Seasonal dynamics of a population of the aphid Uroleucon rudbeckiae (Hemiptera: Aphididae): implications for population regulation

Abstract

AbstractMany aphid species (Hemiptera: Aphididae) that feed on herbaceous crops exhibit a rise and then sudden decline in abundance. Data from a nine-year study of Uroleucon rudbeckiae (Fitch) on Rudbeckia laciniata Linnaeus (Asteraceae) are used to investigate this pattern of seasonal abundance in a non-agricultural aphid. Aphids on a population of tagged and numbered flower stems were counted weekly. Abundance (mean aphids per stem) was partitioned into prevalence (proportion of stems colonised) and mean intensity (aphids per colonised stem), and also considered as the sum of the aphids in individual colonies. Abundance rose in mid-summer to late summer and then declined, peaking between the end of July and mid-September, earlier in years when the peak was higher. Prevalence showed a more uniform and consistent peak than mean intensity. Most of the 949 colonies were small and short-lived, but a small proportion were long-lived and reached 1000 aphids. Large colonies declined more slowly than moderately-sized colonies. Severe weather, shortening day-length, decline in host quality, density-dependent effects on rate of increase, and emigration failed to explain the population decline. An early rise and later decline in immigration, in conjunction with increasing predation through the summer, were consistent with the decline.