The Nature of Population Stability in Eurosta Solidaginis, A Nonoutbreaking Herbivore of Goldenrod

Eurosta solidaginis (Diptera: Tepritidae) is a common, nonoutbreaking herbivore of goldenrod (Solidago altissima) whose populations fluctuate little through time. A large body of theory is based upon the assumption that nonoutbreaking phytophagous insects are regulated at "endemic equilibria" by the temporally density—dependent mortality caused by their natural enemies. This equilibrium viewpoint has been challenged recently by proponents of "nonequilibrium theory," even though the evidence against population regulation is largely observational and likely to be biased by the almost exclusive study of abundant, pest species. To understand the nature of population stability in Eurosta solidaginis, I altered the density of 20 subpopulations, and looked for compensatory changes in mortality that could allow the populations to return to their pre—experimental density. Mortality diminished in those subpopulations whose density was decreased. Moreover, the change in mortality depended significantly on the change in density; subpopulations in which density was most severely diminished enjoyed a greater decrease in mortality. Of four individual mortality agents, only early larval death, which may be partly caused by a defensive reaction by the host plant, showed significant density dependence. No within—generation (spatial) density dependence was detected in either the first or the second (experimental) generation. At the end of the density—manipulation experiment, all Eurosta were removed from the patches. Most patches were recolonized within one generation.