Oak processionary caterpillar goes underground and bypasses control

Oak processionary caterpillars can stay in underground nests for months, only to crawl out en masse in June. They will quickly eat trees bare and scatter their annoying stinging hairs.
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Researchers at the Oak Processional Caterpillar Knowledge Center have discovered that the caterpillars can sit in the ground for ten months without moving on to their next development phase. In those cases, the caterpillars emerge from the ground in the fourth larval stage, with their dreaded stinging hairs. In many places in Drenthe, those hairs, when they come off, cause itching, breathing problems and burning eyes.

‘Much more complicated’

In the ground, the caterpillars can also pupate into butterflies, the researchers report on the Nature Today platform. The new insights make the fight against the caterpillars “a lot more complicated”, they write. “The caterpillars in the soil nests avoid the preventive spraying of oaks with nematodes and bacteria. The spraying is done earlier in the season.”

It is too late for spraying when the caterpillars come out full of stinging hairs in June. “It therefore means that as a tree owner you still have to keep a close eye on the sprayed locations later in the season and you have to be aware that at risk locations you still have to have sufficient capacity to suck away oak processionary caterpillars.”

Good news

In the knowledge center, the University of Wageningen, the RIVM and branch organizations of green companies work together. They also have good news: parasitic wasps and flies, natural enemies of the oak processionary caterpillar, can also go underground. “Stimulating natural enemies is therefore also an interesting option for soil nest control.”

The municipality of Westerveld has been working on improving biodiversity for years, together with the Frisian municipality of Weststellingwerf. For example, roadsides are mowed naturally and sown with flowers. Nesting boxes have also been placed in various places. All with the aim of improving the habitat of the natural enemies of the oak processionary caterpillar.

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