Efficacy of bio and synthetic pesticides against the American bollworm and their natural enemies on cotton

Abstract

Cotton production in Uganda is limited by various factors among which insects pests are the most important. The key insect pests of cotton are the bollworm complex; spotted bollworm, Earias insulana; pink bollworm, Pectinophora gossypiella and American bollworm, Helicoverpa armigera. The American bollworm is an alien species and the most disastrous among all the three bollworm species attacking cotton in Uganda. Management for the bollworm complex in Uganda is largely synthetic chemical use with little or no biopesticide use which reduces natural enemies population and resistance development to continuous use of a single synthetic pesticide product. Therefore this study aimed at determining the efficacy of bio and synthetic pesticides on the management of cotton bollworm pests with special reference to the American bollworm. The pesticides in the study included Nimbecidine (Azaradactin), Engeo K247SC (Lambda Cyhalothrin 106 g l^-1 + Thiamethoxam 141 g l^-1), Talstar (Bifenthrin), and Amdoc3EC (Emamectin and Abamectin), Aster extreme 20SL (Acetamiprid 150g/l and Cypermethrin 50 g l^-1), Agrolambacin (15 g l^-1 Lambda Cyhalothrin + 300 g l-1 Profenofos), Thiodicarb (Larvin 80DF), and Methomyl (Lannate 40SP) on cotton bollworm (Helicoverpa amigera) and its natural enemies in eastern Uganda. The study was conducted at the National Semi-Arid Resources Research Institute (NaSARRI) under both laboratory and field conditions. Under laboratory controlled conditions, recommended doses of these bio and synthetic pesticides were used against third instar larvae of the American bollworm in a completely randomised experimental design with five replicates. Amdoc 3EC and Nimbecidine, showed maximum efficacy, 100 %, in terms of mortality after 72 hours. While Engeo, Aster extreme and Agrolambacin showed maximum mortality of (100%) after 96 hours followed by Larvin 80DF (80%) and Lannate 40SP (67%). In the field conditions, Nimbecidine (0.3 and 1.00) and Talstar (2.0 and 1.7) were the most effective bio-pesticides; while Amdoc with means (1.8 and 1.9), Aster extreme (2.0 and 1.8) and Agrolambacin (1.5 and 1.3) were the most effective synthetic pesticides respectively. Seed cotton yield was significantly higher in biopesticide treated plots, Talstar 1966 , 1935 kg ha^-1, Nimbicidine 1985 and 1715 kg ha^-1 compared to the synthetic pesticide treated plots with amdoc1935 and 1506 , Agrolambacin 1846 and 1722 kg ha^-1 , Aster extreme 1733 and 1930 kg ha^-1 respectively. Computation of the benefit cost ratios showed that these new chemical pesticides were all financially viable and, therefore, adoptable by the cotton farmers, respectively which also performed better than the controls 1012 and 912kg ha^-1 outside the experiment, 892 and 602 kg ha^-1 inside the experimental area. In addition, synthetic pesticides