Effects of Entomopathogenic Fungi and Border Cropping on Bean Aphid Population, Growth, Yield and Quality of French bean

Abstract

French bean (Phaseolus vulgaris L.) is an important crop in Kenya that accounts for 33% of total vegetable export and immensely contributes to the Gross Domestic Product (GDP). Despite the economic value of French bean, growers still experience low yield and quality due to several biotic constraints, including the black bean aphid (Aphis fabae). Efforts to manage aphids have resorted to frequent use of synthetic pesticides, which have negative impact on human health and the environment. Integrating biopesticides use with cropping systems is a promising alternative. This study objectives were; to determine the bioactivity of the entomopathogenic fungus Metarhizium anisopliae against black bean aphid and its interactive effects with border crops on aphid population and damage, growth, yield and quality of French bean. To determine the bioactivity of M. anisopliae against black bean aphid, a laboratory experiment arranged in a Completely Randomized Design (CRD) with five replications was conducted. Different concentrations of Mazao supreme and Metarril wettable powder (WP) formulations were tested against a commercial biopesticide (Biomagic), a synthetic pesticide (alpha-cypermethrin) and water as control. Probit analysis was done to determine the median lethal time (LT50) and median lethal concentration (LC50). To determine the effect of integrating M. anisopliae with border crops on black bean aphid population and damage, growth, yield and quality of French bean, a field experiment was conducted using a 4×3 factorial in Randomized Complete Block Design (RCBD) with three replications. Factor one comprised of four treatments: Metarril (2×108 spores/g) with Biomagic (2×108 spores/ml) and alpha-cypermethrin as positive control and water as negative control. The second factor had three treatments, two border crops (Wheat and sunflower) planted and control (no border crop). Data on black bean aphid population, damage severity yield and quality of French bean were recorded. Data was analyzed using SAS version 9.4 at 5% level. The results on bioactivity showed a time and dose response on the aphid mortality. Metarril (84.2%), Mazao (80.7%) and Biomagic with lowest aphid mortality of 68.4%. In the field experiment, a significant reduction was observed in aphid population (P < 0.0005) and damage severity (P < 0.001) on the plots with border crop. Metarril WP E9 and wheat border crop reduced aphid population, damage and enhanced growth (collar diameter, number of pods), yield (pod yield) and quality of French bean. plots. Border cropping caused an increase in marketable yield (2.7% to 3.1%) compared to control for both seasons, respectively while a sunflower border caused a 4% and 5.4% decrease in the marketable yield compared to no border. This study demonstrates the integrated use of M. ansiopliae and wheat border crop as a viable alternative to synthetic insecticides in the control of black bean aphids for improved yield and quality of French bean.