Effects of botanical insecticides and field margin vegetation on aphids and their natural enemies in dolichos bean (lablab purpureus l.) Productivity

Abstract

black bean aphid (Aphis fabae S.) is a major insect pest that causes significant yield losses in the production of legumes such as dolichos bean (Lablab purpureus L). Synthetic insecticides reduce insect pest damage while ignoring the declining biodiversity of natural enemies such as predators and parasitoids. A field and cage exclusion experiments were conducted in a randomized complete block design (RCBD) to determine the effectiveness of selected commercial botanical insecticides and field margin vegetation (FMV) in the management of bean aphids as well as the abundance of natural enemies in dolichos bean. The treatment comprised of three botanical insecticides: Pyerin75EC® (1% natural pyrethrin + 1% neem oil + 25% garlic extracts), Nimbecidine® (0.03% Azadirachtin + 90.57% neem oil) and Pyeneem 20EC® (1% natural pyrethrin + 1% neem oil) tested alongside synthetic insecticide Duduthrin 1.75EC® (1.75% lambda-cyhalothrin) and untreated as positive and negative controls, respectively. The effect of these treatments was evaluated in plots where the dolichos crop was either surrounded by FMV containing weed species black jack (Bidens Pilosa L.), Mexican marigold (Tagetes minuta L.), goat weed (Ageratum conyzoides L.) and gallant soldier (Galinsoga parviflora Cav.) or with no FMV. The botanical insecticides and positive control (Duduthrin) were applied at the second trifoliate and sixth trifoliate dolichos growth stages. Data collected on aphid abundance, damage severity, and percent incidence and natural enemy population as well as yield and yield components of dolichos were subjected to analysis of variance using Statistical Analysis System version 9.2. Results showed that botanical and synthetic insecticides reduced aphid abundance by 88% and 80%, respectively compared to the untreated control. In plots surrounded with FMV and treated with botanical insecticides, a high number of natural enemies (33.55%) were collected as compared to no FMV (24.88%). High grain yields of 2.55-3.04 and 2.95-3.23 tons/ha were observed for both botanical insecticides and synthetic insecticides in the presence of FMV as compared to no FMV 1.86-2.79 and 2.55-2.85 tons/ha, respectively. In the cage experiment, results revealed that integration of field margin vegetation and botanical insecticides had a higher recovery of ladybird beetles compared application of the botanical insecticides without the FMV. Botanical insecticides resulted in a higher number of ladybird beetles in presence of FMV (3.91-4.43) as compared to no FMV (3.16-3.67). The results demonstrated that commercial botanical insecticides in combination with field margin vegetation are viable alternatives to synthetic insecticides in the control of insect pest.