Harmful butterfly becomes climate change big winner

It is sprayed, vacuumed and recently also bombarded with highly concentrated scents: The oak processionary moth is not a welcome butterfly in parks and forests. Once a rather rare species in Germany, the warmth-loving moth has now spread widely.

A warm spring helps

The oak processionary moth (Thaumetopoea processionea) is favored by climate change. “Certainly, the rapidly changing climatic conditions also have an influence on the development of the oak processionary moth,” says Henrik Hartmann, head of the forest protection institute at the Julius Kühn Institute (JKI) in Quedlinburg. The decisive factors for the development are the weather conditions in late summer when the moths fly and lay their eggs, and in spring when the larvae hatch. If the temperatures rose early in spring, hatching and leaf bursting coincided favorably for the hungry insects.

“Whereas the oak processionary moth used to be found primarily in thinned-out forests or on the outskirts, it is now also found in the area,” says Julian Bethke, Nabu consultant for agriculture, forest and biodiversity. This is initially due to the respective annual climate. “Since 2018 there have been several extremely dry and hot years.”

“This can weaken entire forest stands”

Fighting the oak processionary moth in green areas, on playgrounds and on avenues is the task of the municipalities. The moth is particularly problematic because of the stinging hairs that the caterpillars develop from the third larval stage. They break off easily and can cause rashes, eye irritation, breathing difficulties and allergic reactions if touched. However, the oak leaf-eating butterfly can also cause forest damage if it appears in masses.

“With the accumulation of feeding years, there is an increasing impairment of the vitality of the oaks, which can lead to the death of trees,” explains Sylke Mattersberger from the State Center for Forests in Saxony-Anhalt. “That can weaken entire forest stands.” Weakened oaks are in turn less resistant to drought, mildew or other pests such as the gypsy moth and the oak jewel beetle. According to the state center, the butterfly has been spreading in Saxony-Anhalt for around 15 years.

Banger look at Bavaria

In North Rhine-Westphalian forests, in contrast to parks, for example, the pests have not been a major problem, says Ole Theisinger, a research assistant at the State Forest and Wood Agency in North Rhine-Westphalia. “But if we look at Baden-Württemberg, Bavaria or Brandenburg, we see that the oak processionary moth can also cause major damage in the forest.”

The pest has been appearing in Bavaria since the 1990s, as reported by the biologist Gabriela Lobinger from the Bavarian State Institute for Forestry. Since then, an infestation in forest areas has also been increasingly observed, first in Lower Franconia and in Donauries, and now also in other regions. “The species is quite adaptable,” says Lobinger. The observation shows that the populations fluctuated cyclically. Mass proliferation usually lasts about three to four years in a region, after which the infestation recedes. “After about ten years, mass propagation occurs again,” explains Lobinger.

“An incredible amount is tried out”

Natural regulatory mechanisms are responsible for the cyclical decline in infestation. Insects such as parasitic wasps lay their eggs in the larvae of the moth and thus weaken the population. “These opponents must be strengthened, for example by providing sufficient flowering areas,” says biologist Theisinger. Mowed green areas and a lack of diversity at the edge of the forest, on the other hand, promoted the mass reproduction of the oak processionary moth. The biologist Lobinger confirms this for Bavaria: If there are no opponents, for example in the case of individual trees on the roadside without green spaces, there can be no regulation.

So far, the pests have mostly been combated by suction, but sprayed biocides, nematodes or nesting boxes for tits – natural predators of butterflies – are also used. Experts agree that testing new methods is important when fighting the pest. “An incredible amount is tried out,” says Lobinger. However, some unconventional approaches, such as spraying the trees with hot water or foam, have so far not proven effective. Many procedures are also complex and expensive, says Forest Protection Institute Director Hartmann.

Hoping for the sex attractants

In NRW, the confusion of the male animals is currently being tested by sex attractants, which are shot into the trees in the form of small pellets. This is to prevent mating. The method is new in Germany, but has already been tested in the Netherlands, says Theisinger. There was a decrease in nests by up to 50 percent. Based on the results from NRW, which should be available in the summer of 2024, a nationwide project is to be created to research other possible areas of application for the method.

Nabu advises against biocide treatments from helicopters, as are sometimes carried out in Bavaria and Saxony-Anhalt. “This has fatal consequences for the forest ecosystem, which is already under a lot of stress due to heat and drought,” says Bethke.

You can’t get rid of the pest completely anyway, says Lobinger. Oaks without the oak processionary moth will no longer exist in the future. It is therefore important to remain calm when dealing with the butterfly. Anyone traveling in affected areas should keep their distance from the trees and keep an eye on children.

Giving up oak trees in the long term is not a solution. “The oak is an extremely important tree species,” emphasizes the biologist. The deciduous tree is considered robust against heat and drought. It is therefore often planted to convert monocultures of spruce or pine into mixed forests.