Ecology of the oak processionary caterpillar
The oak processionary moth (Thaumetopoea processionea L.) is a plain grey moth. It has one generation per year with its flying period in August. Moths live for just one or two days and measure just a few centimetres. The male has clearer markings and is often far smaller than the female. There is a clump of short, dark hairs on the female's end segment. She later covers the eggs with these hairs.
After mating the females fly to the oak tree, the common oak being their favourite. There they lay their 30 to 200 eggs in regular batches (August – September) at the top of the crown, mainly on the southern side of one or two-year-old thin shoots (an inch thick). The caterpillars are seldom found on other types of oak (American oak). If there is a shortage of food caterpillars may be discovered on different types of tree such as birch, beech, black cherry, etc.
The batch of eggs is rectangular, with several rows of eggs next to each other. The eggs are covered with a layer of brown glue mixed with hairs from the abdomen. They are therefore well camouflaged and very difficult to distinguish from the bark. The eggs hibernate.
In mid-April or early May, depending on the weather conditions, orange-coloured young caterpillars emerge from the eggs. The caterpillars appear long before the buds come out. Despite the lack of food the caterpillars are capable of surviving this period without any problems. They live in closely knit groups. They are about three mm long and have long hairs spread over their whole body (these hairs are not the microscopic stinging hairs that cause irritation).
Before they are fully grown the caterpillars shed their skin five times. Their colour changes to a dull grey with light coloured sides. From the third larval stage onwards the caterpillars acquire the first stinging hairs on the back of the eleventh body segment. These are just one tenth of a millimetre long. During the fourth, fifth and sixth larval stages, they develop more stinging hairs so that eventually they have stinging hairs all over their body. A fully grown caterpillar has about 700,000 of them.
During the first three larval stages, the caterpillars do not make a distinguishable nest. They spin a few twigs and leaves together and withdraw there. The characteristic sack-shaped nests, consisting of a cocoon of hairs, shed skin and droppings, are not formed until the fifth larval stage. They are mostly about the size of a handball. Some can be up to one and a half metres long. The caterpillars leave these nests, in which pupation also takes place, in long processions at a later stage.
In July the caterpillars pupate in a thick cocoon of hairs and other material and grow into moths in August.
Plagues of insects are a normal natural phenomenon. Nature generally ensures that there is an acceptable number of each species. So explosive growth often results in disease or lack of food. Thanks to natural enemies biological equilibrium is eventually restored. The natural enemies of the oak processionary caterpillar are parasites such as the parasite fly and the ichneumon wasp as well as predators such as birds, beetles and assassin bugs. The caterpillars are not only found in natural and wooded areas but also in built-up areas. The natural enemies obviously need to have the chance to develop in the caterpillars' habitat.