Acute toxicity and sublethal effects of the neonicotinoid imidacloprid on the fitness of Helicoverpa armigera (Lepidoptera: Noctuidae)

The fitness of Helicoverpa armigera (Hübner) was determined using demographic studies on the F₁ generation that survived after exposure to four sublethal doses (LC₅₀, LC₃₀, LC₂₀ and LC₁₀) of imidacloprid. The effects were assessed on the surviving individuals emerged from sixth-instar larvae that had ingested imidacloprid-treated chickpea pods. Age-specific parameters were found to be highest with the commencement of age and gradually decreased with the progression of age in both the treated and non-treated groups. Survivorship was reduced to 37 days after exposure to the sublethal dose of LC₅₀ when compared with 42 days in groups not exposed to the insecticide. The highest number (17%) of unhatched eggs was recorded in insects treated with the LC₅₀ dose of imidacloprid, whereas 98% of the eggs hatched in the unexposed group. The potential fecundity of the F₁ generation females was reduced to 330 eggs/female/generation when treated with the LC₅₀ dose compared with that of the unexposed females. The intrinsic rate of increase was found to be least in insects exposed to the LC₂₀ dose (0.0355 females/female/day) and highest (0.0470) in the unexposed group. It took 38.83 days for H. armigera to complete one generation in the unexposed population, while this was reduced to 33.94 days after exposure to the sublethal dose of LC₅₀. The longevity of adults decreased when the larvae were exposed to the sublethal dose of imidacloprid. The developmental time of pre-pupae and pupae significantly decreased and was least when exposed to the highest sublethal dose of the insecticide. Thus, at sublethal doses, imidacloprid caused a significant reduction in the survival and fecundity as well as increased mortality of H. armigera in the subsequent generation after exposure to the insecticide. In conclusion, imidacloprid can be successfully incorporated into integrated pest management of H. armigera.