Insecticide lethal and sublethal effects on corn silk flies (Diptera: Ulidiidae) followed by ingestion exposure

Euxesta eluta Loew is a picture-winged fly (Diptera: Ulidiidae) pest of sweet corn in Florida. Management of picture-winged flies, locally referred to as corn silk flies, relies primarily on foliar applications of pyrethroids. The increasing number of applications needed for adequate control has become a concern, suggesting the possibility of insecticide resistance development, and emphasizing the need for alternatives. Therefore, laboratory experiments were conducted to determine the effects of six pyrethroids and 14 registered and non-registered insecticides with potential activity against corn silk flies. Glass micropipettes were used to deliver commercial formulations of the insecticides diluted in honey water. In a first experiment, E. eluta mortality associated with pyrethroids was 21-54% whereas spinetoram caused 95% mortality. Imidacloprid and acetamiprid caused ≤ 9% mortality whereas thiamethoxam, dinotefuran, and clothianidin caused ≥ 74% mortality. The other insecticides caused ≤ 7%, except cyantraniliprole (45%). In a second experiment, the effect on E. eluta fertility of ten insecticides causing < 15% mortality was evaluated. After insecticide ingestion, females were allowed to oviposit for 3 days on artificial diet and the number of maggots was counted 10 days after the end of the oviposition period. Novaluron, an insect growth regulator, was associated with a trend for a 48% reduction in the number of E. eluta maggots/female/day. These experiments suggest that spinetoram, selected neonicotinoids, and novaluron might play a role in corn silk fly management, especially if incorporated into effective baits.