Conservation biological control (CBC) is a promising ecological alternative to pesticide use, suppressing agricultural pests by supporting populations of resident natural enemies. The use of generalist predators in CBC has been controversial because they often eat both pest and beneficial species, and so their net influence on crop production may be positive, neutral, or even negative. This talk will report outcomes from a project elucidating the functional role of the American toad (Anaxyrus americanus), a native generalist predator that is common in agroecosystems in the eastern US. Despite a widespread belief that toads are “beneficial” for gardens and farms, there is surprisingly little rigorous research on their net influence in cropping systems. We conducted observational and experimental studies on a 50-acre organic vegetable farm in Pennsylvania from 2015 to the present. The observational study characterized stomach contents of wild-caught toads (n = 46) to characterize their feeding habits. The manipulative experiment involved kale-based mesocosms with three levels of toad density (0, 1, or 2 juvenile toads/m2, n = 3 mesocosms per treatment), in which the response of the invertebrate community and crop plants was measured.
Results/Conclusions
Stomach contents of wild-caught toads contained a diverse range of invertebrates, including both pest and non-pest taxa. The most frequently encountered prey items were ants (Formicidae), ground beetles (Carabidae), and rove beetles (Staphylinidae), taxa that themselves contain many intraguild predators with complex effects in food webs. Consistent with these results, ants and ground beetles were significantly less abundant in mesocosms with the highest toad density (ANOVA, P = 0.019 and P = 0.005, respectively), while herbivorous flea beetles were marginally more abundant in mesocosms with intermediate and high toad densities (ANOVA, P = 0.08). Toad density did not significantly affect crop growth or biomass, although there was a trend for both to increase with toad density. Taken together, these results suggest that this native toad species does not have a strong net influence on crop production in either direction. Ecological theory suggests that weak and diffuse trophic linkages stabilize ecosystems, so the longer-term dynamics in this system deserve further study. Nonetheless we conclude that encouraging populations of the American toad is unlikely to yield immediate benefits for pest management and crop production.
