Volatile cues used by the parasitoid, Aphytis melinus, for host location: California red scale revisited

Aphytis melinus DeBach (Hymenoptera: Aphelinidae) is a biological control a gent of diaspidid scale insects. The parasitoid has a narrow host range but its hosts are polyphagous. We determined the source of volatile cues the w asp uses to locate its few host species when those hosts occur on more than one host plant species. We addressed four questions in regard to the use o f volatile cues in host location of California red scale, Aonidiella aurant ii (Maskell) (Homoptera: Diaspididae): (1) Does A. melinus use volatile cue s to assist in host location? (2) Are these cues innately recognized or lea rned? (3) Are cues produced by female California red scale, or from other s ources? (4) Are the cues specific to the host or host plant? These question s were tested through the use of a Y-tube olfactometer. Female A. melinus u sed volatile cues to orient toward both infested and uninfested host plant material. Wasps learned these cues by associating odors from the host plant with host presence. They had no innate preferences for scale insect or hos t plant volatile stimuli. Contrary to previous studies, we found no evidenc e of orientation toward the female-produced sex pheromone of California red scale, nor to volatile cues from the attacked host stage. Wasps given expe rience with scale insects growing on lemon fruit subsequently oriented towa rd lemon and orange fruit and leaves. The scale species with which the wasp was given experience did not affect this preference. Wasps given experienc e with California red scale growing on squash did not orient toward infeste d lemon fruit. The host ranges of the parasitoid and its hosts are used to explain the adaptive value for the evolution of learned rather than fixed r esponses to cues used in foraging behavior.