Intrinsic Toxicity of Insecticides to Early and Late Instar Larvae of Castor Semilooper, Achaea janata Linn. (Lepidoptera: Noctuidae)

Investigations were carried on intrinsic toxicity of insecticides to early and late instar larvae of castor semilooper, Achaea janata Linn. (Lepidoptera: Noctuidae). Fenvalerate exerted highest toxicity at both 24h and 72h intervals of observation, followed by profenofos. It was 10 and 11.6 times more toxic at 24 h to early stage larvae and 9.9 and 6.53 times to late stage. Toxicity of profenofos to early instar was almost same as methyl parathion, but proved eight times more toxic to late instar. With increase in biomass of larvae from 65 mg (early instar) to 300 mg (late instar), LC₅₀ values also increased in all the cases. While the increase in pesticide quantity was six times more to IV^th instar in the case of fenvalerate and methyl parathion compared to early stage at 24th, it was doubled in the profenofos. The ratio between stages at 72h for the corresponding product were tapered. Sensitivity of IV^th instar to fenvalerate at 72h was reduced to maximum (3.5fold) followed by methyl parathion (1.96fold) and profenophos (1.85fold).

The data on the LC₅₀ values of 13 insecticides to IV^th larvae of A. janata reveal that alphamethrin had highest toxicity (LC₅₀ = 0.00129) at 24h exposure, followed by fenvalerate and methomyl. Cartap hydrochloride was least toxic (0.095). Among the tested compunds at 24h, phosalone, carbaryl, monocrotophos and quinalphos were also less toxic than methyl parathion. At 72h interval, alphamethrin, fenvalerate, methomyl, profenofos, endosulfan and mixture of cypermethrin and profenophos exhibited higher toxicity than recommended insecticide, methyl parathion in the descending order of toxicity. Whereas cartap hydrochloride was least toxic (0.2422%) to A. janata followed by carbaryl, phosalone, monocrotophos, quinolphos and acephate in the ascending order. As the exposure time increased from 24 to 72h, there in LC₅₀ values decreased. The reduction was least in profenofos (2.09 times) and highest (5.47 times) in endosulfan. In case of most toxic insecticides like alphamethrin and fenvalerte, the pesticide requirement to cause mortality in 50 per cent of the population was lowered by 2.11 and 3.29 times vis-a-vis 4.98 times in the recommended insecticide, methyl parathion.