In a year, the oak processionary moth completes its development cycle from the egg through six caterpillar stages, from the pupa to the adult moth.
After the so-called wedding flight, in late summer, immediately after mating, the female lays around one hundred to two hundred eggs of approximately one millimetre in size in the treetops of the oaks. The egg clutch is then camouflaged with a scale-like glandular secretion in brown-grey colour and prepared for wintering.
Hatching time in April / May
When the vegetation awakens in April / May, and the oak leaves emerge, the oak processional caterpillars hatch. The young caterpillars of the processionary moth initially have a brown-yellow colour, which changes to grey-black over time. The caterpillars complete a total of 6 larval stages on their way to pupation over 9-12 weeks.
The caterpillars of the oak processionary moth do not yet have any poisonous stinging hairs in the first and second larval stage. These are only trained in the third development phase.
At this point, the caterpillars have two types of hair: On the one hand, longer caterpillar hairs protruding in bundles from warts without any toxic effect on humans. On the other hand, there are shorter stinging hairs on the eight-limbed abdominal segments, bulging when threatened and secrete the approximately 0.1 to 0.2 mm thin poison hairs. Fully developed oak processionary caterpillars have about half a million poisonous stinging hairs.
Nest building in June
From June, oak processionary moth caterpillars build their typical web nests in forks and on oak trunks. The oak processionary caterpillars use the perch to moult. Filled with excrement and old larval skins, the caterpillar nests of the oak processionary moth reach a size of up to 1.5 meters.
At night, the processionary moths migrate from the web nests in several rows (15-30 caterpillars) up to ten meters long to the top of the tree to eat and return to the porch at dawn.
Pupation in July
At the end of June / beginning of July, the oak processionary caterpillars begin to pupate in the web nests, which lasts about 20 to 35 days. Oval-shaped cocoons are built close to each other, poison hairs are shed during the pupation process but are woven into the cocoons.
For this reason, the nests of the oak processionary moths, as massive structures made of excrement residues, moulting layers and spider threads, remain dangerous in some cases even after years.
Nocturnal moths hatch in August.
The oak processionary moths hatch as nocturnal moths from around mid-August. They are relatively inconspicuous, brown-grey butterflies that fly around from August to September. The colour of the front wings is grey to grey-brown with stripes.
The rear wings of the male processionary moth are white with grey horizontal stripes and a dark spot, and the female is brown-grey. The span of its grey, striated wings is approx. 2.4 to 3.4 cm.