Oak processionary moth: facts, rash treatment & control

Oak processionary moth: facts, rash treatment & control

Oak processionary moth damages oak trees and can cause allergic reactions in humans. In this article, we will show you how to get rid of his moth from the garden.

The fluffy brown caterpillars of the oak processionary moth (Thaumetopoea processionea L.) look quite cute at first glance. They usually form a long line one after the other on their way to the leaves of oaks – one can almost be tempted to stroke them gently. In spite their cuteness, the fluffy larvae of the oak processionary moth are pests that need to be taken seriously because they can harm both animals and humans with their stinging hairs. You can read more about the rash caused by the fluff on the caterpillars in the following.

Oak processionary moth

Here you will learn how to recognise and prevent the oak processionary moth and its caterpillars (also abbreviated as OPM). You will also find all information about the development, distribution and harm that can be caused by the processionary caterpillars. However, for the sake of your health it is better to leave the control to a specialist.

Oak processionary moth: distribution

The warmth-loving oak processionary moth has been on the advance in Europe since about 1990. Its development is favoured by climate change. All types of oak and, in exceptional cases, other deciduous trees, such as hornbeam, beech, birch or black locust, can be infested by it. This pest prefers light, warm forests with a high proportion of oak trees and on the sunny southern edges of these forests. Single trees in park areas are also gladly accepted by the voracious caterpillars.

Identifying oak processionary moth

In order to detect an infestation at an early stage, it is important to be able to detect the moth reliably. The larvae of the oak processionary moth are nocturnal. On their back, there is a broad and dark line with velvet-like hairs. From the third larval stage onwards, the long stinging hairs are formed. From the fifth larval stage onwards, the caterpillars create large webs of caterpillar silk, which serve as protection and nest for pupation. The caterpillars are named after their behaviour: the sociable animals often move directly behind each other in a procession.

The adult moths have a wingspan of about 3 to 3.6 centimetres and are hairy all over. However, their hair is not composed of stinging hairs, so they are not dangerous as their larvae are. While the wings of the males show two clearly visible horizontal stripes, the stripes are less pronounced or not present at all. The moths are also nocturnal, so they are relatively seldom seen.

oak processionary moth
Unlike their larvae, adult moths of oak processionary are not dangerous because they do not have the stinging hairs [Shutterstock.com/Sandra Standbridge]

Oak processionary moth: rash and other types of damage

As a rule, an infestation with the oak processionary moth is very easy to cope with for a healthy tree. Even if completely defoliated, the affected oaks will sprout again well the following year. However, repeated infestation over several years can become problematic. In short, weakening by various abiotic (e.g. drought, heat, lack of water, frost) and biotic (powdery mildew, oak jewel beetles, gypsy moth) factors can lead to an overall loss of vitality, so that in some cases plant protection with pesticides has to be applied in order to prevent the trees from dying. Especially in public green areas, the processionary moth can be threatening to our health, because the stinging hairs can trigger strong reactions of the skin and the respiratory tract.

Development of the oak processionary moth

Below this text, you will find a table showing the development of the oak processionary moth. The female moths lay their eggs, which are about one millimetre in size, in the upper crown area of oaks between the end of July and the beginning of September. A clutch of eggs can consist of 100 to 200 eggs and is arranged in the form of an elongated tray-like structure and carefully camouflaged.

oak processionary moth
The larvae of the oak processionary moths are covered in stinging hairs that can cause allergic reactions in both humans and animals [Shutterstock.com/Serhii Milekhin]

At the beginning of the vegetation period, the first larval stage begins. At this stage, the caterpillars hatch from the eggs and are still yellow to brown in colour. The caterpillars then eat the entire leaves of oaks – only the midrib of the leaf is left undevoured. Depending on weather conditions, the first larvae of the third developmental stage may be present from April onwards. From this stage on they have the typical stinging hairs, which are equipped with barbs and a poison. In June or July, the larvae pupate. After three to six weeks as a pupa, the adult moth finally hatches, and the cycle begins anew.


Oak processionary moth: prevention

Unfortunately, it is almost impossible to use preventative measures to avoid an oak processionary moth infestation. However, you should carry out repeated checks on particularly endangered trees so that you can react in time if infestation occurs. This is because chemical control measures, for example, can only be applied in the first and second stages of development. By mid-May at the latest, such treatment is therefore no longer possible. In general, the promotion of natural predators of the moth leads to a certain reduction in the intensity of infestation. However, a truly effective decimation of the thermophilic oak processionary moth is only possible through longer cold periods.

What causes an infestation with oak processionary moth?

  • Oak trees grown in monocultures or in open landscapes and with little undergrowth of other plants
  • An infestation in previous years
  • Nearby deciduous trees were infested in the previous year
  • Mild winters and warm summers

How to get rid of oak processionary moth

Getting rid of the oak processionary moth is difficult for several reasons. First, the moth prefers to infest already tall trees, and these are very challenging to spray with conventional products. Second, it is a very bad idea to get close to oak processionary moths, especially their larvae. So never try to remove the animal yourself! If you are forced to be near an infested tree, please follow these instructions.

Instructions for staying near an infested tree:

  • All skin areas must be protected as well as possible by clothing
  • Do not touch the caterpillars and webs
  • Wash clothing after contact at 60 °C; rinse hair and body thoroughly
oak processionary larvae
It is best to completely stay away from the tree, if you notice the fluffy caterpillars [Shutterstock.com/Martina Unbehauen]

If problems occur in your private garden due to the presence of oak processionary moths, you have only two options: either you avoid the infested area in order to avoid contact with the stinging hairs of the caterpillars – this is the best option recommended by professionals. Or you arrange for the caterpillars to be controlled by a company specialising in pest control. Suction and scraping are also common methods that are used. Burning or felling the infested plants bears the danger that the stinging hairs spread even further, which is why these methods are not recommended either.

Should an infestation with the oak processionary moth be reported?

Depending on the area you live in, there might be an official rule that obligates you to report the oak processionary moth. However, even if there is no such rule in the place you live in, you can and should voluntarily report an infestation in private as well as public areas to the responsible public order office in your area. There you may receive further tips and, if necessary, measures will be taken to eliminate the infestation.

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