Host selection by Aphidius rosae Haliday (Hym., Braconidae) with respect to assessment of host specificity in biological control

Discrete steps in the process of host selection of Aphidius rosae Haliday (Hym., Braconidae Aphidiinae), a parasitoid of the rose aphid Macrosiphum rosae (L.) (Hem., Aphididae), were studied to assess its specificity in the Australian environment. In a wind tunnel females were strongly attracted only to roses when given a choice between uninfested shoots of hybrid Rosa sp., var. Tea-hybrid and various other plants. The parasitoids were not able to distinguish between infested and uninfested roses when the shoots were 30 cm apart, but they preferred to land on infested plants when shoots were placed only 5 cm apart. These results suggest that aphid-related volatiles involved in host location are detectable by the wasp only over a short distance. Experienced and inexperienced parasitoids were presented with a range of aphid species in choice and no-choice tests. Only the host M. rosae and to a much lesser extent the non-host Macrosiphum euphorbiae (Thomas) were attacked. No larvae of A. rosae were found in M. euphorbiae that were stung when these were dissected. To assess host specificity we argue that more attention should be paid towards the entire host selection process from location of host habitats to location of hosts, host acceptance and host suitability.