Interaction of substerilizing Gamma radiation and thiodicarb treatment for management of the Tobacco caterpillarSpodoptera litura

The F₁ sterility technique (inherited sterility using substerile insects) has been proposed for reproductive suppression of lepidopteran pests. Management of lepidopteran pests using F₁ sterility may be greatly influenced by other possible strategies, including chemical control measures. Within this context, a laboratory investigation was undertaken of the tobacco caterpillarSpodoptera litura (Fabricius) (Lepidoptera: Noctuidae) to assess the bioefficacy of a substerilizing dose (70 Gy) of gamma radiation on bio-characteristics(viz., adult survival, fecundity, egg hatch, F₁ larval weight, F₁ pupation, F₁ sex ratio); to establish the median lethal dose (LD₅₀) of an oxime carbamate insecticide, thiodicarb (Larvin®75 WP), toS. litura; and determine the interaction of thiodicarb with the F₁ insects derived from a substerilizing gamma dose. A dose of 70 Gy administered to 7-day-old male pupae reduced the the eclosion by ~ 10%. When the eclosed males (derived from treated pupae) were mated with normal females, the fecundity and fertility were decreased as compared with control (0 Gy). Analysis of body weight of the F₁ larvae showed no significant influence of radiation,i.e., growth was comparable to that of normal progeny, which is a positive factor in the sterile-insect technique. Bioassay on newly molted 6th instar larvae with thiodicarb (Larvin®75 WP) showed that the LD₅₀ of the insecticide was 6.76µg g^-1 in the combined treatment (radiation to P₁ male pupae + thiodicarb to F₁ larvae), as compared with 28.67 µg g_-1 in the chemical treatment (thiodicarb), indicating that the toxicity of the insecticide to the F₁ progeny was increased 4.24 times by the radiation treatment, which is a markedly pronounced effect. Further studies of the combined effect of radiation and insecticide on pupation and sex ratio of F₁ progeny showed that pupation was affected more by the combined treatment than by the insecticide alone, whereas the sex ratio was skewed in favor of males in both the combined treatment and the chemical treatment, indicating that thiodicarb was selectively more toxic to females. The possible interaction and potential and the feasible approaches of integrating F₁ sterility and thiodicarb treatment are discussed for management of the lepidopteran pestS. litura.