Missing poisonous oak processionary caterpillars may still be dormant underground

The decrease in the number of observed oak processionary caterpillars may be because the insects can spend a year, possibly even longer, underground. Researchers have recently seen an unexpected number of butterflies crawling from out of the ground. This makes combating the plague more difficult, and could potentially lead to greater nuisance later, writes Nature Today.

The caterpillar lives in oak trees in the summer and thrives best in warm weather. The insect has hairs which can cause itching, breathing problems and eye irritation. In 2019, the caterpillar caused a great deal of nuisance, and those fighting off the insects could barely handle the work.

A researcher from the Kennisplatform Processierups think-tank observed this week that both males and females emerged from a testing site which they had burrowed into last year. In other places, caterpillars have been observed that, with a short interruption above ground, have even returned underground twice a year.

The fact that caterpillars remain in an extended diapause, a form of hibernation, for a year or even longer, may explain why the think-tank caught 40% fewer butterflies in 2019, the year in which the caterpillars were at their peak, than a year earlier. The large number of caterpillars meant a further increase of the number of expected butterflies.

Researchers wonder when these “disappeared” caterpillars will reappear and whether they will emerge from the ground as caterpillars or as butterflies. "It will be exciting to see how many butterflies will be caught in placed traps in the coming months," they say.

In the past two years, the nuisance caused by the caterpillars was not too bad. This was due to the relatively low temperatures in the spring. The caterpillar was seen in 17 percent of the about 110,000 trees which were monitored last year.