Factors Associated with Pollinator Foraging and Floral Morphology of Lawn Weeds

Abstract

The decline in pollinator populations poses a significant challenge to global food security, necessitating best management practices that minimize pollinator exposure to harmful pesticides. Many common weeds in managed turfgrass systems attract honey bees (Apis mellifera L.) and other pollinators, increasing their risk of contact with insecticide residues. This dissertation project explored the potential of herbicides, optical brightener, pigmented fungicide, commercial sunscreen, turf colorant, and other deterrent strategies to reduce pollinator foraging on weedy flowers in turfgrass. Field studies were conducted from 2021 to 2023 to evaluate the effect of these treatments on floral morphology and ultraviolet (UV) reflectance of bulbous buttercup (Ranunculus bulbosus L.), dandelion (Taraxacum officinale G. H. Weber ex Wiggers), foxglove beardtongue (Penstemon digitalis Nutt. ex Sims), white clover (Trifolium repens L.), and associated pollinator foraging. Several herbicides, including halosulfuron, sulfentrazone, synthetic auxins, and topramezone reduced floral density, altered UV reflectance, and deterred pollinators to varying degrees on weedy flowers. Synthetic auxin herbicides completely eliminated pollinator visitation within two days and degraded white clover floral quality within five days, suggesting that insecticides could be applied two days post-herbicide treatment to minimize pollinator exposure risk. Pollinator visitation and floral quality were temporarily affected by herbicide application, with some herbicides eliminating food resources while others transiently impacting floral quality and density. While floral UV reflectance affects pollinator visitation, it is not the only factor; as UV-absorbing and UV-reflecting flowers showed distinct reflectance responses to herbicides, yet pollinators were equally deterred. Optical brightener, pigmented fungicide, and commercial sunscreen reduced radiometric UV reflectance and digitally assessed UV-reflecting area of foxglove beardtongue, dandelion, and bulbous buttercup petal apices by 24–79%, significantly deterring pollinator visitation to dandelion and bulbous buttercup flowers. Despite transient impacts on floral reflectance and pollinator foraging visits, optical brightener and sunscreen treatments did not affect floral density or quality, preserving long-term pollinator food resources.

Deterrent practices may reduce the risk of pollinators being exposed to harmful insecticides in turf by decreasing their visits, but the effectiveness of these methods in minimizing contact exposure remains unaddressed. In 2024, an additional study assessed the effectiveness of deterrent practices in preventing contact exposure of actively trapped honey bees and passively trapped insects to white clover inflorescences treated with fluorescent powder in turfgrass. Mowing and pre-treatment with synthetic auxin herbicide reduced fluorescent powder exposure of honey bees by at least 75% and 90%, respectively, but had little effect on passively trapped insects, suggesting that trap attractiveness, rather than white clover flower manipulation, drove insect capture. Blue vane traps captured 1,117 bees from 23 species within 28 hours, with over 96% being native, while yellow sticky cards collected 384 insects from the Lepidoptera, Diptera, and Coleoptera orders. The results indicate that mowing and synthetic auxin herbicides effectively deter honey bees from visiting treated white clover inflorescences, lowering their contact exposure risk. These findings show that herbicides, optical brightener, and mowing can successfully reduce pollinator foraging on treated turfgrass, thereby decreasing the potential for insecticide exposure. This research offers valuable insights for balancing weed management with pollinator conservation, providing practical approaches to minimize insecticide risks while preserving pollinator food sources.