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.
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