Monitoring Insecticide Resistance Development in Beet Armyworm, Spodoptera exigua (Hübner) (Lepidoptera: Noctuidae)

Abstract

Field collected larvae of beet armyworm, Spodoptera exigua (Hübner) were reared and evaluated for resistance against cypermethrin 10 EC, neem extract (Azadirachtin 0.1%) and Bacillus thuringiensis Berliner var. kurstaki (53,000 SU per mg) for 12 generations under laboratory condition by using leaf dip bioassay. Cypermethrin showed LC₅₀ value of 251 ppm at F₁ generation and decreased gradually to onward generations resulting LC₅₀ value of 90 ppm at F₁₂ generation for non-selection, whereas LC₅₀ increased rapidly with a value of 8625 ppm at F₁₂ generation under selection pressure. Neem extract exhibited LC₅₀ value ranging from 3.98 ppm to 9.99 ppm for non-selection, whereas under selection pressure LC₅₀ ranged from 4.38 ppm to 13.04 ppm throughout the generations. Bacillus thuringiensis var. kurstaki showed LC₅₀ value which ranged from 7.3 × 10⁶ SU/L to 15.9 × 10⁶ SU/L for non-selection, and selection for resistance monitoring gave LC₅₀ value ranged from 7.6 × 10⁶ SU/L to 73.9 × 10⁶ SU/L . Compared with non-selection strain, selection strain exhibited 95.83-fold, 1.44-fold and 5.6-fold increase in the LC₅₀ for 12 generations against cypermethrin, neem extract and Bacillus thuringiensis var. kurstaki, respectively.