Study finds birds, ivy, and flowers cut oak processionary caterpillar numbers naturally

A five-year Dutch-Flemish research project involving provinces such as Noord-Brabant and Gelderland has concluded that natural measures can significantly reduce the population of the oak processionary caterpillar, a pest known for causing health problems in humans.

The study highlights that hanging birdhouses in oak trees for insect-eating birds like the great tit and blue tit helps control the caterpillars. Additionally, planting ivy against the trees showed a clear reduction in caterpillar numbers. The presence of flowers and herbs alongside roadsides attracts natural enemies such as parasitic wasps and flies, which also contribute to controlling the pest.

The oak processionary caterpillar is covered with hundreds of thousands of tiny hairs that cause itching, skin rashes, and irritations of the eyes and respiratory system in humans. Currently, municipalities mostly combat the caterpillars using biocides—chemical substances that kill organisms—or by vacuuming the nests.

Hagar Roijackers, provincial executive for nature and environment in Noord-Brabant representing GroenLinks, expressed strong support for the findings. “I am pleasantly surprised that this research confirms how effective biological solutions are in reducing the oak processionary caterpillar’s pest pressure,” she said. The project aims to phase out nearly all biocide use in the future. To help municipalities and residents implement the recommendations, involved agencies have produced instructional videos explaining the measures.

Several municipalities have recently issued new warnings about the oak processionary caterpillar. This year, the caterpillars emerged earlier than usual, at the end of March, due to a warm and dry spring. Before they pupate into moths, the caterpillars develop the harmful hairs that cause health issues. Many municipalities urge residents to report nests so they can be removed by vacuum systems.