Germany and the Netherlands are battling many infestations of oak processionary caterpillars, whose tiny toxic hairs can trigger allergic reactions and skin irritation.

The mild winter and warm spring this year boosted caterpillar numbers. In Louvain, Belgium, firefighters had to destroy nests of the invasive species before a rock concert. The caterpillars turn into pupae, then moths in late July, and the threat diminishes. Germany’s western Ruhr region is densely populated and among the worst affected by the caterpillars. Some schools and parks have been closed to allow specialists to attack the nests in oak trees.

The caterpillars – measuring 2-3cm (about one inch) – march in long processions to the treetops at night, and can wreak havoc in oak trees, as they feast on the young leaves. One mature caterpillar has up to 700,000 hairs, which can be spread by the wind. The Fredenbaumpark in Dortmund was closed for three weeks, as nearly 500 trees were found to be infested there, broadcaster Deutschlandfunk reported.

“The oak processionary infestation this year is very intensive – much more than last year,” said the park’s manager Frank Dartsch. Special teams there and elsewhere have donned protective gear and used firefighters’ lifts to reach the treetops, where they have attacked the caterpillar nests with blowtorches or big vacuum cleaners.(BBC)…[+]