Evaluation of the toxicity of various insecticides to coconut rhinoceros beetle (Oryctes rhinoceros L.) larvae under laboratory conditions

Abstract:

Coconut rhinoceros beetle, Oryctes rhinoceros L., is a major pest of coconut, with its larval stages dwelling in rotten or decomposed organic matter. Insecticides, a part of integrated pest management, act as a curative measure. However, chemicals recommended for managing the rhinoceros beetle grubs have been phased out. Hence this study evaluates comparative toxicity of four different insecticides at two different doses viz., Fipronil 0.3 % GR 0.0375 g kg^-1, 0.075 g kg^-1, Cartap hydrochloride 4 % G 0.05 g kg^-1, 0.1 g kg^-1, Chlorantraniliprole 0.4 % GR 0.025 g kg^-1, 0.05 g kg^-1 and Thiamethoxam 25 % WG 0.0002 g kg^-1, 0.0005 g kg^-1 against the third instar larval stages in laboratory. The experiment was a Completely Randomized Design (CRD) with 9 treatments and 3 replications. The highest mortality was observed with Cartap hydrochloride 4 % G 0.1 g kg^-1 within 15 days after treatment (DAT), followed by chlorantraniliprole 0.4 % GR 0.05 g kg^-1 and Cartap hydrochloride 4 % G 0.05 g kg^-1. The above three treatments showed 100% mortality at 20 DAT. The time required for 50 % mortality (LT50) was lowest with Cartap hydrochloride 4 % G 0.1 g kg^-1 (7 days), followed by chlorantraniliprole 0.4 % GR 0.05 g kg^-1 and Cartap hydrochloride 4 % G 0.05 g kg^-1, as determined by Kaplan-Meier survival analysis, compared to the total microbial count in untreated cow dung, bacterial and actinomycete colony count decreased in all treatments, while fungal count increased in treatments such as fipronil 0.3 % GR 0.0375 g kg^-1, Cartap hydrochloride 4 % G 0.05 g kg^-1, 0.1 g kg^-1, and thiamethoxam 25 % WG 0.0002 g kg^-1, 0.0005 g kg^-1.

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