Now Is The Time Of Year That Oak Trees Are Contributing To Nature’s Food Store By Growing Their Acorns, But A Mediterranean Invader Is Spoiling The Crop.
Oak trees are the great providers in the natural food chain, the carbohydrates built in their photosynthesising leaves form the food supply for so many other species. Hundreds of different insects: aphids, beetles and caterpillars feed on the leaves, only to become the food for larger insects and birds. When the leaves fall to the ground they create a larder to be lived in for worms, woodlice and fungi. There is a whole host of organisms that feed on the wood inside old hollow trees.
The acorns, which are packed with nutrition ready to grow a new young tree, are an important food source, taken from the tree by squirrels and jays, while deer and smaller mammals will feed on them when they fall to the ground.
This natural bounty is coming under threat from the Knopper Gall Wasp Andricus quercuscalicis, a tiny invader from southern Europe. It is only 3mm long but it can cause a lot of damage to the acorn crop.
There are several different species of gall wasps that lay their eggs in various parts of the oak and the insect causes the growth of a gall inside which they live and develop, but the trees have evolved to cope with them and they are no real problem. The knopper gall wasp is a new threat that the native oak trees have not adapted to yet.
The knopper gall wasp is associated with the Turkey oak, a species introduced in the 18th century, but the wasp only made it to Britain in the 1960s, probably floating on a strong southerly wind to cross the channel.
The wasp has a double generation life cycle alternating between Turkey and native oaks. A generation of larvae lives in the male pollen catkins of the Turkey oak, these larvae are all female. They emerge as adults in early summer and, like aphids, are able to lay fertile eggs without the need to mate.
The females lay their eggs in the female flowers of our native oaks. The presence of larvae, both male and female in this generation, trigger a reaction in the flower and, instead of producing a healthy acorn, the cup turns into a lumpy knopper gall. Green at first, this becomes red and sticky, and then hard and brown, usually without a viable acorn.
The knopper galls fall to the ground with male and female wasp pupae inside. These emerge in the spring, mate and the females lay their eggs in the male Turkey oak catkins to start the cycle again.
The populations of the wasps rise and fall naturally, possibly linked to different weather. In a bad year, some of our native oaks produce hardly any viable acorns. Instead, the ground under the trees is littered with the hard knobbly galls to such an extent that it can be painful to walk there.
Given time, the wasp and tree relationship will come into balance, the parasitic wasp has no future if it wipes out the oaks, but the world of trees moves in a different timescale to humans, we will probably have to wait several hundred years for that balance to be achieved.
Ed Dolphin
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