Sublethal effects of insecticides on diamondback moth, Plutella xylostella (L.)

Abstract

The insecticides permethrin, fenvalerate, methamidaphos and carbaryl were tested against diamondback moth, Plutella Xylostella (L.) under the laboratory conditions by caging the larvae on broccoli leaf discs sprayed under Potter's tower and by spraying the adults directly. The LC₅₀'s were determined at 48h. The toxicity of permethrin and fenvalerate to third instar larvae was approximately 10 and 160 times more than methamidaphos and carbaryl respectively. The adult susceptibility to permethrin and fenvalerate was two and 260 times greater than methamidaphos and carbaryl. The effect of post-treatment temperature of 15 and 25⁰C on the toxicity of the four insecticides to larvae was determined. A marked increase in toxicity of pyrethroids and decrease in toxicity of carbaryl was observed at 15⁰C. The effect of sublethal concentrations (LC₁ and LC₅₀) on diamondback moth development, female fecundity and insect behaviour was investigated. All the insecticides had deleterious effects on the number of larvae surviving to pupae duration of pupal period, number of pupae surviving to adulthood and cocoon formation in the pupae. Prolonged duration of larval-period from third instar to pupation and deformed pupae were also observed. Permethrin, fenvalerate and carbaryl caused latent toxicity and deformed wings of the adults. Pyrethroids and methamidaphos reduced the longevity of the adults where as carbaryl increased it. Except for permethrin all the other insecticides inhibited the female fecundity. At LC₁ and LC₅₀ concentrations pyrethroids showed repellent and antifeedant activity against larvae. The female moths did not prefer to oviposit onto leaf discs treated with LC₅₀ of pyrethroids. A field trial was laid down to study the period of crop protection given by single application of permethrin and fevalerate (50 and 150 g AI/ha), methamidaphos (900g AI/ha) and carbaryl (2000g AI/ha). The pyrethroids at a rate 50 and 150g AI/ha effectively controlled P. Xylostella population for 26 and 33 days respectively. The larval density in carbaryl and methamidaphos treated plots were significantly lower than the untreated control for 19 and 33 days respectively.