Continuous trapping of aphids for 13 months in 1985–1986 in a citrus orchard at Kikambala, Kenya coast at an altitude of 15 m indicated that Toxoptera citricida was present in citrus throughout the year. There was a marked seasonal periodicity of flight of alate aphids as shown by two distinct peaks in population density constituting about 72% of the total catch of aphids for the 13 month period. Effect of climatic factors on population levels of aphids is illustrated. Rainfall was found to have an indirect effect on aphid population increase. The negative binomial distribution fitted very well the spatial distribution of the aphids in the field and provided an explanation of the patchy spread of tristeza disease in the citrus orchard at Kikambala. The possibility of reducing the rate of spread of citrus tristeza virus in the field is discussed.
