September News Update – Blean Woods NNR

My most pleasing recent discovery was of two heath fritillaries, on a day when it was too overcast for them to contemplate flight, and so windy that the mauve flowers of devil’s bit scabious on which they were feeding swung wildly from side to side.  In a normal year the first fritillaries appear in late May or early June, and can then be seen on any sunny day through to July.

The caterpillars hatch and start feeding on cow-wheat leaves, but shut down in early autumn, hibernating while still tiny, and only resuming growth when cow-wheat reappears the following spring.  However, this simple univoltine (one flight period per year) process is not immutable and in some, possibly most, years there is a small second flight period in late August and September.  My best year for viewing a second generation was in 2009 when one site produced a staggering peak count of 1338 first generation butterflies, so the 73 second generation butterflies seen one day in September that year was a feeble 5% of the peak in the main flight period, and in a more normal year, when far fewer early butterflies are around, the chances of seeing any later in the season are pretty remote.  Quite why a minority of the caterpillars should opt for fast-track development, whilst the great majority of their contemporaries take the slow route, simply isn’t known.  By late summer their food-plant is usually shrivelling up, so the speedy growers may have to switch to another plant in order to continue their rapid development.   And what becomes of any eggs laid by second generation females?  If they hatch this year the caterpillars certainly won’t find any cow-wheat and so would have to turn to an alternative plant or starve.  If, on the other hand, they overwinter as eggs, which seems quite a reasonable strategy when time is running out, the caterpillars emerging the following spring will be at a disadvantage when they find themselves in competition with the larger first generation caterpillars that had overwintered, if you’re still following me!  Back in 2009 I had hoped to unravel part of the mystery by monitoring the development of two egg batches laid by second generation females – would they hatch and feed in the autumn? – but unfortunately both clumps disappeared, possibly eaten by mice.  So I was none the wiser and still don’t know if these second generation individuals are simply flying down a blind alley, or if they have a potential competitive advantage.  If the former is the case, then you would expect that behaviour to be weeded out by natural selection as no offspring are produced:  but if the latter, why are second generation adults the exception rather than the rule?

It has been a bumper year for knopper galls.  Like oak apples, robin’s pin cushion, marble galls and a host of other weird excrescences, knopper galls are produced by a tiny solitary, parasitic wasp laying its egg in a bud that, in this instance, was destined to become an acorn.  In response to chemicals emitted by the hatched grub, the plant is stimulated to surround the organism with new growth that both protects it and provides it with its food supply.  Each species of gall wasp causes the plant to respond in different but characteristic ways, such that the position and shape of the gall can be used to determine the wasp species without actually seeing the insect.  The wasps appear to be very selective, and a single tree can lose virtually all its acorns to knopper galls, while those on either side aren’t “knoppered” at all.  As the parasitized acorns tend to drop prematurely, a stripy pattern can appear along paths, with a sudden patch of galls scattered over the ground, then a bare stretch followed by another carpet of galls.  Initially bright green and sticky, the galls quickly lose their lustre when dehisced, turning a dark brown which makes them far less obvious on the woodland floor.  To add insult to injury, this particular wasp is a recent addition to our fauna, apparently making its way from Europe in the late 1970s.  And, in case you’re wondering, Brexit won’t put a stop to any future immigrations of this nature.

2nd generation 1 - Copy 2nd generation 2 - Copy Knopper 3 Oak 1