From early summer many trees start to develop strange looking warty structures from their leaves and buds. These odd looking growths are homes to the developing larvae of a range of insects including wasps, mites and flies. Many gardeners are worried when they spot these galls often in abundance covering their trees. While a heavy infestation can reduce the trees efficiency to photosynthesise it is worth remembering that trees are part of a complex ecosystem that supports and interacts with many different species. Oak trees can support hundreds of different insect species and this is a natural process.
In most cases the female insect lays her eggs under the epidermis of the developing leaf using her ovipositor. The larvae hatch out and cause the strange growths that surround them, keeping them safe from predators and with a supply of energy and nutrients.
The aptly named ‘Robin’s Pincushon’ is caused by a tiny wasp that is commonly seen on rose plants, creating a large fluffy structure around the larvae. The larvae will continue to feed on the host plant throughout the winter before pupating and emerging as an adult in the spring.
Oaks support many gall forming insects including the common ‘Oak Apple’ which is caused by another species of wasp. The smaller ‘spangle galls’ can be seen on the underside of the leaves are created by a tiny gall wasp larvae which falls to the ground within the gal in the autumn and continues to complete its lifecycle within the leaf litter on the ground.
Sycamores also support a range of insect species including the spiky red ‘nail gall’ and many tiny species of gall mite.
Next time you are out in the garden or going for a walk in the countryside have a look and see how many types of gall you can see and be amazed by the complexity of nature!