Crafty caterpillars have repurposed an acid normally deployed against predators as a defence against plants that ooze a sticky, dangerous goo.

Before they transform into moths, Theroa zethus larvae gorge themselves on plants in the Euphorbiaceae family. This intrigued David Dussourd at the University of Central Arkansas in Conway and his colleagues because, when damaged, the plants exude large amounts of gummy latex that can trap caterpillars or clog up their works.

Observation of the species revealed that the caterpillar uses its mandibles to scrape off some of the wax that coats the plant’s leaves and stems, then secretes an acid, which softens cell walls, into the opening. The caterpillars use the same acid to deter predators such as birds.

The resulting withering of the plant’s tissue, along with the caterpillar’s manual compression of the area with its mandibles, dams the leaf’s latex channels. The larvae can then feed on the leaf, downstream of the obstruction, without being hit by a tsunami of latex.

Related caterpillar species clamp leaf stems, probably to stop defensive compounds from rushing into damaged leaves. T. zethus might have adapted this trick, allowing it to feed on otherwise inaccessible plants, the researchers suggest.