Malathion-resistant Tribolium castaneum has enhanced response to oxidative stress, immunity, and fitness

Many cases of insecticide resistance in insect pests give resulting no-cost strains that retain the resistance genes even in the absence of the toxic stressor. Malathion has been widely used against the red flour beetle, Tribolium castaneum Herbst. in stored products although no longer used. Malathion specific resistance in this pest has provided resistance that is long lasting and widely distributed. To understand this resistance a malathion resistant strain was challenged with a range of stressors including starvation, hyperoxia, malathion and a pathogen and the antioxidant responses and some lifecycle parameters were determined. Adult life span of malathion-specific resistant strain of T. castaneum was significantly shorter than the susceptible. Starvation and/or high oxygen reduced adult life span of both strains. Starving with and without 100% oxygen gave longer lifespan for the resistant strain, but for oxygen alone there was no difference. Under oxygen the proportional survival of the resistant strain to the adult stage was significantly higher, for both larvae and pupae, than the susceptible. The resistant strain when stressed with malathion and/or oxygen significantly increased catalase activity, but the susceptible did not. The resistant strain stressed with Paranosema whitei infection had significantly higher survival compared to the susceptible, and with almost no mortality. The malathion resistant strain of T. castaneum showed greater vigour than the susceptible in most oxidative stress situations and especially where stressors were combined. The induction of the antioxidant enzyme catalase could have helped the resistant strain to withstand oxidative stresses, including insecticidal and importantly those from pathogens. These adaptations, in the absence of insecticide, seem to support the increased immunity of host insects to pathogens seen in other insect species, such as mosquitoes. By increasing the responses to a range of stressors the resistant strain could be considered as having enhanced fitness.