Saturday, November 21, 2009

Gooseberry sawfly Nematus ribesii

I am an amateur naturalist trying to learn something about everything living in my garden.

I have a Gooseberry bush in my garden. In previous years I've had a good berry crop. But not this year! In May something attacked the bush, totally stripping the leaves. The culprit is shown in photo 1.

From closely similar photo's on the web I'm fairly confident this is the larva of the Gooseberry sawfly (Nematus ribesii). I should add my customary 'health warning' however: I'm not an expert on plant pests. As an amateur, you quickly learn that in natural history, identification of insects (fungi, spiders, lichen etc. etc.) solely on the basis of a photograph is always risky. There are more than a dozen species in the Nematus genus for example (see Bioimages site). I've found photos of only a handful. My caterpillar certainly looks like N.ribesii, but I don't know for certain it isn't one of the others. Can anyone more expert comment?

The damage done to my poor Gooseberry bush is shown in Photo 2. The branches would normally have been covered in leaves at this point in the year.

I did not spot any adult N. ribesii. From the photo's on Faroe Nature however it would appear they are squat, orange insects. This site has a drawing of the wing veinature taken from the book 'The Wings of Insects' (J.H. Comstock, 1918). (Wing veins are an important guide to insect identification - see my posting here).

In searching for articles on my sawfly I came across the admirable site of the Journal of Cell Science. This carries a large, searchable database of freely downloadable scientific papers. My searches turned up three on gooseberry sawflies (L. Doncaster, 1907, L. Doncaster 1905 and S. Shafiq, 1954). All get rather technical in places and I don't pretend to have followed all the details. From a quick read however, an interesting snippet I picked up is that eggs from both fertilised and unfertilised N. ribesii females can hatch but that larvae hatching from unfertilised eggs are overwhelmingly male. Eggs are laid in rows on the lower side of leaves at intervals of about a minute incidentally.

The main topic of the papers above relates to embryogensis i.e. the truly miraculous feat that Mother Nature manages of beginning with a single cell (an egg) and by a processes of repeated cell-replication and inter-cell communication constructs a complete insect (say) comprising hundreds-of-thousands of cells of countless types, all located in the right places, and all in an incredibly short period of time (about 4 days in the case of N. ribesii). Trying to undestand how she does this remains one of the great challenges for modern bioscience. It is perhaps fitting for this posting, that one of the most intensely studied creatures in all of science is a fruit fly (Drosophila melanogaster), flies being an ideal test-animal for such studies (along with nematode worms - see my post here) since dozens can be kept in a test tube where they will breed copiously and the offspring hatch rapidly. Armies of biologists have published innumerable articles about D. melanogaster - this site gives a flavour.

So there you have it, a seemingly humble garden pest with a rich natural history. Mind you, it might have been nice to have had a gooseberry crumble this year!


Mizz Bee said...

This species is found both in Europe and North America, where it is called "imported currant worm". I am wondering if that means that it was introduced to the US from Europe or vice-versa. Did you find anything about this in your search?

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