Knopper gall wasps have been extra busy on the oaks this year.

Many acorns were contorted into unsightly, sticky, bright green ribbed lumps which kill forming acorns, sometime splitting or completely enveloping them in the process. The infestation is so bad that many oaks have not produced a single viable acorn.

Walking under such trees we can see piles of knopper galls which have now dropped from their host oaks, turning brownish grey and resembling mildewed walnut kernals that crunch noisily under foot.

Spangle

Over thousands of years, the English oak has nobly withstood annual attacks from a range of gall wasps, whose activities are familiar to us either in winter as hard malteser-like galls adhering to twigs, or in summer as bunches of puffy pinkish white oak apples or silk button and spangle galls glued under leaves, writes Tony Drakeford.

All are caused by ant sized wasps with complex life-histories involving alternate sexual and asexual generations. Eggs soon hatch causing the tree's tissues to react by swelling around the grubs which then feed within.

Jeopardised

The comparatively recent arrival in Britain of the knopper gall, however, worried scientists in the1970s as it was feared that the long term future of our oaks might be jeopardised as this new' insect seemed to belong to a different, more virulent league altogether, unlike established galls. But oaks have survived more attention from invertebrates than any other tree.

Although this autumn seems to be exceptional, oddly there are always some trees which escape attention of the knopper wasp.

Cutting one of the fallen galls in half will reveal either a tiny white developing grub or one that has already pupated all set to emerge as an adult next spring.

November 6, 2001 17:00