Initial activity and persistence of insecticides for the control of bollworms (Lepidoptera: Noctuidae) in cotton crops

We evaluated six insecticides for their initial activity against the cotton bollworms Helicoverpa armigera (Hübner), Diparopsis watersi (Rotschild), and Earias spp., in sub-Saharan Africa (Cameroon). Residual activity and effect of simulated rainfall were also assessed in the case of H. armigera. Bioassays were conducted by transferring larvae on leaf discs collected from sprayed plots. Thiodicarb, endosulfan, and to a lesser extent emamectin-benzoate and indoxacarb had high initial activity against H. armigera, regardless of larval instars. Spinosad and cypermethrin–profenofos (CP) mix were effective at controlling larvae of first and second instars but not larvae of third to fifth instars. All tested insecticides effectively controlled Earias larvae (87–98% mortality). Regarding D. watersi, indoxacarb and endosulfan (77 and 82% mortality respectively) were less effective than spinosad and thiodicarb (95–99% mortality). Persistence was quantified by the duration after which an insecticide kills less than 50% of H. armigera neonates. Rain had a significant detrimental effect on insecticide persistence, except in the case of thiodicarb and emamectin-benzoate. In rainy conditions, thiodicarb (17.2 d) was the most persistent insecticide, followed by emamectin-benzoate and spinosad (10.6 and 8.9 d), and endosulfan, indoxacarb, and CP (2.7–5.2 d). Indoxacarb should be recommended for controlling sporadic outbreaks of H. armigera due to its high efficacy and low persistence, while CP should be used to control D. watersi infestations. More persistent insecticides such as spinosad and emamectin-benzoate should be recommended to control continuous and mixed-species populations of bollworms in the field.