Population dynamics of aphids

Abstract

9.1 INTRODUCTION Most aphid species reproduce both asexually and sexually, with several overlapping parthenogenetic generations between each period of sexual reproduction. This is known as cyclical parthenogenesis and, in temper-ate regions, sexual reproduction occurs in autumn and results in the pro-duction of over-wintering eggs, which hatch the following spring and initiate another cycle. For their size, aphids have very short developmen-tal times and potentially prodigious rates of increase and, as a conse-quence, are typically little affected by the activity of insect natural enemies (Kindlmann and Dixon, 1989; Dixon, 1992; Dixon et ai., 1995, 1997). That is, aphids show very complex and rapidly changing within-year dynamics, with each clone going through several overlapping gen-erations during the vegetative season, and potentially being made up of many individuals which can be widely scattered spatially. The produc-tion and survival rates of the eggs determine the numbers of aphids pre-sent the following spring. The study of the population dynamics of aphids living on wild herba-ceous plants is difficult because their host plants vary in abundance and distribution from year to year. Tree-living aphids, in addition to being very host specific, live in a habitat that is relatively stable both spatially and temporally. Therefore, it is not surprising that most long-term popu-lation studies on aphids have been on tree-dwelling species. The studies on deciduous tree-dwelling species