CONTINUOUS, ALTERNATING, AND MIXED INSECTICIDES AFFECT DEVELOPMENT OFRESISTANCE IN THE HORN FLY (DIPTERA, MUSCIDAE)

Susceptible populations of horn flies, Haematobia irritans (L.), were selected with six insecticide treatment regimens: continuous use of pe rmethrin, diazinon, or ivermectin; permethrin/diazinon (1:2) mixture; permethrin-diazinon rotation; and permethrin-ivermectin rotation. Inse cticide selection was applied as low-volume sprays on steers infested with horn flies housed in environmentally controlled rooms. During the study period, horn fly populations developed an apparent resistance t o all insecticide treatment regimens. Selection with permethrin diazin on, and ivermectin resulted in development of resistance in generation s 21, 31, and 30, respectively. Selection with insecticide mixtures an d rotations resulted in the delay of any apparent resistance developme nt for an additional one to seven generations. The magnitude of this r esistance ranged from 1.470-fold for the single continuous use of perm ethrin to <3-fold for ivermectin in the permethrin-ivermectin rotation . Compared with single continuous insecticide selections, the onset an d degree of resistance development was significantly reduced by altern ating and mixing insecticides.