Biological control of the spotted alfalfa aphid, Therioaphis trifolii F. maculata, on lucerne crops in Australia, by the introduced parasitic hymenopteran Trioxys complanatus

TLDR: The drought, which developed concurrently with the decline and may have reduced source populations on non-irrigated lucerne plants thus lowering immigration rates into irrigated crops, broke in 1983 with no indication of aphid resurgence since.

Abstract: SUMMARY satisfactorily account for it. The capacity of the aphid to infest and increase on susceptible lucerne remains unchanged. The drought, which developed concurrently with the decline and may have reduced source populations on non-irrigated lucerne plants thus lowering immigration rates into irrigated crops, broke in 1983 with no indication of aphid resurgence since. Insecticide use, although high at first, decreased rapidly with the reduction in infestations. Relative to the rate of decline, the replacement of aphid- susceptible lucerne with aphid-resistant varieties proceeded slowly to reach only 10%/o in 1981 and 26%/o in 1984. The majority of lucerne grown in Australia is still susceptible to the aphid. Although native predatory insects became numerous in aphid-infested crops, they did not reduce aphid numbers sufficiently consistently to account for the decline. (4) Of the hymenopterous parasites, only one, Trioxys cornplanatus, became generally widespread and numerous throughout the lucerne-growing areas of Australia. (5) Parasitization by T. complanatus increased from one lucerne growth cycle to the next and higher levels were associated with aphid infestations that did not increase to damaging levels. (6) Observed reductions in the proportion of apterous adult female aphids in field populations were consistent with estimates of parasitization by T. complanatus. (7) When the properties of the aphid and parasite, as measured in the laboratory, were assembled into a simulation model of their interacting life systems on irrigated lucerne crops, outputs from the model closely matched the observed field data. Furthermore, the simulated population at harvest from initial lucerne growth cycles, when used as input for other runs of the model, resulted in good simulations of aphid and parasite populations in subsequent cycles in the same season. (8) It is concluded that Therioaphis trifolii on irrigated lucerne crops was biologically controlled by the introduced hymenopterous parasite Trioxys complanatus. (9) The short-term (say 5-year) value of the biological control, in terms of the reduction in the cost of insecticide applications needed to prevent loss and spoilage of the N.S.W. lucerne-hay crop, has probably been of the order of 1 million dollars a year. In terms of reduced urgency for replanting with aphid-resistant varieties, the value was similar. Over the whole of Australia, the value would be proportionally greater. (10) Although T. complanatus was known to be less effective at high summer temperatures, this did not prevent an overall reduction in immigration rates, although some alates still infest many crops.