Effects of groundnut crop density on the population dynamics of Aphis craccivora Koch (Hemiptera, Aphididae) in Malawi

Three plant spacings, 30 cm, 7·5 cm and 2·5 cm within rows 90 cm apart, equivalent to about 36, 143 and 430 thousand plants/ha respectively, were used in three field trials on the population dynamics of Aphis craccivora Koch on groundnuts at Chitedze, Malawi in 1964–1967. Aphid density per infested plant was consistently higher, while numbers of aphid-infested plants per unit area were consistently lower, in 30 cm than in 2·5 cm spacing. Aphid numbers per unit area increased to similar levels at all spacings in crops under 40–50 days old, but subsequently aphid density was consistently lower in 2·5 cm than in 30 cm spacing. This effect could not be related to the intensity of fungus disease infection, parasitism, predation, the availability of feeding sites or the rate of emigration. Aphid groups were, however, consistently smaller in 2·5 cm than in 30 cm spacing in one trial. The potential rate of increase was studied in two trials using the methods of Hughes (1962), and was found to be consistently lower in 2·5 cm than in 30 cm spacing. The rate of plant growth was relatively depressed in 2·5 cm spacing, particularly after 40 days. The effect of spacing on aphid numbers was attributed to the effect of host-plant condition on the potential rate of increase.