THE ROLE OF FIELD MARGIN VEGETATION AND NATURAL ENEMIES IN THE MANAGEMENT OF BEAN APHIDS ON DOLICHOS BEAN (Lablab purpureus L.)

Abstract

Bean aphid (Aphis fabae) is a major pest of dolichos bean (Lablab purpureus L.) in Kenya. The over-reliance on synthetic insecticides to manage this insect pest poses serious risks to human health and the environment. Use of natural enemies in biological control provides a sustainable option of controlling aphids in dolichos. However, lack of diversity and abundance of natural enemies is a key constraint in effecting biological control. There is need for conservation of natural enemies to increase their diversity and abundance and enhance their ability to regulate pest populations below economic threshold levels. The objectives of this study were to determine the; i) effect of diverse field margin vegetation and cropping systems on bean aphids and their natural enemies in dolichos, ii) effectiveness of planted field margin vegetation species in supporting natural enemy populations and reducing bean aphid damage in dolichos and iii) diversity and abundance of floral resources that support natural enemies outside the main crop growing season. Field and cage exclusion experiments were conducted to evaluate the effect of field margin vegetation in conserving and enhancing the diversity and abundance of bean aphid natural enemies. The on-farm experiments were conducted in two agro- ecological zones: Njoro (LH3) and Rongai (LH4) while the on-station trials were carried at the Agronomy Field Station, Egerton University, during the 2019 and 2020 cropping seasons. Results from on station trials showed that maize-dolichos intercrop had 15.5% lower abundance of natural enemies than the lablab monocrop. Dolichos monocrop showed a higher bean aphid abundance 54.4, incidence 27.1% and severity of damage 9.1% compared to doichos-maize intercrop at 26.0, 15.6% and 5.9% for aphid abundance, incidence and severity of damage, respectively. On farmer’s fields dolichos monocrop, a higher grain yield was observed in 2019 (2.8 tha-1) compared to 2020 (2.0 tha-1). Similarly, for maize-dolichos intercrop a higher land equivalent ratio (LER) of 1.27 was observed in 2019 compared to 1.02 in 2020. In the cage experiment, a local annual weed, Bidens pilosa, was associated with the highest number of aphid mummies (8.3) whereas the lowest number of mummies were observed in cages with Tagetes minuta (3.4). Through molecular identification, Aphidius colemani was found to be the most abundant parasitoid responsible for bean aphid control. During the off-season, results showed a positive significant correlation of natural enemy abundance and plant diversity (P=0.002). These results demonstrate the potential of plant-rich field margins to conserve and support natural enemy populations for natural pest regulation of aphids.