Author(s): Karem Ghoneim, Khalid Hamadah
The present study was conducted to assess the antifeedant activity of Nimbecidine (0.03% Azadirachtin) against 4th instar larvae of the destructive phytophagous pest Spodoptera littoralis and investigate its disruptive effects on different nutritional parameters in both 4th and 6th (last) instar larvae. Fresh clean castor bean leaf discs were treated with sublethal concentrations (500, 100 & 10 ppm) of Nimbecidine and offered to the early 4th instar larvae for 24 hrs. Nimbecidine exhibited a serious antifeedant activity against 4th instar larvae in a dose-dependent course. A significant reduction of food consumed by the 4th and 6th instar larvae was recorded in an inverse relation to the concentrations. Enhanced approximate digestibility (AD) was recorded for 4th instar larvae, but remarkably prohibited for last instar larvae. A general inhibitory effect was exhibited by Nimbecidine on ECI and ECD of both 4th and 6th instar larvae with an exceptional case. Assimilation rate of 4th instar larvae was significantly induced at the higher two concentrations but considerably or slightly suppressed in last instar larvae. Significantly or slightly increasing relative metabolic rate was recorded. The relative weight gain was reduced, regardless the instar. The growth rate of 4th instar larvae was reduced parallel to the increasing concentration while a generally enhanced rate was recorded for last instar larvae. Nimbecidine exerted a prohibiting action on the excretory function in the 4th and 6th instar larvae which discharged drastically reduced amounts of fecal pellets.
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