Effects of Physical and Chemical Signals on Host Foraging Behavior of Drino inconspicua (Diptera: Tachinidae), a Generalist Parasitoid

Abstract

Host-foraging behavior of the tachinid parasitoid, Drino inconspicua Meigen (Diptera: Tachinidae), was studied by laboratory bioassays to elucidate the role of physical and chemical factors for host finding and recognition. Larvae of the European spruce sawfly, Gilpinia hercyniae Hartig (Hymenoptera: Diprionidae), were used as hosts. While volatiles of spruce twigs with feeding sawfly larvae did not attract the parasitoid, movements of host larvae and of dummies of the host significantly elicited a positive response at short range. Females of D. inconspicua showed a direct walking behavior to a moving object at distances of 9-10 cm. The maximum distance at which the parasitoids were attracted by movement was 16 cm. At short range, parasitoid females preferred to contact a moving larval dummy made of filter paper rather than an unmoved dummy or an unmoved freshly killed larva of C hercyniae. The tachinid females which were reared from G. hercyniae larvae showed no preference for this host species when compared with larvae of Diprion pini L. (Hymenoptera: Diprionidae) and the unnatural host species Galleria mellonella L. (Lepidoptera: Pyralidae). Furthermore, D. inconspicua did not prefer certain instars of G. hercyniae for oviposition. The role of cuticular chemicals of C. hercyniae larvae for host acceptance was tested by offering host larvae washed with solvents of different polarities and unwashed larvae, Each tested parasitoid female readily accepted both treated and untreated larvae for oviposition. When a hexane extract of the cuticle of host larvae was applied to a dummy, ovipositing D. inconspicua did not significantly distinguish between a treated dummy and a control dummy. Whereas the parasitoid's response to dummies offered as oviposition sites did not depend on the presence of cuticular extracts of hosts, the softness of the dummies was crucial. The parasitoid females significantly preferred soft dummies (hardiness unit [HU]:0.05± 0.01 Nmm -2 ) to hard ones (HU: 207.72± 6.16 Nmm -2 ) for oviposition. Thus, physical factors like movement of the host and its softness clearly play a major role in host foraging of D. inconspicua rather than chemical signals of the host or its host plant.