Sublethal effects of bistrifluron on key biological traits, macronutrients contents and vitellogenin (SeVg) expression in Spodoptera exigua (Hübner).

The beet armyworm, Spodoptera exigua, is a highly polyphagous pest originated from Southeast Asia but has spread globally, attacking economically important crops and fruits. Bistrifluron insecticide is one of the highly active insect growth regulators that has been reported to inhibit development and longevity in other lepidopteran species and could be used in the control of S. exigua. In the present study, the age-stage, two-sex life table technique was applied to assess the sublethal effects of bistrifluron on biological traits and vitellogenin gene (SeVg) expression when 2nd instar larvae fed to sublethal concentrations (LC, LC and LC) of bistrifluron. Mean generation time from eggs to adults was longer at LC (37.79 ± 0.81 d) and LC (37.04 ± 0.72) compared to the LC (36.89 ± 0.63 d) and control groups (36.07 ± 0.38 d). Fecundity of female at LC (279.17 ± 42.8 eggs), LC (347 ± 35.4 eggs) and LC (411.58 ± 42.38 eggs) were significantly lower than the control treatment (532.47 ± 7.13). Furthermore, the lower intrinsic rates of increase (LC; r = 0.1207 ± 0.009, LC; r = 0.1329 ± 0.009 and LC; r = 0.14398 ± 0.009 compared to the control r = 0.164 ± 0.0076), was observed along with significantly extended mean generation times (LC; T = 34.825 ± 0.317 days, LC; T = 33.27 ± 0.368 days and LC; T = 31.899 ± 0.398 days compared to the control 30.927 ± 0.255 days). Furthermore, the contents of energy reserve macronutrients (carbohydrate, lipid and protein) significantly reduced in dose and time dependent manner in treated insects as compared to control. Furthermore, the expression level of SeVg mRNA significantly decreased by 43.8% in the female adults when one-day-old second instar larvae were treated with sublethal concentrations of bistrifluron in comparison with the control. Documenting these sublethal effects is a vital, and often overlooked factor, in assessing the overall efficacy of insecticides in the management of pest populations.