FIELD APPLICATION AND EFFECTS OF CHLORPYRIFOS RESIDUES ON SITONA DISCOIDEUS (COLEOPTERA, CURCULIONIDAE) ADULTS AND IMPLICATIONS FOR BIOLOGICAL-CONTROL BY MICROCTONUS-AETHIOPOIDES (HYMENOPTERA, BRACONIDAE)

Chlorpyrifos was applied to field lucerne at a rate (0.3 kg/ha) recomm ended to control the weevil Sitona discoideus Gyllenhal. The fate of t he weevil population and that of the immature stages of its parasitoid Microctonus aethiopoides Loan were examined by field sampling. A sing le application of chlorpyrifos reduced the weevil population by 96.2% 14 days after application. There was no significant decline in the con trol population between sampling intervals nor a change in the parasit ism levels. However, in the insecticide-treated plots c. 88% of surviv ors (n=8) were parasitized. The effects of chlorpyrifos residues at 0. 3 and 1.0 kg/ha were also examined. At the lower rate, residues produc ed c. 40-60% weevil mortality for up to 15 days after application alth ough a significant decline in activity only occurred between days 1 an d 3. At 1.0 kg/ha, the activity decay curve was much slower with no si gnificant decline in weevil mortality for the first 15 days of the exp eriment. Weevil gender was shown to be a significant factor influencin g mortality. Mortality of unparasitized weevils was also significantly greater than for parasitized hosts while the stage of parasitoid deve lopment had a significant effect on mortality. Where parasitoid develo pment was sufficiently advanced, late-instar larva emerged and success fully pupated from cadavers up to 18 h following host mortality. The c onsequences of applying insecticide to control S. discoideus as they r elate to the parasitoid and the success of biological control are disc ussed.