Estimates of the Direct and Indirect Effects of Red Imported Fire Ants on Biological Control in Field Crops

Red imported fire ants, Solenopsis invicta Buren (Hymenoptera: Formicidae), are usually considered serious pests. S. invicta workers, however, are voracious predators and are frequently among the most abundant predators in agroecosystems within their range. Unfortunately, fire ant workers may also attack beneficial insects and arthropods. The goal of this study was to quantify the relationship between the abundance of fire ant workers and the abundance of insect pests and their natural enemies in cotton and soybean. In addition, I used path analysis to estimate the direct and indirect effects (potential loss of pest control due to suppression of other natural enemies) of fire ants. Densities of S. invicta workers were negatively associated with all 16 herbivore taxa sampled in cotton and 13 of the 16 herbivore taxa sampled in soybean. These data suggest that red imported fire ants are important predators of the major insect pests of these crops. The abundance of fire ants, however, was also negatively correlated with the densities of 22 of 24 natural enemy taxa in cotton and 14 of 16 natural enemy taxa in soybean. It appears that fire ants are significant intraguild predators of some of the most important biological control agents in these crops. These indirect interactions were often complex because fire ants not only suppressed populations of beneficial natural enemies (i.e., natural enemies that had negative impacts on pest populations), but also suppressed natural enemies that interfered with biological control (i.e., intraguild predators that had net positive effects on pest populations). Detailed experimental work is needed to determine whether the benefits of pest suppression by fire ants outweigh the negative impact of fire ants on natural enemies.