Comparison of Honey Bee (Hymenoptera: Apidae) Mortality as a Result of Diurnal and Nocturnal Applications of Dimethoate

Abstract

Applications of dimethoate in Arizona are currently being made to citrus orchards at night to reduce the hazard to pollinating honey bees, Apis mellifera L. Over a 9-d period, an application of dimethoate sprayed during the day produced a mean number of 13,414 dead honey bees per colony, but an application sprayed at night produced a mean loss of 6,735 honey bees per colony. Over the same 9-d period, colonies in an unsprayed orchard had a mean loss of only 1,792 honey bees. At one point, pollen collection was reduced by 100% in two sprayed orchards but was unchanged in an orchard that was not sprayed. Colonies in an orchard sprayed during the day lost a mean of 9.2 frames of honey bees, whereas those in an orchard sprayed at night lost a mean of 4.2 frames. Colonies in an unsprayed orchard, however, gained a mean of 3.2 frames of honey bees. Honey bee mortality from applications sprayed at night, although significantly lower than the applications sprayed during the day, is probably still at a level unacceptable to beekeepers.