wasps are minute (around 2 mm long), so the larval body can support large numbers of them. I counted 40+ wasp cocoons from a single Large White. The shot of the veins is a microscope shot (x 40).