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Recognizing oak processionary moths and their caterpillars is not easy but important. Because they can not only be dangerous for plants. We show which features help to differentiate.

In a nutshell

  • adults are difficult to distinguish from other moths
  • Locomotion of the caterpillars is unusual and conspicuous
  • Foliage is almost never attacked
  • Nests are found on oak trees
  • Period of occurrence can be an indication

What are oak processionary moths?

It is a species of moth that feeds on oak leaves and shoots, causing the trees to become completely bare. In some regions, they therefore pose a significant threat to entire sections of forest.
Because when the oaks no longer have any leaves, photosynthesis can no longer take place. In addition, the regulation of the moisture balance is negatively influenced.
This weakens the trees and can directly cause them to die. This is often the case, especially with very young plants. However, by reducing the processes taking place, the oaks also become more susceptible to other pests and diseases. Mainly the caterpillars of the oak processionary moth can therefore indirectly ensure that the trees die.

damage picture

Oak processionary moths are particularly recognizable by the damage they leave behind. Even in mixed forests, they feed on oak leaves and the young shoots of the trees, which is extremely noticeable.
If there are only signs of feeding and bare oaks, this is a clear indication of a heavy infestation with this moth species and its caterpillars.

size of the caterpillars

The larvae go through several stages of development. Of course, the size can vary. In the last stage they reach a length of up to five centimeters. Due to their size, they are very noticeable. This is not just due to the length alone. Due to their grouping when moving, they are sometimes hard to miss.

color

Depending on the stage of development, the color of the caterpillars can change. Possible are:

  • brown with a light line on the back
  • dark brown all over
  • light sides with dark brown back

This makes it difficult to distinguish between harmless species. The color alone should therefore not be used as a criterion.

hairiness

One of the most important distinguishing features is the hairiness of the caterpillars. This consists of different hairs or bristles:

  • stinging hairs
  • dark, short hairs
  • long, light to white hairs

The most striking are the long, light-colored bristles. They stand out clearly and form a kind of ring around the entire body.

Notice: Beware of the stinging hairs, these can be dangerous for both humans and pets.

locomotion

Regarding the most noticeable characteristics, the behavior of the caterpillars during locomotion is particularly striking. Because these are rarely found individually. Instead, they migrate in groups. For this reason they also have the name oak processionary moth. Because they move on as a procession.
At the top is a single larva that acts as a sort of leader. This results in the following characteristics:

The caterpillars of the oak processionary moth always move in large groups.
  • formed as an arrow
  • Occur in larger groups
  • easily recognizable and distinguishable from other species
  • mainly found on tree trunks

nests

The caterpillars spin nests that are vaguely reminiscent of very dense cobwebs. These can mainly be found on:

  • branches
  • drives
  • tribes
  • drives

The weaves of the oak processionary moth can be recognized by their different shapes. For example:

  • as a belt around an oak trunk
  • rarely stuck under leaves
  • wrapped around branches and twigs

The weaves can sometimes also hang down in threads and are so dense that they initially appear silky.

Notice: The nests change color relatively quickly and are then difficult to distinguish from the bark. On closer inspection, however, the threads are occasionally noticeable.

The nests of oak processionary caterpillars are found on oak trees.

propagation time

The caterpillars are found mainly in late spring to early summer. However, the activity and speed of development depends on the climate and the current weather. In years with mild winters and warm springs or a generally mild climate, the larvae can therefore hatch very early and pupate quickly. They spread rapidly as early as March and at the latest by the end of April.

activity

The adult moths are nocturnal, while the caterpillars move and feed mainly during the day. It is therefore extremely difficult to recognize the moths. However, the caterpillars can be observed on a walk in the woods, in parks or avenues.

Rash & allergic reactions

Due to the stinging hairs of the caterpillars, the oak processionary moth also poses a danger to humans and animals. Direct skin contact can cause a rash. In addition, the hair can also fly through the air. This means that they can get into the respiratory tract and eyes even without direct contact. This can lead to allergic reactions. Below:

  • burning, watering and reddened eyes
  • to cough
  • itching
  • Sneeze
  • redness
  • swelling
  • difficulty breathing
  • increased production of secretions and tears

Therefore:

  • avoid direct proximity to the caterpillars
  • Urgently keep pets and children away
  • Do not touch
  • wash after accidental touches or short distance

If the symptoms persist or are very severe, a doctor or veterinarian must be consulted.

butterfly

Recognizing the adult oak processionary moth is not only difficult for laypeople. Because the animals can hardly be distinguished from other nocturnal butterflies due to their size and coloration. The characteristics are:

Source: Ben Sale from UK, (2022) Oak Processionary (Thaumetopoea processionea) (35482535524), edited by Plantopedia, CC BY 2.0
  • brown-grey coloring of the wings
  • Pattern in the form of lines
  • nocturnal
  • reddish brown body
  • 25 to 35 millimeters long

frequently asked Questions

Do oak processionary moths have to be reported?

There is no obligation to do so. Nevertheless, it makes sense to report an infestation on private property as well as in forests and parks. This means that appropriate measures can be taken at an early stage to keep the damage as low as possible. Possible reporting points are the public order office and the green space office.

Can the moth species be combated?

Yes, the oak processionary moth can be controlled by various means and measures. These include, for example, sucking off or using natural enemies. However, the infestation is often not noticed until late, so that trees already show considerable damage and may have to be felled. This may be necessary to prevent further spread.

When and where does egg laying take place?

Around late summer, the females of the species fly between two and ten kilometers. They look for a suitable host tree and lay their eggs in packets of 30 to 300 on the one to two year old branches.

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Category:

Garden pests