Models of intraguild predation (IGP) predict that adding an intraguild predator will act to increase a shared herbivore’s population, yet empirical investigation demonstrates that adding an intraguild predator can have a wide range of effects on herbivore density. The availability of multiple herbivorous prey may be one of the most likely explanations for this disagreement between empirical and theoretical investigations. We briefly present a simple theoretical model that predicts that the addition of a second herbivore, an alternative prey, can either enhance or diminish the effects of IGP. Moreover, the model suggests that the qualitative outcome may depend on the relative preferences of the intraguild predator. We tested this idea in the cotton agroecosystem, a model system of IGP. Using field cages, we investigated how the addition of a second, herbivorous prey affected IGP dynamics for multiple community modules, each with a different species of intraguild predator. Our results indicate the potential importance of the intraguild predator’s preference in determining the dynamics of intraguild predation, especially in more realistic multi-prey systems. Moreover, we suggest that investigating the effects of alternative prey on IGP may improve our ability to predict the long-term dynamics of predator-prey systems and understand why some systems are more or less capable of continuous pest control.