Predator-prey interaction between greenhead ants and processionary caterpillars is mediated by chemical defence

The predatory greenhead ant, Rhytidoponera metallica, readily attacks some insects but not the processionary caterpillar, Ochrogaster lunifer. This urticarial lepidopteran larva is covered in long hairs and has numerous smaller, detachable barbed setae present on the third instar and older larvae. These setae are a health risk to humans and other mammals. In this study, we tested whether O. lunifer larvae are defended against invertebrate predators, using R. metallica as an example. Field experiments at R. metallica nests and laboratory olfactometer experiments were used to assess possible behavioural, morphological and chemical defence mechanisms. We found that a volatile chemical component from O. lunifer larvae inhibited attack/approach by R. metallica. This odour was associated with O. lunifer cuticular components: larval hairs, setae and exoskeleton. Behavioural defence was not found to inhibit R. metallica attack. Furthermore, despite the dense hairs and setae covering O. lunifer larvae, these did not act as a physical barrier to ant attack. Our study demonstrates the key role of chemical ecology in invertebrate interactions involving such hairy caterpillars. Future isolation of the deterrent chemical(s) produced by O. lunifer larvae may offer novel possibilities for managing ant behaviour.