Experts have counted far fewer oak procession butterflies in recent months than they did in the same period last year. This is a 65% decrease. This brings us back to pre-2013 levels, Knowledge Center Eikenprocessionierup reports on Tuesday. As a result, the nuisance caused by caterpillars in 2023 is expected to decrease further.
There were already 18% fewer oak processionary moth caterpillars this year than in 2021. According to the knowledge center, the so-called pest pressure “now reaches acceptable nuisance level”. The caterpillars release their prickly hairs in spring. They can cause itching, bumps, and eye discomfort.
Fewer caterpillars usually mean fewer butterflies. To measure the number of butterflies, Kenniscentrum Eikenprocessionierupper has installed pheromone traps throughout the Netherlands. These are traps with a female attractant (pheromone) that attract male oak processionary moths.
On average, there were 8.5 butterflies in such a trap. The northern provinces had the most butterflies. But this is where the decrease has been greater than in the rest of the Netherlands. Nationwide, the number of butterflies caught has decreased for the fourth consecutive year.
Prior checking pays off
The knowledge center provides various possible explanations for the decrease. For example, preventive spraying of oak trees has already resulted in fewer caterpillars in spring. Many nests have also been removed over time.
Also, success has been achieved with natural caterpillar control. For example, birds, parasitic wasps, and parasitic flies are used to reduce processionary caterpillar populations.
There is a possibility that many caterpillars will crawl out of the ground next spring. Sometimes they disappear into ground nests for up to ten months, only to suddenly crawl out en masse in the spring. To monitor this development, experts have collected ground nests in cages.
In many nests, the butterflies have not yet hatched. When and if they will fly again is not yet clear, according to the knowledge center.