Wednesday, 9 September 2009

Another Angle on the Spangle

Catching up on various blogs after being out of circulation for a while, I've been enjoying the various posts (eg by Greenfingers and Rambling Rob) on plant galls.

By coincidence, I must have been taking my blurry close-up of galls on oak at about the same time as Greenfingers was taking his sharp ones (see his post of 17th August) except that my measly oak leaf had but one type of gall on it - the spangle gall - not the four that he found.

That apart, it set me reading up about the life cycle of Neuroterus quercusbaccarum, the small-wasp-big-name that causes this, and its a fascinating story.

Phil explains that the oak tree is induced to produce the spangle gall around the wasp larva by insects that appear in the summer, lay eggs and inject a chemical into the leaf. What follows next is intriguing. The gall, with its tiny grub inside falls off in October. The larva completes its growth during the winter and pupates. Adults emerge in Feb/March but they are all asexual females. They lay eggs but this time the larvae that come from these grow inside a different type of gall - the currant gall - on leaves or on male catkins.
This time, in May/June, it is both male and female adult gall wasps that emerge, mate and the eggs laid in the oak leaves produce the next asexual generation via the spangle gall.

Questions: Why such a tangled life cycle and why are two different generations per year, one asexual and the other sexual, of benefit to this and other insects?


  1. Interesting question. Quite a few insects alternate between sexual and asexual generations - greenfly for example. One likely explanation is that individuals in the sexual generation that overwinters are genetically variable and will include some that are better adapted to survive the rigours of winter than others - the species is, if you like, spreading its bets. Come spring, the genetically superior survivors reproduce parthenogenetically - cloning themselves - because this allows them to propagate their successful genotype with maximum efficiency. Interesting to contemplate the scenario for human evolution if our species could do this......

  2. Now there's a thought. Wonder why we don't do it then?
    Your explanation sounds perfectly plausible - except that I'm sure I read that its the asexual generation that over-winters in this species of gall wasp.