Pieris rapae larvae secrete small oil droplets from their dorsal setae, which adhere to objects that touch them. The function of the droplets was studied in terms of both generalist and specialist predators. We tested the function of the droplets against ants tinder laboratory and field conditions. In the laboratory observation, Formica japonica ants that touched the larvae with their antennae initiated antenna) cleaning and did not prey on the larvae. In the field, predation pressure by ants on larvae with oil droplets was significantly lower than on larvae from which oil droplets were removed. Thus, we concluded that the droplets had a defensive function. By contrast, oviposition by Cotesia glomerata, the specialist parasitic wasp of P. rapae larvae, was not affected by the presence of oil droplets. Furthermore, the wasps exhibited searching behavior and oviposition behavior towards filter paper which had been impregnated with the droplets' substance, suggesting that the oil functions as a host-searching cue and an oviposition stimulant for C. glomerata. According to these results, the functions of the droplets are discussed with regard to the prey-predator interaction. none