Pattern and rate of within-field dispersal and bionomics of the cowpea aphid, Aphis craccivora (Aphididae), on selected cowpea cultivars

Abstract

Comparative field studies were conducted during the rainy and dry seasons at the Mbita Point Field Station of ICIPE in southwestern Kenya, to investigate the pattern and rate of dispersal of Aphis craccivora on aphid-resistant (ICV-12) and aphid-susceptible (ICV-1) cowpea cultivars in relation to key weather factors. The effects of the dispersal trends on crop performance and aphid population dynamics were analysed. Treatments consisted of initial aphid releases at the north, south, west, east and centre of test plots and uninfested controls maintained on plants for 22 days. Parameters recorded included: pattern (direction of spread of aphids) and rate (number of aphid-infested plants at a given time) in test plots; crop growth and yields and associated factors such as incidence of sooty mould and plant mortality, and aphid density and associated factors, including the incidence of natural enemy species, particularly coccinellids. Dispersal was fastest when releases were made in the west, north and centre of plots, and resulted in adveVse effects on ICV-1 growth and yields. Infestations of ICV-12 did not significantly affect crop performance. There was an apparent direct, positive relationship between wind direction and pattern of spread of aphid infestations, but this pattern was more apparent during the rainy season, when wind speeds were higher, than during the dry season. The incidence of natural enemy species correlated with the spread of aphid infestations, while the abundance of coccinellids correlated with aphid density.