Abstract
Conopomorpha sinensis Bradley is the dominant borer pest of Litchi chinesis and Euphoria longan. Current management of C. sinensis relies upon insecticide application to adult moths. In addition to the direct mortality induced by insecticides, a sublethal dose of insecticides also affects growth, survival, and reproduction in the exposed insects. Vitellogenin (Vg) and vitellogenin receptor (VgR) are normally identified as essential reproduction-related proteins in insects. In this study, we characterized these two genes from C. sinensis, and investigated their differential responses to sublethal concentrations of insecticide. Cloned CsVg and CsVgR consist of 5391 and 5424-bp open reading frames, which encode proteins of 1796 and 1807 amino acid residues, respectively. The CsVg protein contains the typical vitellogenin, DUF1943 and VWFD domains as other reported lepidopteran Vgs. The CsVgR was characterized as a typical low density lipoprotein receptor with two highly conserved LBD and EGF precursor domains, one hydrophobic transmembrane domain, one cytoplasmic domain, and 13 putative N-glycosylation sites. We next assessed the sublethal effect of four major insecticides on egg-laying in C. sinensis. The toxicity against C. sinensis varied among the insecticides tested, with LC50 values ranging from 0.23 ppm for chlorpyrifos to 20.00 ppm for β-cypermethrin, among which emamectin benzoate (EB) showed a significant negative impact on egg-laying, survival rate, ovarian development, and mating rate of C. sinensis at LC30 doses. Further investigation showed that the transcriptional level of CsVg and CsVgR were impaired in different way at 24, 48, and 72 h after EB exposure, and this result was in agreement with the diminished egg-laying of C. sinensis in the sublethal concentration EB-treated group. A repressed transcription level of CsVgR was observed at 48 h after treatment, suggesting that EB elicits a delayed response in the abundance of CsVgR. These results established different roles of CsVg and CsVgR in response to the sublethal effect of insecticides. CsVg might be a better parameter than CsVgR for assessing the effect of sublethal insecticides on reproduction in C. sinensis.
Highlights
Conopomorpha sinensis Bradley (Lepidoptera: Gracilariidae) is the most destructive borer pest of Litchi chinesis and Euphoria longan, and causes severe economic loss in litchi and longan cultivated areas, including India, Nepal, Thailand, Vietnam, and China (Menzel, 2002; Schulte et al, 2007)
Probit analyses of concentration-mortality data showed that after 24 h of exposure to chlorpyrifos, emamectin benzoate (EB), triazophos and β-cypermethrin, the LC50 values were estimated to be 0.23, 1.88, 2.11, and 20.00 ppm, respectively (Table 1). These results revealed that chlorpyrifos had the highest toxicity to C. sinensis, followed by EB and triazophos, whose LC50 values were more than 10 times higher than that of β-cypermethrin
20 Vg and 9 vitellogenin receptor (VgR) sequences from lepidopteran are available in the GenBank database, but no information has been reported for Vg and its receptor from gracillariiae insects
Summary
Conopomorpha sinensis Bradley (Lepidoptera: Gracilariidae) is the most destructive borer pest of Litchi chinesis and Euphoria longan, and causes severe economic loss in litchi and longan cultivated areas, including India, Nepal, Thailand, Vietnam, and China (Menzel, 2002; Schulte et al, 2007). Frequent application of insecticides to adult C. sinensis is the most effective strategy for this borer pest, since the egg laying amount can be effectively reduced by decreasing the density of adult C. sinensis in orchards. In addition to the direct mortality due to acute toxicity (lethal effect) after initial insecticide application, insects are exposed to low-lethal or sublethal doses of insecticides for a long period in fields (Biondi et al, 2013). Sublethal doses of insecticides may affect various the physiological, biochemical, and behavioral traits of exposed insects (He et al, 2013; AbduAllah and Pittendrigh, 2018). The sublethal effects of insecticides on beneficial arthropods have received considerable attention, while their impacts on pest insects are poorly studied (Desneux et al, 2007; Liu et al, 2016). Identifying the sublethal effects of insecticides on pest insects could help us better understand the overall insecticide efficacy in controlling the insect population, thereby optimizing the insecticide usage and in turn delaying resistance in pest insects
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