The oak processionary moth (Thaumetopoea processionea) has infested many parks in Germany, causing disruption and significant health risks. The spread of this invasive species is so severe this year that Estonian scientists are monitoring the situation with concern.
Despite its common name, the scientific name Thaumetopoea means 'miracle worker' because the unusual procession of the insects amazed a German entomologist who named the species over 100 years ago.
The caterpillars go through six developmental stages before pupating. In the third stage, they grow short, sharp defensive hairs. These airborne poisonous bristles cause severe health problems, including skin rash, eye inflammation, strong respiratory irritation, and asthma. In very sensitive people, shock can lead to death.
Even abandoned nests remain dangerous for years because wind carries the hair dust hundreds of meters. Cleaning infested trees is dangerous, expensive, and time-consuming, and it does not guarantee complete removal of harmful hairs. Areas where the insect has been active remain a source of danger for several years even after thorough cleaning.
Milder winters facilitate the northward spread of the species. Previously, hard frost killed most eggs, but milder conditions allow them to survive. The moth also spreads via imported infested oak saplings.
In 2024, a strong population was found in Denmark, likely from imported saplings. The Danish population was discovered by a nature observer who reported it, leading to the identification of over a thousand dangerous nests.
Natural enemies, such as birds, ground beetles, and parasitoids, do not spread as quickly with the species. Associate Professor Ivar Sibul advised that if people see unusual webs or hairy caterpillars moving in procession on tree trunks, they should take a photo from a safe distance and report it to the Estonian Agricultural and Food Board or Environmental Agency.