Intraguild Predation Usually does not Disrupt Biological Control

Abstract

Intraguild predation is claimed to be ubiquitous in nature. It also occurs among natural enemies in biological control systems, where one natural enemy (the intraguild predator) attacks another species of natural enemy (the intraguild prey), whereas they also compete for the same pest. We review the theory of intraguild predation and its consequences for biological control for two different scenarios. 1. The intraguild predator is the superior natural enemy (i.e. reduces the pest population the most). In this case, the intraguild predator will exclude the intraguild prey, thus there will be no intraguild predation in the long term. 2. The intraguild prey is the superior natural enemy. In this case, the intraguild predator and intraguild prey may coexist or the intraguild predator can exclude the intraguild prey. Theory predicts for this scenario that pest numbers will always be lowest when only the intraguild prey is present. Hence, the occurrence of intraguild predation in cropping systems would never result in increased control, but can result in decreased control. We subsequently review experimental tests of the effect of intraguild predation among natural enemies on the population dynamics of pests. Contrary to expectations, we find that intraguild predation often did not result in an increase of pest populations, even when the intraguild predator was the inferior natural enemy. Often, the presence of the intraguild predator had no effect or even resulted in a decrease of pest populations. Although the number of studies was limited, we scanned the literature to identify possible causes for the discrepancy of experimental results with theoretical predictions. We specifically evaluated trends in the effects with respect to the length of the study period, the spatial scale at which experiments were carried out, the number of species involved in the studies and the spatial complexity of the experimental arenas. There was a slight trend towards experiments of longer duration showing less positive effects on pest densities, but no clear effect of spatial scale. All studies that showed positive effects on pest densities were studies with 3 species, but the number of studies with more than 3 species was small. Spatial complexity had mixed effects on experimental results. In conclusion, it is clear that intraguild predation most often does not increase pest densities as was predicted from theory, but more research is needed to reveal why theory does not meet practice.