Vienna is experiencing a decline in toxic caterpillars that can harm oak trees and trigger allergic reactions in humans.

While global warming has led to an overall increase in oak processionary moths (Thaumetopoea processionea), the wet spring has resulted in fewer moths this year in Vienna, according to Alexander Mrkvicka from the City of Vienna’s forestry department (MA49).

“The caterpillars don’t fare well in damp conditions because fungi, bacteria and viruses thrive on wet leaves, leading to many caterpillars dying,” Mrkvicka told ORF.

Vienna is home to some 2,500 oak trees, which are treated preventatively each spring using a special biological treatment.

Before transforming into moths, the caterpillars attack oak trees. Measuring 2-3 centimetres and moving in long processions from tree to tree (hence the name), their toxic hairs can cause “caterpillar dermatitis” in humans.

Symptoms resemble an allergic reaction, causing extremely itchy rashes, while inhaling the hairs can trigger severe asthma attacks. Due to the health risks, property owners must take protective measures and report any infestations to the authorities immediately.

Although the caterpillars usually cause only moderate damage to their host trees, repeated infestations over several years can lead to lasting damage.