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Evaluating the impact of insecticides on Scaptomyza flava and its parasitoid, Asobara persimilis

Abstract The leaf miner, Scaptomyza flava (Diptera: Drosophilidae), causes commercial losses to brassica crops, particularly in unsprayed Asian brassicas such as ‘Joi Choi’, ‘Pak Choi Sum’, and Chinese cabbage. The objective of this study was to assess the susceptibility of a larval/pupal parasitoid, Asobara persimilis (Hymenoptera: Braconidae), of this pest to insecticides registered for use on brassicas and to an insecticide, abamectin, used to control agromyzid leaf miners overseas, as a contribution to the development of an integrated pest management programme. Adults of A. persimilis were exposed to leaf discs dipped in solutions of field rates of abamectin and 13 insecticides registered for use on vegetable brassicas. Mortality after 24 h varied from 0% to 100%. In four replicated field trials in Pukekohe (South Auckland, New Zealand), Asian brassicas ‘Joi Choi’, ‘Pak Choi Sum’, and Chinese cabbage were sprayed with insecticides and numbers of leaves with leaf mines and fly damage were recorded from 5 plants per plot at harvest. Fly pupae emerging from leaf mines during the 4 days after sampling were kept to assess fly and parasitoid emergence rates. All insecticides tested reduced the numbers of leaves with blotch mines, although results achieved with spinosad were inconsistent. Parasitism by A. persimilis varied from 7.6% to 82% in untreated plants. Parasitoids emerged from pupae reared from plants exposed to all insecticide treatments. Although A. persimilis can reduce populations of leaf miners in summer and prevent crop damage, insecticide applications are required in spring and early summer.