Oak processionary moth: how to successfully combat this pest

Oak processionary moths damage oak trees and can cause allergic reactions in humans. Find out how to get rid of the oak processionary moth in your garden.

The fluffy, brown caterpillars of the oak processionary moth (Thaumetopoea processionea L.) look quite cute at first glance. As they march in a long line on their way to the oak leaves, you might even be tempted to touch them. Despite how cute they may seem, the fluffy larvae of the oak processionary moth are pests that need to be taken seriously, as their setae (stinging hairs) can be harmful to both animals and humans. Read on to learn how to recognise, control, and prevent the oak processionary moth and caterpillar.

Oak processionary moth: distribution and diet

Oak processionary moths, OPM for short, have been on the rise in the UK since about 1990. The oak moth loves heat, so its spread has been favoured by climate change. It attacks all types of oak trees and, in exceptional cases, other deciduous trees, for example hornbeam (Carpinus betulus), beech (Fagus spec.), birch (Betula spec.) or black locust (Robinia pseudoacacia). This pest occurs predominantly in sparse, warm forests with a lot of oak trees, especially on the sunny, southern edges. However, they can also infest individual trees in urban parks.

Identifying the oak processionary moth

In order to detect an infestation at an early stage, you need to be able to identify the oak moth and its larvae. Oak processionary moth larvae are nocturnal creatures with a broad, dark line with velvet-like hairs that runs down their back. From the third larval stage onwards, they form the typical long stinging hairs. From the fifth larval stage onwards, the caterpillars create large webs of caterpillar silk, which serve as protection and nests for pupation. Oak processionary moth caterpillars are named after their behaviour, as these sociable insects often move in single file processions.

The adult moths have a wingspan of about 3 to 3.6 cm and are hairy all over. However, this hair is not stinging hair, so the adults are not dangerous like the larvae. While the male wings bear two visible horizontal stripes, the stripes on females are less pronounced or not present at all. As the moths are nocturnal, they are seldomly seen.

Harmful effects of the oak processionary moth

Generally speaking, a healthy tree will not have a hard time coping with an oak processionary moth infestation. Even if completely defoliated, affected oaks will usually sprout again the following year. However, repeat infestations over several years are problematic and can lead to a significant loss of vitality. This is why foresters sometimes use pesticides to control the oak processionary moth in order to prevent forests from dying.

How dangerous is the oak processionary moth for humans and animals? The oak processionary moth can cause health problems in humans and animals from the third larval stage until pupation. Its stinging hairs contain a toxic protein, known as thaumetopoein, which is an allergen that can lead to various allergic reactions, including skin rashes, conjunctivitis of the eyes, coughing due to inhalation of the stinging hairs, and even anaphylactic shock. What is more, the hairs are not only a danger when directly attached to the caterpillars, but can also fall off and accumulate in bushes or in abandoned caterpillar nests and lead to incidents even years later.

Oak processionary moth life cycle

The female moths lay their eggs, which are about one millimetre in size, between the end of July and the beginning of September in the upper crown area of oak trees. A clutch usually consists of 100 to 200 eggs, is arranged in the form of an elongated, flat plate and is well camouflaged. Caterpillars in the first larval stage, which are still yellow or brown in colour, hatch from the eggs at the beginning of the growing season. They feed on the oak leaves and consume almost the entire leaf, leaving only the midrib behind. Depending on the weather, the first larvae of the third developmental stage may be found from the end of April. From this stage onwards, they have the typical stinging hairs, which are equipped with barbs and the previously mentioned poisonous protein. In June or July, they pupate and then, after three to six weeks, the adult moth emerges and produces the next generation.

Developmental stages of the oak processionary moth:

1. The caterpillar hatches from the egg from April to May.
2. The caterpillar goes through 5 to 6 developmental stages.
3. Between June and July, the caterpillar pupates.
4. From July on, adult moths hatch, only living for a few days and reproducing.
5. Oak processionary moths overwinter as eggs.

Combatting the oak processionary moth

Combatting the oak processionary moth is difficult for a couple of reasons. Firstly, the moths prefer to attack trees that have already grown to be quite tall, so they can be hard to reach with conventional spraying equipment. Secondly, it is a very bad idea to approach a group of oak processionary moths. Never try to remove the caterpillars yourself! Here’s what to do if you ever find yourself near an infested tree.

What to do if you find yourself near an OPM infested tree:

• Do not touch OPM caterpillars or webs
• Cover all of your skin as best you can with clothing to protect it
• If you do come into contact with the larvae, wash clothing at 60 °C and thoroughly rinse hair and body with water

Do I need to report oak processionary moth sightings and who deals with them? Although there is no legal obligation to report an infestation, the government strongly urges you to report any sighting to the Forestry Commission. In public areas, the responsible authorities will take further action to control the oak processionary moth. If you find OPM on your own private property, you may be asked to deal with the infestation yourself or the government will help you depending on whether you live in a pest free area, an established area or in the buffer zone. But be careful! As this pest poses a health risk, we strongly recommend finding a professional, such as an arborist, to deal with the oak processionary moth for you. See the Arboricultural Association website to find a moth management specialist in your area.

Controlling the oak processionary moth with plant protection products

If oak processionary moths are causing problems in your garden, you have one of two options. You either avoid the infested area altogether, thereby avoiding contact with the stinging hairs, or you arrange for the oak processionary moth to be controlled by a company that specialises in pest control (see above). A good way of getting rid of OPM is by using biological controls. Bacillus thuringiensis preparations are also used by farms and specialist companies as a way of biological controlling the oak processionary moth. However, private individuals without a certificate of expertise are not permitted to use this against oak processionary moths.

Other common methods are vacuuming and scraping off the OPM caterpillars. Burning or felling the infested plants are ineffective methods as they only risk spreading the stinging hairs even further.

Nematodes against oak processionary moths

Nematodes are microscopic, thread-like organisms which, depending on the species, are considered either pests or beneficial insects. The nematodes of the species Steinernema feltiae are beneficial organisms for us in that they attack the larvae of the oak processionary moth and kill them from the inside. You can start treatment as soon as the caterpillars have hatched and continue control until the end of May. Early larval stages are much more sensitive than later ones, so it is better to start using nematodes early.

As nematodes are sensitive to light, you should only apply them in the evening or on overcast days. In order to reliably control OPM using nematodes, you should repeat the treatment within a fortnight.

Caution: If you know that you react strongly to allergens, or if you have problems with your lungs or skin, do not under any circumstances approach OPM caterpillars. Just leave it to the professionals to take care of the problem. For everyone else, wear long, tight clothing and put on gloves, a breathing mask and protective goggles to be on the safe side.

Preventing the oak processionary moth

In order to catch an oak moth infestation early on, you should regularly check on trees that are particularly at risk, which of course includes oak trees. Traps tend to be set up in public areas where and when the caterpillars are expected to appear. Chemical control measures can only be used in the first and second stages of development. But as of mid-May at the latest, such treatments are no longer effective. Promoting natural predator populations can help keep an OPM infestation somewhat under control, but the only thing that can effectively eradicate the heat-loving oak processionary moths is a prolonged period of cold weather.

What increases the risk of an oak processionary moth infestation?

• If oaks are growing in a monoculture or in an open landscape with few other plants in the undergrowth.
• If there was a previous OPM infestation in the last few years.
• If nearby deciduous trees were infested the previous year.
• Mild winters and warm summers, as OPM love warmth.
• If there are too few natural predators such as robins or titmice.

Tip: Providing nesting aids for birds can help to support the oak processionary moths’ natural enemies. Initial observations suggest that supporting birds with nesting aids has had a positive effect in affected areas.