SHORT-RANGE HOST LOCATION MECHANISMS OF BARK BEETLE PARASITOIDS

The host searching behaviour and short-range host location by parasitoids of the Eurasian spruce bark beetle, Ips typographus, were examined in order to repeat, using another scolytid-parasitoid system, that observation by RICHERSON and BORDEN (1972b) that host metabolic heat radiation is the principle method of host location by scolytid parasitoids. Differences in searching behaviour of the braconid parasitoids, Coeloides bostrychorum and Dendrosoter middendorffii, and the pteromalid, Rhopalicus tutela, are described. Experiments using both an infra-red scanner and thermistor probes failed to detect "hot spots" associated with susceptible host beneath the spruce bark. Deepfreezing and host removal treatments of scolytid infested spruce logs indicated that host vibrational cues were unnecessary for successful host location by parasitoids. The role of volatile chemical cues was investigated using both barriers and implantations on logs. Cellulose barriers did not disrupt host location, while wax barriers disrupted host location for up to 24 h. Similarly, bark from which hosts were removed sustained parasitoid probing for up to 24 h while implantation of hosts into uninfested logs required a considerable lag time before becoming attractive to parasitoids. These results indicate that volatiles are more probably the proximal cues used for host location by the parasitoids of I. typographus and that metabolic heat radiation from hosts was undetectable.