On Tooting Common yesterday I came across an oak whose leaves were encrusted on their undersides with small, flat discs:
My friends on Twitter helped me identify them as spangle galls, home to the larval stage of Neuroterus quercusbaccarum, a tiny cynipid wasp. In fact, oak trees are host to over 30 species of gall wasp, each of which creates a different type of gall.
Although they are made from the leaf's own lipids and proteins, spangle gall infestations rarely have an adverse affect on their host tree. Each leaf may carry up to 80 or even 100 galls on its underside, which (unlike the ones I found) will usually detach themselves and fall to the ground before the leaves are shed: that can mean millions of larva, overwintering on the ground beneath an oak in their protective casings.
In April, tiny wasps will emerge from the galls that haven't been eaten by birds or attacked by their own parasites. The wasps will lay unfertilised eggs on the oak's male catkins, which will produce small, spherical galls like little redcurrants ('currant galls') before hatching out in June, mating and laying fertilised eggs on the leaf undersides again. In this way, Neuroterus quercusbaccarum produces two generations per year, one reproducing asexually, one bisexually, and two forms of gall.
It is a life cycle, and a relationship, that has been perfected over millennia, and it goes on all around us, unattended and largely unobserved. The gall wasp is proof that the world in which we share tenure is so much more various and complex than we, with our recklessly selfish, inward-looking lives, can ever understand; our view of the richness we blunder through so heartbreakingly narrow.
With thanks to Phil Gates for his cecidological expertise