Abstract
The maize weevil Sitophilus zeamais is a primary pest of stored grains, and it is primarily controlled through the use of synthetic insecticides, which are directly connected to increases in the resistance of these insects. In this study, populations of S. zeamais acquired from small farmers in southern Piaui were obtained, and an experiment was performed with the insecticide K-Obiol EC25, with dose-response concentrations ranging from 3.5 ´ 10-4 to 0.05 g i.a/cm2. Five replicates containing 20 insects were used. For the amylase, lipase and total protease assays, 20 adult insects from each population were used separately for each treatment. The experiments performed with K-Obiol for the insect populations from different locations showed different LC50 values. The highest LC50 value was from Currais at 94.0 µg i.a/cm2, and the lowest value was from Bom Jesus at 14.0 µg i.a/cm2. The specific activities of the energy metabolism enzymes (lipase and amylase) responsible for sugar hydrolysis in four populations of S. zeamais exposed to the insecticide K-Obiol at different times (1, 6, 12 and 24 h) were determined. In all of the populations, the greatest amylase activity was observed in the group of insects exposed to the insecticides for 12 h. The population from Currais showed greater lipase activity in the treatment with 1 h of exposure to deltamethrin, whereas the population from Colonia showed the lowest lipase activity in the treatment with 24 h of exposure. Key words: Deltamethrin, enzyme activities, amylase, lipase, protease.
Highlights
The maize weevil, Sitophilus zeamais Motsch. (Coleoptera: Curculionidae), is one of the most important pests of stored grains in Brazil and is well-distributed globally
The population with the highest LC50 value (94.0 μg i.a/cm2) was from Currais, and the population with the lowest value was from Bom Jesus at 14.0 μg i.a/cm2
After 24 h of exposure to deltamethrin, the population from Currais showed higher amylase and lipase activity relative to the other populations, which may be directly related to resistance because in the dose response concentrations of this same experiment, this population had the highest LC50. These findings show that these insects increase their metabolism over time, thereby increasing their defence mechanisms and conferring resistance.Specific activities of digestive system enzymes were determined in the four populations of S. zeamais (Table 3)
Summary
The maize weevil, Sitophilus zeamais Motsch. (Coleoptera: Curculionidae), is one of the most important pests of stored grains in Brazil and is well-distributed globally. (Coleoptera: Curculionidae), is one of the most important pests of stored grains in Brazil and is well-distributed globally. The primary control of this insect depends directly on chemical insecticides. The intensive use of insecticides has resulted in the emergence of generalised resistance in insects of this species, which has caused serious problems related to their control (Suthisut et al, 2011; Ishii et al, 2010; Araújo et al, 2011). Metabolism or detoxification are the most likely mechanisms of resistance in insects to the most frequently studied insecticides. These mechanisms are performed through enzymes such as hydrolases, monooxygenases that are dependent on cytochrome P450, and transferases, which allow the insect to detoxify.
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