ESTIMATING THE RELATIVE CONTRIBUTION OF WILD BEES IN THE POLLINATION OF APPLE, CRANBERRY, AND SQUASH IN WISCONSIN (USA)

Abstract

Farmers growing pollinator-dependent crops such as fruits, vegetables, nuts, and oilseeds traditionally rely on managed bees for pollination. However, managed bee populations have suffered significant colony losses due to agricultural intensification, pesticide exposure, diseases, parasites, habitat loss, and climate change. Wild bees, although often underestimated, play a vital role in pollination services and can enhance fruit quality, particularly in diverse landscapes. Despite their potential, farmers are cautious about relying solely on wild bees due to their inconsistent abundance and uncertain contributions to crop pollination. This study evaluated the contributions of wild bee morphogroups to the pollination of apple (Malus × domestica), cranberry (Vaccinium macrocarpon), and squash (Cucurbita spp.) in Wisconsin (USA). Flower visitation rates on farms were measured using standardized 5-minute visual surveys during the flowering period of each crop in Wisconsin in 2024. Additionally, we obtained single-visit pollen deposition data through empirical observations in our cropping systems and combined these with additional information from published studies. Honey bees were the most frequent visitors in apple and cranberry, attributed to the abundance of managed hives, whereas large dark bees dominated visitation in squash. Large dark bees deposited the highest pollen grains per visit in apple (𝑥̄ = 253), while bumble bees deposited the highest pollen grains per visit in cranberry (𝑥̄ = 36) and in squash (𝑥̄ = 209). We estimated the expected full-day pollen deposition to flowers of each crop based on visitation rates and single-visit pollen deposition (pollen supply, Ps) and compared this to the minimum pollen requirements for full pollination from known studies (pollen demand, Pd), to estimate a pollination index Pi = (Ps/Pd). The Pi values varied across farms within each crop: apple (Pi =1.3 to 12.6), cranberry (Pi =2 to 49.2), and squash (Pi =

1.8 to 13), indicating the potential of wild bees to provide effective pollination services, even in the absence of honey bees. However, variability in visitation and deposition across farms suggests potential shortfalls in some farms, especially in apple and cranberry, without supplementation from honey bees. By utilizing tools like the pollination index (Pi), growers can make informed decisions and promote sustainable, resilient pollination systems. Future research should further validate these results and explore the broader scalability of wild bees’ contributions. Keywords: Pollination, Pollination service index, Single-visit pollen deposition, Visitation, Wild bees