BERLIN—Ever wondered what kind of weasel would vandalize a perfectly good car left outside at night?

Germany has the answer: Its technical name is the stone marten. This sharp-toothed critter has a habit of squeezing into the innards of parked cars and feasting on plastic hoses and tubes. And Germany’s marten population is exploding.

Weasel damage is the fourth most frequent cause for non-collision auto insurance claims in Germany. Last year, drivers here filed 198,000 claims for weasel-inflicted damage, a 42% increase since 2005. And that probably underestimates the carnage.

“We only have data from those insurers that offer a weasel policy,” said Henning Engelage, a spokesman for the insurers’ federation.

While other Europeans may dismiss weasel attacks as a mere annoyance, no one messes with a German’s car without consequences, says Michael Schönthal, owner of MS MarderSicher GmbH, a maker of popular weasel deterrent systems.