Field margins and cropping system influence diversity and abundance of aphid natural enemies in Lablab purpureus

Field margin plants around crops are postulated to increase natural enemy abundance to enhance pest management. A trial was conducted to determine the contribution of field margin vegetation and cropping systems to natural enemies of bean aphids (Aphis fabae) on Lablab purpureus, an orphan crop legume. Natural enemy populations were surveyed in plots with a combination of lablab monocrop or maize-lablab intercrop and with or without planted field margins comprising four plant species (Chenopodium album, Bidens pilosa, Galinsoga parviflora and Tagetes minuta), arranged in a randomized complete block design with four replicates. A cage experiment was also conducted to understand the extent to which these field margin plants supported the parasitoid wasp, Aphidius colemani, to parasitize bean aphids. A total of 2029 insects from 10 families were collected using sticky cards and pan traps. In comparison to plots with no field margin plants, the presence of plant-rich field margins increased abundance of natural enemies by 9.5% and supported higher populations of Braconidae (parasitoids) and damsel bugs (predators). The maize-lablab intercrop had 15.5% lower abundance of natural enemies than the lablab monocrop. Higher grain yield was recorded in plots with a field margin vegetation (300 kg ha(-1)) compared to plots without field margin vegetation (210 kg ha(-1)). The presence of a companion plant did not significantly affect the performance of parasitoids, in the cage experiment, indicating that parasitoids were not limited by access to food. In supporting aphid parasitization, B. pilosa was associated with the highest number of mummies (8.28). The lowest number of mummies were observed in cages with T. minuta (3.44). These results demonstrate the potential of plant-rich field margins to augment natural enemy populations and enhance conservation biocontrol of aphids.