Abstract
Esterases (ESTs) play important roles in metabolizing various physiologically endogenous and exogenous compounds, and various environmental xenobiotics in insects. The psocid, Liposcelis bostrychophila is a major pest of stored products worldwide and rapidly develops resistance to commonly insecticides. However, the involvement of ESTs in insecticide metabolization and the application of RNAi approach in psocids have not been well elucidated. In this study, we characterized four LbEST genes and investigated the transcriptional levels of these genes at different developmental stages and under different insecticides exposures to assess their potential roles in response to insecticides. The four LbESTs contain a catalytic triad (Ser-His-Glu) linked to an oxyanion hole and acyl pocket involved in substrate stabilization during its hydrolysis. Synergism observed with the esterase-inhibitor DEF suggests the involvement of esterases in malathion detoxification. LbESTs were expressed during the whole of developmental stages, but predominant abundance in the first nymphal instar and adult stage. The mRNA level of three LbEST genes (except for LbEST4) was induced (1.29- to 5.60 fold) in response to malathion or deltamethrin exposures, indicating that these esterases are involved in the detoxification process. Silencing of LbEST1, LbEST2 or LbEST3 through dsRNA feeding led to a higher mortality of psocids upon the malathion treatment compared to controls (1.83 to 2.69-fold), demonstrating that these esterase genes play roles in malathion detoxification in L. bostrychophila. Our study provides new evidence for understanding of the function and regulation mechanism of esterases in L. bostrychophila in insecticide detoxification. The current study also suggests that the present RNAi method could be applied for gene functional studies in psocids.
Highlights
The psocids from Liposcelis genus (Psocoptera: Liposcelididae), are stored-product pests worldwide that pose new risks for global food safety and security (Nayak et al, 2014)
Addition of triphenyl phosphate (TPP) and diethyl maleate (DEM) marginally increased the toxicity of malathion against L. bostrychophila with a synergism ratio of 1.66- and 1.56-fold, respectively, whereas DEF showed higher levels of synergism, 5.90-fold
Only addition with TPP enhanced its lethal effect on L. bostrychophila; LC50 values were reduced from 4.51 mg/m2 to 2.57 mg/m2
Summary
The psocids from Liposcelis genus (Psocoptera: Liposcelididae), are stored-product pests worldwide that pose new risks for global food safety and security (Nayak et al, 2014). Intensive and unreasonable use of insecticides has caused high levels of resistance in psocids, which has created a serious obstacle for the chemical control of these pests (Ding et al, 2004; Nayak et al, 2014). Variable degrees of tolerance of L. bostrychophila to pyrethroids (permethrin, cypermethrin and deltamethrin), organophosphates (fenitrothion, malathion, and pirimiphos-methyl) and insect growth regulators (methoprene and fenoxycarb) were detected (Leong and Ho, 1994; Nayak, 2010). Different Liposcelis species usually exhibit various degrees of tolerance to chemical treatments, and this has made it difficult to control in the field, since psocid infestations often involve more than one species. The above studies demonstrate that L. bostrychophia possess naturally higher tolerance to contact insecticides compared with other insect pests of stored products. The molecular mechanisms underlying tolerance or resistance to insecticides in L. bostrychophia have not been well characterized
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