Biodegradation of insecticides by gut bacteria isolated from stored grain beetles and its implication in host insecticide resistance

Stored grain pests have developed widespread resistance to currently available grain protectants. Although symbiotic microbes played an important role in insecticide resistance of insect hosts, it has not been reported in stored grain pests. In this study, five gut bacteria were excised from adult Sitophilus oryzae (L.) (Coleoptera: Curculionidae), Cryptolestes ferrugineus (Stephens) (Coleoptera: Laemophloeidae), and Rhyzopertha dominica (F.) (Coleoptera: Bostrichidae). These bacteria can degrade malathion, pirimiphos-methyl, and deltamethrin and utilize these insecticides as the carbon source in vitro. When treated with 0.5–10 mg/kg of malathion, pirimiphos-methyl and 0.3–0.75 mg/kg of deltamethrin, gnotobiotic reinoculation of a given bacterial strain significantly enhanced survival rates of its hosts. The significance of gut bacteria in host insecticide tolerance was insecticide dosage-dependent. Furthermore, the in vitro insecticide biodegradability of gut bacteria was not completely consistent with their in vivo performance in host insecticide resistance. This suggests that there may be other physiological processes responsible for host insecticide tolerance mediated by symbiotic bacteria except for direct degradation of insecticides. These results provide insight into insecticide resistance management of stored grain pests and further study of the association between gut bacteria and insecticide resistance.