Wednesday, 6 November 2013

7 year identification

On 11th October 2006, I published photographs of an unknown Chalcid wasp ovipositing into galls of Neuroterus numismalis, a Cynipid wasp that creates galls on oak leaves. In common with many Cynipid wasps, the life-cycle of Neuroterus numismalis is complex, with different generations making different galls on oak leaves. The autumn generation creates doughnut-shaped disks, variously called 'Silk Button' or 'Spangle gall' on the underside of the leaf. These disks are the home and food for the larva of N. numismalis, and they eventually drop off and overwinter on the ground.

Galls are frequently visited by other wasps for a number of reasons: they contain food and at least one larva which, itself, can be food or a host for a parasite's egg. Some wasps treat the gall as a home for their own larva, which will coexist with the host. These are known as 'inquilines', and are harmless to the creator of the original gall. Different parasitic wasps can target either the original larva or any inquilines which may be present.

Here's the original shot of the egg-laying process from 2006:


Chalcid wasp ovipositing
And here's a shot that shows the scale of the whole event:

Oak leaf, showing galls and wasp
The Chalcid is the dark mark which can be seen to the left of the pair of galls at the lower left. The galls are 3 mm. in diameter, so the wasp is around 2 mm. in length.

The identification literature on Chalcids has been out of print for some years, and has been very difficult to obtain, so for 7 years I have had to put this down as 'a Chalcid Wasp'. Since we have been in a phase of perma-rain for almost a month, I have had plenty of time to carry out background research, and I finally managed to get a copy of parts a and b of volume 8 of the RES guides:


This was originally published in 1958 (almost half a century ago!) and contains keys and descriptions of 1400+ species of UK Chalcids known at that time, along with a cross-reference showing the target host(s) of each species, if known.

The keys depend on microscopic characters, which always require a specimen and a microscope, so I couldn't key out my specimen from my photographs. The cross-reference, however, showed only three species that specifically target Neuroterus numismalis, so that narrowed things down a bit. Two of the species could quickly be eliminated due to gross features - such as colour - leaving me with Cirrospilus diallus as the only viable option. The description for Cirrospilus diallus fits what can be seen from my photographs and also mentions that it's the most common species of Chalcid in Oak woodlands, so it doesn't seem too unlikely.

Since I haven't keyed out the specimen from first principles, this can't be seen as a definitive identification, but it's not entirely impossible. There is one previous record from Ireland: from county Kildare in 1946. This record was made on the 13th October, which also neatly ties in with the date of my specimen.  


7 comments:

Gill said...

Fascinating. I'm seeing lots of fallen oak leaves with galls on them now, but have not noticed any insects.

Incidentally I would not call your "doughnut" or silk button gall a spangle gall - to me those are the flat round ones also very common on oak leaves.

stuart dunlop said...

I agree entirely about the naming of galls, which is why I put the names in single quotes. 'Spangle gall' is more usually used for the spring generation of Neuroterus numismalis or for the galls of Neuroterus quercusbaccarum, but sadly, I see them all used more or less interchangeably. The only way to be specific is to say "the autumn generation of Neuroterus numismalis" or even "the agamic generation of Neuroterus numismalis", etc. It seems to me that the deeper we dig into our wildlife, the more we have to depart from common usage(s).

amanda peters said...

Hi Stuart, just found your blog,I'm into all things to do with nature....wanting to learn more about what I see, just got into fungi.Have been out most days photograph as many as I can and trying to I D them all. Have had a quick look at your blog and have managed to ID a few more of my fungi from your photos.
Will be popping back.

stuart dunlop said...

Welcome, Amanda. Some fungi are easy enough from photos, but many are just impossible to get to species without a microscope and some decent books. I started off with Jordan and Rogers, but had to get single family monographs, since the general purpose books only contain 1500 or so out of 25k+ species.I know there's a fungal group in N. Yorkshire. Gill will have the details.

amanda peters said...

Thanks Stuart,The more I look the more I realise how complex nature is...
I have realised there will be allot I can't name, but hope to get to grips with the more common ones..

stuart dunlop said...

"The more I look the more I realise how complex nature is..."

I could have said that....wait....I do.....regularly!

Kevin Gill said...

Good work Stuart. This is a great blog keep is up.