Insect tolerance to insecticides is closely related to the host plant. Migratory insects flying downwind and landing randomly may face host-plant switching after migration. However, it is not clear whether host-plant switching affects the tolerance of migratory insects to insecticides. In the present work, Cnaphalocrocis medinalis, a migratory pest destructive to rice, was studied to explore this issue. The results show that the host-plant switch reduced the susceptibility of rice and wheat larvae populations to abamectin but increased that of wheat larvae populations to chlorpyrifos, indicating that host switching resulted in different tolerance to different insecticides. Enzyme activity determination showed that, although abamectin and chlorpyrifos affect the activities of detoxification enzymes (carboxylesterase, multifunctional oxidase, and glutathione S-transferases), antioxidant enzymes (superoxide dismutase, peroxidase, and catalase), and acetylcholinesterase at sublethal dosages, the effect depends on the host plant and insecticide species. Overall, our findings show that the induction of C. medinalis by host switching affects its susceptibility and biochemical responses to abamectin and chlorpyrifos. Insecticides against C. medinalis should be used with consideration of the potential impact of host switching. Reasonable selection and usages of insecticides can help in the resistance management and control of migratory insects.

Different Cry toxins derived from Bacillus thuringiensis (Bt) possess different insecticidal spectra, whereas insects show variations in their susceptibilities to different Cry toxins. Degradation of Cry toxins by insect midgut extracts was involved in the action of toxins. In this study, we explored the processing patterns of different Cry toxins in Cnaphalocrocis medinalis (Lepidoptera: Crambidae) midgut extracts and evaluated the impact of Cry toxins degradation on their potency against C. medinalis to better understand the function of midgut extracts in the action of different Cry toxins. The results indicated that Cry1Ac, Cry1Aa, and Cry1C toxins could be degraded by C. medinalis midgut extracts, and degradation of Cry toxins by midgut extracts differed among time or concentration effects. Bioassays demonstrated that the toxicity of Cry1Ac, Cry1Aa, and Cry1C toxins decreased after digestion by midgut extracts of C. medinalis. Our findings in this study suggested that midgut extracts play an important role in the action of Cry toxins against C. medinalis, and the degradation of Cry toxins by C. medinalis midgut extracts could reduce their toxicities to C. medinalis. They will provide insights into the action of Cry toxins and the application of Cry toxins in C. medinalis management in paddy fields.

Pomacea canaliculata is one of the most destructive threats to the safe production of rice. Molluscicides are the primary measure for controlling this pest species. Tea saponin (TS) is a plant secondary metabolite with a high snail-killing activity. However, some individuals have enhanced detoxification metabolism against TS, leading to a weakened control efficiency of TS to P. canaliculata. To clarify the detoxification and metabolic mechanisms of TS in P. canaliculata, we characterized the toxicity of TS against the snails. Additionally, we examined its biochemical and molecular effects on glutathione S-transferases (GSTs) and monoamine oxidase (MAO) activities, as well as their corresponding gene expression levels. The results showed that the LC₅₀ values were 6.12, 9.03, and 9.35 mg/L for small, medium, and large snails following 24 h exposure. The LT₅₀ values were 16.85, 20.96, and 29.85 h for small, medium, and large snails at a concentration of 9 mg/L exposure, respectively. Furthermore, as the TS concentration increased or the treatment duration was prolonged, the activities of GSTs and MAO increased, along with the corresponding gene expression levels of PcGSTs1, PcGSTo1, PcmGST1 and PcMAO, compared to those of untreated snails. These results indicated that TS displayed dose- and time-dependent toxicity to P. canaliculata. Overall, the biochemical and molecular results suggested that GSTs and MAO were probably involved in the detoxification metabolism of P. canaliculata against TS. These findings preliminarily clarified the detoxification mechanism of P. canaliculata against TS.