Relative effectiveness of topical, filter paper and grain applications of deltamethrin, and associated behaviour of Rhyzopertha dominica (F.) strains

The susceptibility of stored grain pests to insecticides may be assessed by different types of bioassay application methods such as topical, impregnated filter papers and grain treatments. Pests may behave differently when in contact with pesticides and these changes in behaviour may contribute to their tolerance of pesticide. To compare the three application methods and determine how Rhyzopertha dominica (F.) behaved on contaminated filter papers and treated grain, experiments were carried out on four strains of this species. The strains were of known susceptibility to deltamethrin; three were from Brazil (BR4, BR6 and BR7) and one from the United Kingdom (UK1). BR4 and UK1 were susceptible strains, and BR6 and BR7 were resistant strains. Behavioural responses of the pest to deltamethrin were compared between resistant and susceptible strains by measuring the walking response of individuals on deltamethrin impregnated filter papers and the repellent effect of treated wheat grain. The body weights of strains were compared. The results showed resistance ratios of 21, 175 and 17 times between the most susceptible and the most resistant strains, following topical, filter paper and grain applications, respectively. The most resistant strain walked less on contaminated filter papers while the most susceptible strain did not change its walking pattern. Deltamethrin showed no repellent effect, neither on impregnated filter papers or in treated wheat grain. The body weights differed among the strains but were not associated with the level of tolerance to deltamethrin.