- Intraguild predators can have unpredictable effects in food webs: they can either disrupt or enhance herbivore suppression, depending on their direct effects on herbivores. Furthermore, intraguild predation is not always unidirectional, with top predators eating more effective intermediate predators.
- In a vineyard field experiment and a series of laboratory experiments, the effect of intraguild interactions between a likely top predator (the spider Cheiracanthium mildei) and an intermediate predator (the spider Anyphaena pacifica) on suppression of their shared leafhopper prey (Erythroneura spp.) was examined.
- In the field experiment, C. mildei drove the suppression of leafhoppers while reducing numbers of the other spider A. pacifica. The results of laboratory experiments confirmed the predatory impacts of C. mildei on both leafhoppers and A. pacifica. As a top intraguild predator, C. mildei appears to dominate predator–prey interactions in the vineyard ecosystem, although results indicate that its impact may be dampened by cannibalism.
- By contrast, A. pacifica preyed on leafhoppers and smaller C. mildei in laboratory experiments, but had no measurable impacts in the field experiment.
- The larger average size and wide-ranging hunting mode of C. mildei may provide a predatory advantage in the field that did not emerge over short time periods in small laboratory cages.
- These results highlight the pitfalls of extrapolating from the controlled setting of the laboratory to more heterogeneous conditions in the field.