COMPARATIVE TOXICITY OF CERTAIN INSECTICIDES AGAINST LARVAE AND EGG STAGES OF COTTON LEAFWORM LABORATORY AND FIELD STRAINS

Resistance to major insecticide classes was diagnosed in the cotton leafworm (CLW) Spodoptera littoralis (Boisd.) (Lepidoptera: Noctuidae). This study was carried out to monitor the resistance levels in field strain collected from Abou- Homos, El-Behira government to some insecticides in comparison with laboratory strain. In addition, ovicidal activity of the formulated tested insecticides against laboratory and field strain of CLW egg masses was determined. The activities of glutathione S transferases (GST) and total esterases were determined in both strains. The results revealed that 2 nd instar larvae of the field strain exhibited different levels of resistance to cypermethrin, chlorpyrifos, and methoxfenozide with resistance ratios (RR) 12.7, 48.0, and 13.8, respectively, while 2 nd instar larvae of field strain showed tolerance ratios 4.6 and 5.0 to chlorantraniliprole and chlorfluazuron, respectively. Fourth instar larvae exhibited high resistance levels to cypermethrin (120.8) and moderate resistance to chlorpyrifos (19.0). On the other hand, 4 th instar larvae of field strain showed tolerance ratios to chlorantraniliprole methoxfenozide and chlorfluazuron (3.3). Concerning the ovicidal and residual toxicity, cypermethrin, chlorpyrifos, chlorantraniliprole and chlorfluazuron at the recommended field rate achieved 95.3, 98.9, 93.3 and 96.8% mortality of treated eggs and neonates of the laboratory strain, respectively. The same treatments achieved 75.6, 70.0, 85.2 and 91.6%, respectively, for the egg masses of field strain. On the other hand, there is no significant difference between the ovicidal and residual toxicity of methoxfenozide against neonates of both strains. The field strain exerted elevated GST and esterases activity compared to the laboratory one. Therefore, monitoring of insecticide resistance is the first step for the resistance management programs.