Spiders are generalist feeders, predating on a wide variety of animals. This is especially true for the Lycosid (wolf) spiders, who employ a sit and wait strategy in pursuit of prey. This reduces their ability to be selective in feeding. Toft and Wise (1999, Oecologia 119 pp. 191 – 197) have shown that prey of similar morphology and behaviour can have wildly different effects on wolf spider health. Spiders fed the collembolan Tomocerus bidentatus showed excellent growth rates, while those fed on an alternate collembolan, Folsomia candida, had lower survial rates than spiders who were starved.
Similar results were reported by Oelbermann and Scheu (2002 Basic Applied Ecology 3 pp. 285 – 291) in a study on Pardosa lugubris.
P. lugubris is a common wolf spider, found widespread on leaf litter and beneath vegetation. They are most visable in early to mid summer when the female can be seen bearing a cocoon of eggs attached to the spinnerets at the tips of the abdomen (see picture above and below). After the brood hatches they remain on the mother's opisthosoma, attached to the spiny, knobbed hairs that are peculiar to adult female Lycosids for 3 to 6 days before dispersing (Rovner et al., 1973 Science 182 pp. 1153 – 1155).
Oelbermann and Scheu found that P. lugubris was unable to recognize and refuse detrimental prey species (such as F. candida) in mixed diets. The presence of these toxic or low quality prey species can counteract the perceived benefits of mixed diets (e.g. good fatty acid and amino acid intake) for such generalist predators. This has implications in the use of generalist predators as biological control agents, as the presence of toxic prey species would negate any effect on target prey species.