Understanding the biology of Oak processionary moth (scientific name Thaumetopoea processionea) can help you to manage it.
It gets part of its common and scientific names from its distinctive habit of forming nose-to-tail processions of larvae on its host trees, and sometimes on the ground beneath host trees, as they move around to feed on the leaves.
Both of Britain’s native species of oak, pedunculate and sessile oak, and several other oak species grown here are susceptible to OPM attack.
In a very broadly descending order of susceptibility, they are:
Turkey oak (Quercus cerris), English or pendunculate oak (Q. robur), chestnut-leaved oak (Q. castaneifolia), white oak (Q. alba), Turner's oak (Q. x turneri), Holm oak (Q. ilex), Algerian oak (Q. canariensis), Hungarian or Italian oak (Q. frainetto), sessile oak (Q. petraea) and cork oak (Q. suber).
OPM larvae will attack other species of broadleaved tree, but usually only if they are running short of oak leaves to feed on. They have been observed feeding on sweet chestnut, hazel, beech, birch and hornbeam.
There is only one generation of OPM each year. The moth generally lays its eggs about mid-August in masses, or plaques, about 2-3cm long on branches and twigs. The eggs hatch from about mid-April into May, and the emerging larvae will go through six stages, or 'instars', until the pupation stage, when they become adult moths able to mate and breed. These are often referred to as stages L1 – L6.
The damage which OPM’s feeding does to oak trees is quite distinctive and noticeable, because the larvae tend to leave the leaves skeletonised, with the main veins remaining.
Larvae can remain hidden in the soil around the base of the tree, so any trees seen with feeding damage, but no signs of larvae, should still be treated with suspicion and checked carefully. If the tree is in a pot or container, these should also be thoroughly checked.
Eventually, the larvae moult (from about late June to early August) to the pupal stage, again within their nest. Sometimes a nest can become dislodged from the tree and can be found on the ground, as in the picture above.
Pictured above is a rear view of a fallen nest showing the pupal chambers.
The final stage in the life cycle is the adult moth, which will emerge and fly from about the middle of July to early September. Males are strong flyers, the females less so. Both sexes live only for 3 to 4 days as adults.
Knowing the approximate timings of the different life stages will help you understand what to look for, and what action is best, and this is set out in more detail in the next section.