Why can arrowhead scales, Unaspis yanonensis Kuwana (Homoptera: Diaspididae), which burrow and settle below conspecifics, successfully avoid attack by its parasitoid, Coccobius fulvus Compere et Annecke (Hymenoptera: Aphelinidae)?

Abstract

When arrowhead scales, Unaspis yanonensis Kuwana (Homoptera: Diaspididae), burrow and settle underneath conspecifics (i.e., burrowers), the burrowers are less vulnerable to attack by the parasitoid wasp Coccobius fulvus Compere et Annecke (Hymenoptera: Aphelinidae) than scales that settle singly (i.e., singletons). To examine how this differential susceptibility between settling modes occurs during the host-finding and oviposition processes of the parasitoid, we observed the foraging behavior of the parasitoids in the laboratory. Encounter rates between parasitoids and hosts were similar between settling modes. When scales that had been burrowed under (burrowees) were alive, the parasitoids oviposited in them as frequently as in singletons, however, the wasps did not lay eggs in burrowers. When burrowees were dead or parasitized, the parasitoids did not exhibit oviposition behavior and left soon after they had drummed these individuals several times with their antennae. Our results strongly suggest that the parasitoids were unable to identify the burrowers. Based on this study, we discuss which stimuli may be used by the parasitoids as cues to locate hosts.