Beetles and citrus share a common defense

In the journal Naturwissenschaften (The Science of Nature), scientists report that certain beetles produce a substance called limonene to defend against predators. Limonene is what gives lemons and oranges their characteristic citrusy odor. It’s also found in pine needles and many other plants, which use it to defend against grazers.

Scientists classify limonene as a terpene. Certain ants, sawflies, and termites also use terpenes to defend against predators. But this is the first documented case of a ­terpene-producing beetle. Two closely related ground beetles, Ardistomis schaumii and Semiardistomis puncticollis, use slightly different versions of the limonene compound, the scientists find.

Currently, products containing limonene are used for pet flea and tick control, as an insecticide, and as an insect repellent for use on humans. The repellent kills insects directly by damaging the protective wax coating of their respiratory system.

But in the beetles’ case, the authors think that limonene protects not by killing would-be predators directly, but by enabling the absorption of toluquinone, another noxious compound that the beetles secrete.