Growth, development, and survival of a generalist predator fed single- andmixed-species diets of different quality

It is often assumed that prey species consumed by generalist predators are largely, though not entirely, equivalent in terms of their value to the pre dators. In contrast to this expectation, laboratory feeding experiments unc overed distinctly varied developmental responses of a generalist predator, the wolf spider Schizocosa, to different experimental diets. Naive Schizoco sa attacked and fed upon all the prey species offered, however, highly dive rgent patterns of survival, development, and growth of Schizocosa spiderlin gs reared on different single-prey diets revealed a wide spectrum of prey q ualities. Spiderlings fed the collembolan Tomocerus bidentatus sustained th e highest overall rates of survival, growth, and development. Fruit flies ( Drosophila melanogaster) were intermediate-quality prey: spiders fed Drosop hila initially exhibited rates of survival, growth, and development equal t o those of spiders on a diet of T. bidentatus, but after about 3 months, ra tes declined markedly. Fungus gnats (Sciaridae; Bradysia sp.) and conspecif ic spiderlings are low-quality prey for Schizocosa: a sole diet of either o f these prey types resulted in positive but markedly submaximal rates of gr owth, retarded rates of development, and survival rates much lower than tha t supported by a diet of Drosophila. Worst were the collembolans Folsomia c andida and Isotoma trispinata, and the aphid Aphis nerii: spiderlings fed s olely one of these species did not grow and died without molting, A. nerii is classified as poor quality because survival was no better than that of s tarved controls. F. candida and I. trispinata were toxic: survival of Schiz ocosa hatchlings fed these collembolans was lower than that of starved cont rols. A mixed diet of T. bidentatus and fruit flies yielded positive synerg istic effects with respect to growth, but development and rate of survival were similar to those of spiders on a sole diet of T. bidentatus. Including toxic prey did not produce a better diet, while inclusion of toxic prey wi th prey of higher quality created diets that were no better than the toxic prey alone. The results of these experiments suggest that prey species that are similar in morphology and behavior, and that are initially killed and consumed, may differ dramatically in their suitability as food for generali st arthropod predators.