Influence of Simulated Rain on Residual Activity of Ultra-Low-Volume Applications of Carbaryl and Malathion Against Grasshoppers1234

Aerial ultra-low-volume applications of Seven®-4-oil formulation of carbaryl (8 oz AI/acre) or of technical malathion 9.74 oz AI/acre were made to rangeland infested with grasshoppers. Then, 8-9 hr posttreatment, ⅛, ¼, or ½ in. simulated rainfall was applied to grasshoppers caged in the area. The control obtained with carbaryl was reduced at 6 and at 9 days posttreatment by ½ in. of rainfall, but at 12 days there were no significant differences among treatments. The control obtained with malathion was reduced significantly by even ⅛ and ¼ in. of rainfall. Also, mortality of grasshoppers confined to treated vegetation that had not been exposed to rain was substantially higher after treatment with carbaryl than after treatment with malathion, and the carbaryl was still effective at 18 days after treatment. Caging affected the results obtained with malathion but had little or no effect on the results obtained with carbaryl, because the confinement limited contact with malathion which is toxic for a shorter period.