Plants besieged by caterpillars release a chemical alarm that summons an army of defenders, such as parasitoid wasps whose voracious larvae devour caterpillars from within. But this plant-defence system is open to sabotage from the parasite's parasites.

Erik Poelman of Wageningen University in the Netherlands and his colleagues found that, in some cases, volatile compounds released by plants damaged by infected caterpillars also attract another set of wasps, called hyperparasitoids, that attack the cocoons of the caterpillar-eating wasp larvae. Infection with Cotesia glomerata wasps affects caterpillars' oral secretions, causing the plants they eat to give off a different blend of chemicals from those colonized by unaffected caterpillars. These chemicals seem to call in the hyperparasitoids. In the lab, the hyperparasitoid Lysibia nana (pictured) preferred plants munched by caterpillars infected with C. glomerata to those eaten by their uninfected counterparts.

PLoS Biol. 10, e1001435 (2012)