Fitness in a malathion resistant Tribolium castaneum strain; feeding, growth and digestion

Extensive historical use of malathion in grain stores has led to the universal development of resistant strains of the red flour beetle, Tribolium castaneum Herbst. Adaptation to insecticide stress often reduces general fitness. To investigate non-defensive attributes physiological and biochemical properties (that may underlie fitness) of a field-derived malathion-resistant strain (larval resistance factor of 490) were measured against those of a susceptible strain and also with susceptible T. confusum. Further inhibition in vitro of gut proteolysis was determined for the resistant and susceptible strains of T. castaneum. The weights of whole gut tracts of the susceptible T. castaneum strain and T. confusum were similar but the resistant strain had significantly lower weight and width of whole gut tract. The gut of the resistant strain had more, and more intense, electrophoretic protein staining bands. Food consumed through the life cycle was significantly less for the resistant strain. Inhibitors for cysteine proteases had less effect in vitro in the resistant strain than in the susceptible strain. An inhibitor for serine proteases had almost no effect on proteolysis in the resistant strain. The reduced gut dimensions and food consumed, together with lower larval protein, appear to indicate a fitness cost that may be associated with malathion resistance. However, this may be offset by enhanced gut proteins and a relative greater protein content of the adult stage. The latter may be related to the reported increased reproductive capacity of the resistant adult and consequent superior fitness, compared to the susceptible. The lowered response of the resistant strain to cysteine and serine protease inhibitors may interact with adaptations to dietary inhibition in cases where resistant strains need managing, if this result is confirmed to be widespread. •Fitness in a malathion resistant strain based on underlying physiological processes.•Wheat flour ingestion in the resistant Tribolium castaneum was significantly reduced compared to the susceptible strain.•Gut tract size and protein contents were lower in the resistant strain than in both the susceptible strain and T.confusum.•Inhibition of gut proteolysis was less in the resistant than in the susceptible strain.•Resistant larvae had lower total soluble protein, while resistant adults had higher levels.