Response and genetic analysis of malathion-specific resistant Tribolium castaneum (Herbst) in relation to population density

Extensive use of malathion for pest control on stored cereals has resulted in worldwide resistance in red flour beetles, Tribolium castaneum. In this paper we investigate population density effects on the malathionspecific resistance in PRm, a strain from the Philippines, in an integrated resistance management framework. Two populations of malathion-specific resistant (PRm) and a malathion- susceptible strain of T castanewn were bred at different densities: low (4 adults/g) and high (12 adults/g) density on wheat plus brewer's yeast in the laboratory. After eight generations, slopes of probit regression lines and LC50 values were used to monitor the effect of insect rearing density on the progression of malathion-specific resistance. The LC50 of the malathion- susceptible strain (Asm) did not change significantly during selection while LC(50)s varied for both the high-density and low-density lines of PRm, the LC50 of malathion ranged from 27.51 to 34.06 and from 21.14 to 29.39 Vg malathion cm(-2) for high and low density, respectively. More than 33 generations were required to achieve a 10-fold increase of resistance for the low-density line compared to only 17 generations for the high-density line. Calculations from published formulae suggested that the malathion-specific resistance of both high- and low-density lines was under monofactorial control, with complete dominance. The data showed that environmental factors such as population density differences in insect rearing and development may influence the heritability of resistance. Furthermore, the variability in results published worldwide on resistance emphasises the need to standardize test conditions across laboratories. (c) 2005 Published by Elsevier Ltd.