By Robert Koch & Marla Spivak, Extension Entomologists
Honey bees and native bees forage in and near soybean and cornfields, especially during dry weather. When treatment decisions are being made for pests of these crops, it is important to consider minimizing the risk to these pollinators. Bees are the most important pollinators of our fruits, vegetables and crops like alfalfa hay that feed our farm animals. Honey bees and the thousands of native bee species all rely on the flowers they pollinate for good nutrition and health. Bees are being pushed to the tipping point by various factors, such as disruption of natural habitats, diseases and parasites, and widespread overuse of pesticides.
Knowledge of basic bee biology can go a long way in helping to protect bees from pesticides. Bees forage throughout the growing season from sun up to sun down when temperatures are above 50-55°F. Honey bees fly an average of 2 miles on each foraging trip, searching for flowers over an 8,000 acre area surrounding the hive or apiary. Native bees generally fly within ½ mile of their nest. If flowers are blooming, bees will be foraging on them. Beekeepers cannot close up the hive during a pesticide application because the bees will suffocate. Moving the bees is not a viable option because bees home back to their original site unless they are moved out of their foraging range, over 3 miles away. Locations to place bee hives are difficult to come by, and it is even harder to find locations where the bees are out of range of pesticide application.
In a July 3, 2013 article in the NDSU Crop and Pest Report, Janet Knodel provided several general recommendations for reducing pesticide risks to pollinators:
• Know and communicate with beekeepers about pesticide application schedules and products.
• Use economic thresholds and other IPM strategies. Economic thresholds ensure that pesticides are used only when crop losses prevented by pesticide use are greater than the cost of the pesticide and the application.
• Use pesticides with low toxicity and low residual to bees. For example, avoid using dusts or wettable powder insecticide formulations because they generally are more toxic to bees.
• Evening or early morning applications are the least harmful to bees, because fewer bees are foraging.
• Never apply pesticides outdoors on a windy day (winds higher than 10 mph) which could cause spray drift problems.
Honey bees and native bees visit the flowers of soybean and many flowering weeds growing in and near soybean fields. As soybean aphid populations continue building throughout the state, aphid management should take these pollinators into consideration. The University of Minnesota recommends making treatment decisions based on scouting program (i.e., getting into the field and counting aphids) and the following economic threshold for R1 (beginning bloom) to R5.5 (seeds expanding in pods) soybean: treat if populations are increasing, the majority (at least 80%) of plants are infested, and average aphid counts exceed 250 aphids per plant. This threshold can protect yields, reduce costs, conserve natural enemies of aphids and other pests, and reduce the risk of pests developing resistance to pesticides. In addition, treating soybean aphid populations with pesticide when they exceed this threshold will minimize unnecessary pesticide applications and reduce the exposure of pollinators to pesticides.
Honey bees and native bees can be found foraging in cornfields, especially when pollen is available during tasseling and silking. Pesticide applications made during these growth stages, may put pollinators at risk. The emerging problem of corn rootworm resistance to Bt traits may result in increased pesticide applications during these crop growth stages, when the adult rootworm beetles are active. Furthermore, these growth stages are critical for protection of sweet corn against certain caterpillar pests. Consider pollinators when making management decisions for certain corn pests.
When using pesticides, always read and follow the label directions. Labels for some products/formulations with high toxicity to bees will provide specific directions for minimizing risk to pollinators. The label is the law.
Keeping these recommendations in mind will help you to protect honey bees and other pollinators while you use pesticides to protect your crops.