Weeds Impose Unique Outcomes for Pests, Natural Enemies, and Yield in Two Vegetable Crops.

Weed management requires enormous labor investments from vegetable farmers, yet crops vary in how much weed pressure they can tolerate without yield loss. Moreover, until weeds reach a point where they threaten yield or approach seed production, they can increase biodiversity and provision food and habitat to attract predatory insects. In two related field experiments, we quantified impacts of weed presence and diversity on pests, predators, and biocontrol of both weed seeds and insect prey. We also measured yields of two vegetables that vary in competitiveness (eggplants and turnips) across two weed management treatments (weedy and weed-free), to determine productivity costs of tolerating weeds. Allowing weeds to grow adjacent to rows of eggplants increased abundances of predators and reduced pests. Surprisingly, relaxing weed management came at no cost to eggplant yield. In contrast, tolerating weeds in turnips had strong yield costs, and did not benefit predators or decrease pest pressure. On both crops, pests declined as weed diversity increased. Yet, weed treatments had no impact on consumption of weed seeds or sentinel prey by soil-surface insects, which were dominated by red imported fire ants. Our results suggest that highly competitive crops might benefit from stronger natural pest control when weeds are less-aggressively managed. However, herbivores and predators had unique responses to weeds that were crop-specific. To help farmers allocate limited weed management labor resources, future work should examine the relative competitiveness of a wider variety of vegetables over a gradient of weed pressure while measuring corresponding impacts on pest control.