Flower diversity on organic farms increases invasive ants rather than non-invasive natural enemies

Abstract

Increasing plant diversity in agroecosystems is well-known to strengthen natural pest suppression, yet reliably predicting which non-crop management practices best support natural enemies remains a challenge. Farmers often supplement diversity with managed flower patches or by allowing weedy non-crop plant growth, both of which may support diverse, abundant and balanced communities of insect predators that provide biocontrol. At the same time, evenness among predators and effectiveness of biocontrol may be compromised by dominant arthropod invaders that co-benefit from non-crop diversity. We examined whether floral resources or adjacent weeds had stronger effects on the evenness and abundance of natural enemies by surveying plants and insects on zucchini crops at 37 organic farms. Neither flower richness nor weed richness affected insect predator evenness. Instead, the abundance of an invasive predator, the red imported fire ant (Solenopsis invicta Buren), increased with floral resource diversity, which negatively predicted the evenness and abundance of non-invasive predators. These results suggest that floral diversity might be a less reliable predictor of natural enemy diversity when invasive predators interfere. Habitat management recommendations for bolstering biocontrol services should consider and potentially mitigate unintended consequences of floral resource provisions on farms where invasive predators dominate.