Diversity and convergence of mechanisms involved in pyrethroid resistance in the stored grain weevils, Sitophilus spp.
Published: 11-05-2018 In Publication
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.
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. Regarding the pyrethroid insecticides, the resistance management is complicated because resistance occurs in a variety of forms, including reduced insecticide penetration, metabolic resistance (through detoxification enzymes), behavioral resistance and target-site alterations1,2. Although the pyrethroid insecticides exert their toxicity primarily by disrupting the function of the voltage-gated sodium channels in excitable cells3,4,5,6,7,8, these compounds also have secondary action targets (e.g., ionic imbalance and osmoregulatory dysfunction) that contribute to their activity9,10,11.
Multiple and distinct pyrethroid resistance mechanisms have been investigated in toxicological studies with focus on the contribution of the major mechanism, which includes target-site mutations (known as knockdown “kdr” resistance) and/or metabolic-based resistance12,13,14,15,16,17. The co-occurrence of distinct and multiple pyrethroid resistance mechanisms threatens resistance management strategies, with the threat particularly acute when alternative compounds are scarce, as is the case with stored grain weevils. Thus, it is essential to evaluate the potential of other classes of insecticides such as neonicotinoids, oxidiazines and spinosyns to control resistance populations of stored grain weevils.
Most of the losses in stored grains are caused by insect pests among which the grain weevils of the genus Sitophilus (e.g., the maize weevil Sitophilus zeamais Motsch. and the rice weevil Sitophilus oryzae L.) are particularly destructive18,19. The maize weevils, Sitophilus zeamais Motsch., and the rice weevil, Sitophilus oryzae L., are cosmopolitan and a major concern in tropical and subtropical regions, conditions that also occur in the Neotropical region20,21. Despite the economic importance of insecticide resistance in stored grain insect pests in general and grain weevils in particular22,23, studies on insecticide resistance are relatively limited for grain weevil species and do not usually explore the underlying molecular basis of the phenomenon24,25,26.
The mechanisms of pyrethroid resistance in the maize weevil S. zeamais as well as the fitness cost associated with it have been investigated20,25,27,28 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 pyrethroids27, with secondary involvement of increased detoxification by glutathione-S-transferases28. However, this single mutation alone does not explain the high levels of resistance observed in maize weevil strains, and therefore, additional effort is required to understand the molecular basis of the resistance mechanisms involved in this species. The rice weevil is the subject of even greater neglect but also deserves attention because of the importance as a pest species and the relatively close phylogenetic relationship with the maize weevil29,30,31. 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 aphids32 and Sitophilus spp.33,34 but still need confirmation.
Thus, the present study was conducted to assess the physiological (e.g., occurrence of mutations in the sodium channel gene and activities of metabolic enzymes) and behavioral mechanisms (e.g., changes in walking patterns) of pyrethroid resistance in the maize and rice weevils (S. zeamais and S. oryzae, respectively). A series of toxicity, enzymatic, molecular and behavioral bioassays were conducted with a diverse and representative set of populations from both weevil species to achieve this objective. Our findings clearly demonstrated diversity and convergence of mechanisms involved in the pyrethroid resistance among strains of both species of grain weevils.
The probit model satisfactorily described the concentration-mortality data (goodness-of-fit tests exhibited low χ2-values [<9.5] and high P-values [>0.05]). The resistance ratios were estimated relative to the LD50 for the most susceptible strain for each insecticide (Tables 1 and 2). Based on the LD50 values obtained for the 14 maize weevil strains, the pyrethroid lambda-cyhalothrin and the neonicotinoid thiamethoxam were the most potent (i.e. lowest LD50 values) insecticides followed by the neonicotinoid imidacloprid and the spynosin spinosad (Table 1). Furthermore, the most susceptible maize weevil strain varied with insecticide. Individuals from E. S. Pinhal-SP were the most susceptible to both neonicotinoid insecticides (i.e., thiamethoxam and imidacloprid); while individuals from Teresina-PI (for the pyrethroid lambda-cyhalothrin) and Cristalina-GO (for the spynosin spinosad) were the most susceptible to other insecticides (Table 1). Regarding the pyrethroid insecticide lambda-cyhalothrin, and based on the 95% confidence intervals for resistance ratios (RR), five strains (total of 14) exhibited moderate to high resistance (i.e., RR > 5.0; Table 1). No resistance was found for spinosad, with the resistance ratios (RR) all below 2.8. Regarding the neonicotinoid insecticides, only three populations (Amambai-MS, Piracicaba-SP and Sao João-PR) exhibited (low) resistance levels to imidacloprid with resistance ratios (RR) between 2.7 and 3.6, while six populations (Amambai-MS, Balsas-MA, Ipojuca-PE, Jacarezinho-PR, Juiz de Fora-MG and Piracicaba-SP) exhibited low levels of thiamethoxam resistance (RR between 2.5 and 3.8). Generally, resistance to both neonicotinoids was either absent or very low among the strains tested.
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