Studies on the host-finding ability of the aphid parasitoid,Trioxys complanatus (Hym.: Braconidae) in lucerne and clover

The spotted alfalfa aphid (SAA),Therioaphis trifolii (Monell)f. maculata, was first recorded in Australia in 1977. It is a major pest of lucerne and other pasture medics but not of clover (Trifolium spp.). The parasitoidTrioxys complanatus Quilis was introduced into Australia in late 1977 as part of a biological control program for SAA and made a substantial contribution to the management of the pest. Since 1989, aphids which are individually indistinguishable from SAA have been causing substantial yield loss to clover pastures in western and south-eastern Australia. This aphid, now known in Australia as the spotted clover aphid (SCA), is genetically distinct from SAA and has a much wider host range. Populations of SCA in clover are rarely parasitised byT. complanatus. A series of experiments was undertaken to determine whether the greater susceptibility of SAA in lucerne than of SCA in clover to parasitisation byT. complanatus is due to the attraction of the parasitoid preferentially to lucerne or to the differential attractiveness of the aphids themselves. SAA and SCA were offered to the parasitoids in arenas of increasing complexity from single trifoliate leaves of lucerne and clover on agar through single potted plants in small cages to groups of potted plants in a large cage. Results showed that the parasitoids exhibited a preference for lucerne though, in the cage situation, they did find and parasitise SCA on clover. They did not differentiate between SAA and SCA on lucerne.