Influence of plant allelochemicals on growth rate, nutritional physiology and mid-gut esterase activity in fifth instar larvae ofSpodoptera litura(F.) (Lepidoptera: Noctuidae)

Summary The influence of seven plant allelochemicals on growth rate, nutritional physiology and mid-gut esterase activity was studied in fifth instar larvae of Spodoptera litura. Relative growth rates were highest in larvae fed on the control diet followed by diet treated with β-sitosterol (0.618± 0.012 mg d−1 mg−1) and cineole (0.618 ± 0.054 mg d−1 mg−1). Similarly, relative consumption rate was highest in larvae fed on diet treated with (-)-β-pinene (3.20 ± 0.10 mg d−1 mg−1) which was higher than the control. Gross growth efficiency was highest in larvae fed on diet treated with stigmasterol (27.26 ± 0.042%). Efficiencies of conversion of both ingested and digested food was highest in larvae fed on a stigmasterol-treated diet, in spite of approximate digestibility being the lowest (46.36 ± 4.37%). Mid-gut esterase activity in fifth instar larvae of S. litura was significantly elevated in the case of larvae fed on diet containing (-)-β-pinene, β-myrcene or D-limonene. Mid-gut esterase activity decreased in insects fed with diets containing the following compounds: (-)-β-pinene > β-myrcene > D-limonene > (-)-α-pinene > control > stigmasterol > β-sitosterol > cineole. These results suggest that plant allelochemicals exert their antiherbivore pressures by influencing growth and nutritional parameters. S. litura reacts by a simultaneous increase in detoxifying enzyme activity (mid-gut esterases, in this case), excessive defecation and dealkylation, possibly reflecting an adaptive plasticity with respect to the insect's fitness.