RELATIVE FITNESS OF A MALATHION-RESISTANT STRAIN OF CRYPTOLESTES-FERRUGINEUS (COLEOPTERA, CUCUJIDAE) WHEN DEVELOPMENT AND OVIPOSITION OCCUR IN MALATHION-TREATED AND UNTREATED WHEAT KERNELS

Malathion resistance in an English strain of Cryptolestes ferrugineus (Stephens) was triphenyl phosphate suppressible, autosomal, monofactorial, and semi-dominant. The resistant strain (RR) was crossed with a susceptible strain (SS) and heterozygotes were backcrossed to the SS strain for 6 generations then reselected on malathion-treated feed to produce another homozygous resistant strain (BC6RR) with most of the genome derived from the SS strain. Intrinsic rates of increase on whole-kernel wheat at 30°C, 75% r.h. were similar for the SS strain, the F1 reciprocal hybrids and the BC6RR strain but 22-28% lower for the original RR strain. On wheat kernels treated with 4, 8 and 16 ppm malathion, the intrinsic rates of increase of the BC6RR strain and the heterozygous strains were equal to or greater than the intrinsic rates of increase of the RR strain. Therefore, lower fitness of the RR strain relative to the SS strain could not be attributed to the R gene itself. The tendency of larvae to wander from the germ of the wheat kernel (treated or untreated) in which they had emerged from the egg was greatly increased in the presence of the R allele but only when it was homozygous. © 1990.