Factors affecting ovipositor probing in Trybliographa rapae, a parasitoid of the cabbage root fly

The effect of allelochemicals from its host, the larva of the cabbage root fly, Delia radicum (L.) (Diptera: Anthomyiidae), and the host's food plant on the ovipositor probing response of the parasitoid Trybliographa rapae (W estw.) (Hymenoptera: Cynipidae) were investigated. Trybliographa rapae prob ed both cabbage root fly infested and uninfested swede (Brassica napus var. napobrassica), although significantly more wasps responded to infested swe de. Antennal sensilla are likely to be the mediators of this response. The synomones and kairomones involved are extractable in water, diethyl ether a nd methanol. No response was observed to washed, starved cabbage root fly l arvae. Wasps spent significantly longer searching infested swede than uninf ested, although probing frequency remained constant. It is suggested that t he initiation of probing in T. rapae is dependent on a threshold concentrat ion of general synomones or host related synomones and kairomones, whereas time spent searching a particular area is dependent on the environment perc eived by sensilla on the ovipositor.