Plant-derived semiochemicals as contact host location stimuli for a parasitoid of leafminers

We elucidated the source of chemical cues in a system where the host is concealed and the parasitoid has no direct contact with the host larvae or its frass. Behavioral bioassays with Pholetesor bicolor, a larval parasitoid of the apple leafminer, Phyllonorycter pomonella, showed that the herbivore-damaged leaf epidermis (mine) elicited ovipositional probing of parasitoid females. Probing on larvae or frass was seldom observed. Hexane extracts of mines elicited the same ovipositional probing behavior while no response was observed with hexane extracts of larvae or frass or with methanol and diethyl ether extracts. In addition, gas chromatographic analyses showed qualitatively and quantitatively different profiles of these three components of the host-plant complex. By far the highest quantities and also the highest number of compounds was recovered from mine extracts. Identified compounds in the mine included six alkanes (n-C-27 to n-C-33) and squalene (C30H50) A synthetic blend of the seven compounds was slightly less active in biotests than the equivalent natural blend, as shown by a time delay in female response. We conclude that this leafminer parasitoid does not rely on host-derived kairomones but instead uses plant-derived semiochemicals for host location and ovipositional probing behavior.