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4. Biology, life-cycle, habitat and spread - OPM manual

Understanding the biology of oak processionary moth can help you to manage it.

Oak processionary moth (Thaumetopoea processionae) larvae in procession across the ground

Both of Britain’s native species of oak, pedunculate or 'English' oak (Quercus robur) and sessile oak (Q. petraea), and several other oak species grown here are susceptible to OPM attack. In a very broadly descending order of susceptibility, they are:

Turkey oak (Q. cerris), pendunculate oak, 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 and cork oak (Q. suber).

OPM larvae (caterpillars) 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 (Castanea species), hazel (Corylus avellana), beech (Fagus spp.), birch (Betula spp.) and hornbeam (‎Carpinus betulus).

Egg plaqueThere is only one generation of OPM each year. The moths emerge from pupation, mate and lay eggs from late July to early September. Eggs are laid in masses, or plaques (pictured) on branches and twigs. These plaques are usually about  2-3cm long.

The eggs remain on the trees over the winter, and the larvae emerge from them from mid-April into May. However, emergence can begin as early as March in warmer weather.

Ideally the larvae emerge after bud burst, when there will be oak leaves for them to feed on. However, if the weather turns cold and/or there is a lack of food after they have emerged, the larvae can go into a hibernation-like torpor until conditions improve.

As they grow, the larvae descend lower in the trees to feed and build nests, and this is when they are most likely to be seen by the public. This is also when they develop the irritating hairs which pose the health problems.

The larvae will go through six stages, known as 'instars', until the pupation stage, when they metamorphose into adult moths able to mate and breed. These instars are referred to as stages L1 – L6. 

The damage which OPM’s feeding does to oak trees is quite distinctive and noticeable, because it tends 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.

Pupal nest dislodged

Eventually, from about late June to early August, the larvae retreat into their nests to moult to the pupal stage.  Sometimes a nest can become dislodged from the tree and be found on the ground, as in the picture above.

Pupal nest

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 for only three to four days as adults, during which time they mate and lay eggs in healthy oak trees.

The moths are difficult to distinguish from some other species, so the Forestry Commission does not need reports of them. However, people with expertise, such as moth recorders, are welcome to submit reports of finds, especially if they in areas where the pest has not previously been found.

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.

Last updated: 31st August 2018