Veronica Yurchak, Vegetable Specialist, UMD; vjohnso4@umd.edu
The goal of this series is to increase awareness and knowledge of beneficial insects in cropping systems and help differentiate them from pests of concern. This week’s feature beneficial is a diverse group of unmanaged pollinators collectively referred to as “wild bees”. The Delmarva region is home to a diverse assemblage of wild bees, with over 400 species found in Maryland alone. Wild bees are highly effective pollinators for many fruit and vegetable crops, with some species known to visit thousands of flowers in their lifetime. In many cases, wild bees are able to pollinate crops more effectively than honeybees, with one study finding that an increase in wild bee visitation enhanced fruit set by twice as much as an equivalent increase in honeybee visitation.
Figure 1. Bumblebee visiting blueberry flower.
In the Delmarva region, crops including blueberries, strawberries, cherries, apples, and cucurbits require insect pollination to produce fruit. Mason bees, also known as orchard bees, are well known for their ability to efficiently pollinate fruit trees. Also contributing significantly to crop pollination are bumblebees, which can be easily recognized by their large size and furry bodies (Figure 1). The Mid-Atlantic region is home to 19 different species of bumblebees. Bumblebees enhance pollination in blueberries, tomatoes, peppers, eggplant and other crops through a process known as “buzz pollination” during which the bumblebees vibrate the flowers at a specific frequency to dislodge the pollen. Halictid bees (Figure 2) are also effective at buzz pollination and can further increase fruit production and quality in tomatoes and other “self-pollinating” crops. Even crops that do not benefit from insect pollination, like sweet corn, can still serve as an important food source for many wild bees. A study recently conducted in Maryland recorded frequent sweet corn pollen collection by halictid bees, the two-spotted longhorn bee, and honeybees during the pollen-shed period. Consideration should therefore be given to protecting pollinators in all cropping systems.
Figure 2. Halictid bee visiting tomato flower.
Many of the same challenges commonly attributed to honeybee declines are also causing reductions in wild bee populations. Land use changes, pesticides, pathogens, and weather variability are among the leading causes of wild bee decline. Reducing tillage and other forms of soil disturbance can help preserve nesting sites, as most wild bee species build their nests underground. Minimizing pesticide exposure is also essential for wild bee survival in cropping systems. Studies have revealed a strong correlation between pesticide use and declining wild bee populations. In addition to direct bee kill, sublethal effects including disrupted navigation, reduced memory, and weakened immune systems are also attributed to bee exposure to many broad-spectrum products (ex. neonicotinoids, pyrethroids, and others). Whenever possible, choose pesticides with lower bee toxicity ratings. When the use of a product with a higher bee toxicity rating is necessary, applications should be restricted to non-flowering stage crops and made shortly after dark, when bees are not actively foraging. Providing diverse habitats, including natural areas and wildflower plantings (Figure 3), is also crucial for supporting wild bee populations. These areas provide additional food, nest sides, and refuge areas from pesticide-treated fields. Many of the other beneficial insects discussed throughout this series will also benefit from any measures taken to protect and promote wild bees in crop fields.
Figure 3. Pollinator refuge area at the Wye Research and Education Center in Queenstown, MD.