Monitoring of an existing leucaena crop for the arrival of the psyllid began at Muguga in Kiambu District, Central Province of Kenya late in 1992. An infestation was noted by December the same year and a psyllid population monitoring programme began. This paper presents the findings of a 3-year study of the psyllid population monitored weekly throughout this period. The population was observed to build up gradually over the normal period of growth of leucaena foliage but then collapsed during the hot dry weather (February). This pattern of infestation has been repeated over the years but has been gradually shifting towards