Comparative Population Dynamics of Two Goldenrod Aphids: Spatial Patterns and Temporal Constancy

Two aphid species, Uroleucon nigrotuberculatum and U. caligatum, that feed on the goldenrod Solidago altissima, are similar in life histories and feeding habits, but differ markedly in their spatial distribution and temporal variability. In this study, U. caligatum typically occurred in small colonies scattered throughout goldenrod fields, and its numbers remained relatively constant through time, whereas U. nigrotuberculatum occurred in dense colonies and exhibited greater fluctuations in population size. The aggregated spatial pattern of U. nigrotuberculatum resulted from both active aggregation of alates and lack of dispersal by apterae and nymphs. Field experiments in which colony size was held constant revealed that U. nigrotuberculatum enjoyed no clear reproductive advantage that might explain its greater tendency to outbreak. In addition, U. nigrotubercultatum was more susceptible to generalist predators (such as cantharid beetles, mirid bugs, and mites) than U. Caligatum. Aphid dispersion was then manipulated in the field to determine how the different spatial patterns of the two species influenced reproduction and mortality, and hence the relative tendencies of their populations to irrupt. Feeding in large colonies provided no reproductive advantage to either species. Aggregation did, however, enhance aphid survivorship when generalist predators were the main mortality agents. The advantage was reversed later in the season when the aphids' fungal pathogen was present; aggregation then led to a decrease in survivorship. Thus, aggregation allowed a numerical escape from generalist predators but also promoted fungal epidemics, so that the temporal variability in U. nigrotuberculatum populations was largely a consequence of its spatial distribution.