It’s been four or five weeks since I’ve been to see my oak tree and last time there was no sign of any acorns growing, so I was pretty sure the tree was not mature enough to produce them (see Acorns are not the only fruit…). I was surprised then, yesterday, to find a few juicy-looking acorns adorning my tree.
Not all looked as healthy as this one however – some are a very odd shape.
These peculiar, ridged acorns are actually Knopper galls, chemically-induced by a tiny wasp called Andricus quercuscalicis which lays its eggs in oak buds. Galls then grow between the cup and the acorn as the wasp larvae develop inside. The wasp larvae is in the small white sphere in the centre of the gall cut open for the photo below.
In autumn the galls will become brown and woody and will be shed from the tree and, next spring, adult female gall wasps will emerge through a hole in the top of the gall. So far so straightforward, but these female wasps will not go on to lay their eggs in more English oak buds. Instead, they rely on a second host species – the Turkey oak, Quercus cerris. Eggs are laid in the male catkins and, this time, both male and female wasps develop inside – the sexual phase of the two-stage life cycle. Because of this need for a second, non-native, host, Knopper galls are a relatively recent arrival in the UK. The wasp, Andricus quercuscalicis, first appeared in Devon in the 1950s – unsurprisingly, in gardens where Turkey oak had been planted as an ornamental, alongside English oaks.
Knopper galls are not the only pest affecting my young tree, though. Many of the leaves have bright yellow blotches, marking the site of Oak spangle galls on the underside. These are produced by another gall wasp, Neuroterus quercusbaccarum – the same wasp which produced the Currant galls I found in May (see Oaks in flower). In fact, some 70 % of all gall wasp species use oak trees as a host.
Oak spangle galls
Spangle galls each contain a single wasp larva and, when they mature in a month or so, they will fall to the ground – before the tree loses its leaves. The larvae inside will continue to develop over winter, protected by a blanket of fallen leaves, until they emerge as female wasps next spring. These females will lay eggs (which don’t need fertilisation) in catkins or on new leaves, giving rise to Currant galls from which both male and female wasps will emerge, in due course.
Currant galls, May 2017
After mating, eggs laid on the leaves will develop into next year’s Spangle galls and the cycle starts again. Unlike the Knopper gall, only one oak species is required to complete the whole life cycle.
Despite their prevalence, none of these gall wasps really harm the oak trees which host them, though Knopper galls will reduce the number of fertile acorns a tree can produce. A healthy host is much more use to a parasite than a dead one, after all!