Differential Effects of Some Insect Growth Regulators on the Reproductive Potential of Lepidopteran Pest, Spodoptera littoralis
Ahmed M. El-Sabrout, Samah M. Hassan and Sahar E. Eldesouky
Laboratory experiments were conducted to evaluate the lethal and sublethal effects of novaluron, methoxyfenozide and chromafenozide on the fecundity and fertility of Spodoptera littoralis adults when applied on the newly molted 6th instar larvae. The effect of least concentration (LC10) of tested IGRs on testes weight (mg), eupyrene and apyrene sperm numbers of treated male (48h-old) were also carried out. Different abnormalities of adults, deformed ovaries and testes of the treated S. littoralis females and males were clarified. There was no significant difference between the toxicity of the three IGR compounds with LC50 values 9.6, 7.6 and 11.8 mg L-1, respectively, after 96 hrs post-treatment. Methoxyfenozide at LC50 value caused highly significant decrease in the average number of eggs laid per female (fecundity) and the percentages of hatched eggs (fertility) followed by novaluron and chromafenozide compared with LC25 and LC10 values. The hatchability percentages were 2.6, 7.8 and 15.2 % after the mating of treated female (TF) × treated male (TM), treated female (TF) × untreated male (UTM) and untreated female (UTF) × treated male (TM), respectively, for the treatment with LC50 value of methoxyfenozide. While, the hatchability percentages were 9.0, 17.4 and 26.8 % for LC50 value of novaluron and 45.8, 50.5 and 58.3 % for LC50 value of chromafenozide after the same previous mating possibilities, respectively, compared to 97.3 % in the control. The LC10 values of novaluron, methoxyfenozide and chromafenozide appeared to be dramatic when reducing the average number of eupyrene sperm to be 13074 (61.2%), 9335.67 (72.3%) and 19017.08 (43.55%), respectively, when compared to 33691.81 eupyrene sperm in the control. Furthermore, the LC10 of three tested IGRs decreased the average number of apyrene sperm by 1089257 (13.7%), 1005230.80 (20.37%), 1154880.35 (8.5%), respectively, when compared to 1262480.7 apyrene sperm in the control. Finally, the obtained results emphasized that novaluron, methoxyfenozide and chromafenozide are promising insecticides and suitable for IPM programs directed against lepidopteran pests.