Sunday, September 6, 2009

Exploding Acorns

Knopper galls (above), uninfected normal acorn below 

These weird objects are knopper galls, caused by a tiny wasp called Andricus quercuscalicis, that I found on oaks growing beside the River Tyne at Wylam this morning. The wasp lays its eggs in flower buds which, instead of developing into acorns, grow into these popcorn-like galls that provide a home for the wasp larvae and pupae through the winter. The young galls are bright green and covered in sticky sap, but as they age they turn brown before dropping off. If you collect a few and keep then in a dry container over winter you can watch the minute wasps emerge through the pore in the gall in spring. The causative gall wasp first arrived in Devon sometime in the early 1960s, having slowly spread across Europe from Turkey, and colonised rapidly once it arrived in England, so it’s now well established throughout most of Britain and well into Scotland. It first arrived here in the North East in the 1980s. The conspicuous damage that it does to acorns led to widespread speculation that it would destroy the acorn crop and would be a major threat to the future of oaks in Britain, but infestation levels vary a lot from year to year and it’s unlikely that this minute wasp poses a long-term threat to our national tree. Whenever you see a knopper gall (usually on pedunculate oak Quercus robur), there’s sure to be a Turkey oak Quercus cerris (identifiable by its hairy acorn cups) somewhere nearby, because the wasp spends half of its life cycle on this introduced tree, which is widely grown in parks, arboreta and large gardens, and the remainder on native pedunculate oak.


Turkey oak acorns

6 comments:

  1. I will have to keep an eye open for these, Phil. They certainly do transform the acorn.

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  2. I've seen these before, and never knew what they were; thanks. Fascinating post again Phil.

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  3. Complicated things these gall wasps. Not only does this one have alternating sexual and asexual generations - like the spangle gall wasp we have discussed before (http://standandstare-nyctalus.blogspot.com/2009/09/another-angle-on-spangle.html) but it needs 2 different trees as well.

    I remember you saying Phil, in an earlier post, that no-one really understood the process by which the galls are formed. I wonder then if anyone understands exactly why this wasp needs 2 different species of oak to complete its life cycle?

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  4. Hi Emma,this seems to be quite a good year for them (or a bad year, if you happen to be an oak tree) - I've seen quite a few trees that are infected in Northumberland, although in Durham city last year as a mast year with a bumper crop of acorns, and there are very few this year.... so not many knopper galls either.

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  5. Ho Keith, this year I've collected a few and will try to hatch out the wasps in spring, to see what they look like..

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  6. Hi Nyctalus, no idea why it has evolved to use two different trees because the two generations use the flowers of both (although different sexes of flowers - the much smaller sexual galls are on the male flowers of Turkey oak. What I also find fascinating is how these gall wasps can sometimes produce galls of different shapes on different organs of the the same tree species...

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