Abstract
Target-site mutations and changes in insect metabolism or behavior are common mechanisms in insecticide-resistant insects. The co-occurrence of such mechanisms in a pest strain is a prominent threat to their management, particularly when alternative compounds are scarce. Pyrethroid resistance among stored grain weevils (i.e., Sitophilus spp.) is an example of a long-standing concern, for which reports of resistance generally focus on a single mechanism in a single species. Here, we investigated pyrethroid resistance in maize and rice weevils (i.e., Sitophilus zeamais and S. oryzae), exploring potential knockdown resistance (kdr) mutations in their sodium channels (primary site for pyrethroid actions) and potential changes in their detoxification and walking processes. Resistance in pyrethroid-resistant rice weevils was associated with the combination of a kdr mutation (L1014F) and increases in walking and detoxification activities, while another kdr mutation (T929I) combined with increases in walking activity were the primary pyrethroid resistance mechanisms in maize weevils. Our results suggest that the selection of pyrethroid-resistant individuals in these weevil species may result from multiple and differential mechanisms because the L1014F mutation was only detected in Latin American rice weevils (e.g., Brazil, Argentina and Uruguay), not in Australian and Turkish rice weevils or Brazilian maize weevils.
Highlights
The overuse of dichlorodiphenyltrichloroethane (i.e., DDT) up to the 1980’s and more recently of other synthetic insecticides for controlling stored product insect pests has contributed to the selection of insecticide-resistant strains, leading to severe economic losses in storage facilities worldwide
Based on the LD50 values obtained for the 14 maize weevil strains, the pyrethroid lambda-cyhalothrin and the neonicotinoid thiamethoxam were the most potent insecticides followed by the neonicotinoid imidacloprid and the spynosin spinosad (Table 1)
The frequent and indiscriminate use of synthetic insecticides for the control of stored product insect pests contributes to the selection of stored grain weevils (i.e., Sitophilus spp.) exhibiting high levels of pyrethroid resistance
Summary
The overuse of dichlorodiphenyltrichloroethane (i.e., DDT) up to the 1980’s and more recently of other synthetic insecticides (e.g., pyrethroids) for controlling stored product insect pests has contributed to the selection of insecticide-resistant strains, leading to severe economic losses in storage facilities worldwide. The mechanisms of pyrethroid resistance in the maize weevil S. zeamais as well as the fitness cost associated with it have been investigated[20,25,27,28] but not those of the rice weevil These studies with the maize weevil suggest that the primary resistance mechanism involves a single mutation in sodium channels (i.e., the kdr mutation T929I) that reduces the susceptibility to pyrethroids[27], with secondary involvement of increased detoxification by glutathione-S-transferases[28]. This single mutation alone does not explain the high levels of resistance observed in maize weevil strains, and additional effort is required to understand the molecular basis of the resistance mechanisms involved in this species. Besides the resistance to insecticides resulting from the target site and metabolic alterations, other mechanisms associated with behavioral modification such as change in locomotory parameters have been reported in aphids[32] and Sitophilus spp.[33,34] but still need confirmation
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