Blue orchard mason bees are one of the few bees native to North America that can be a managed pollinator for orchard crops like apples, cherries and blueberries. Mason bees are considered a wild and solitary species. Insecticides used to manage pest insects represent a risk to beneficial insects including pollinators.

Recently developed neonicotinoid insecticides are very effective in managing pest insects. Christine Fortuin, a post-doctorate researcher at the University of Georgia, has studied the impact of this class of insecticides on mason bees. According to Fortuin, “Imidacloprid is a neonicotinoid, which is a group of pesticides that are highly toxic to bees. It has several common uses but my research was focused on the soil-drench application method. This is when it is applied directly to the soil and soaked up through the roots of the tree to prevent beetles and other pests.”

Fortuin developed a more accurate understanding of the lethal and sublethal effects of neonicotinoid exposure on blue orchard mason bees by studying multiple pathways of imidacloprid exposure.

Because mason bees carry loose, dry pollen rather than forming it into a ball like honeybees, they have a much higher pollination rate. They also collect pollen from a variety of sources, where honey bees tend to focus on one area until it is depleted. According to Fortuin, these tendencies allow one female mason bee to pollinate as many as 120 honeybees in her lifetime.

Mason bees get their name because they do not build a wax-comb hive but instead use mud to build their nests within naturally existing holes. Blue orchard mason bees are comfortable using holes created by other insects or any other small opening such as hollow reeds, straw or small paper tubes. Their adaptability in nesting allows farmers to create habitats.

“After a female bee selects a nesting site, the female collects a pollen provision big enough for one egg then uses mud to seal off an egg with the pollen, creating individual partitions or cells for each egg within the nest. The eggs hatch and mature within the cells of the nest and eventually spin a cocoon where it pupates and turns into an adult which emerges in the spring,” said Fortuin.

According to the U.S. Department of Agriculture Forestry Service, these bees have a strong preference for fruit trees and are known for their efficiency when pollinating along with a willingness to pollinate in colder conditions than honey bees. Despite their importance in orchards, little is known about the impact that soil-drench applications of neonicotinoid insecticides have on blue orchard mason bees.

Previous studies on interactions between wild bees and this class of pesticide have focused on direct contact from pollen or foliar applications of the pesticide. Although wild bees regularly interact with the soil, little research has been conducted on the risks of exposure through soil-based contact.

A two-year research project assessed the impact of imidacloprid-treated soil on adult female mason bees and their nests. “Residue levels in a field can vary widely depending on the type of soil, amount of organic matter and the time since the pesticide was applied. Limited studies have shown that up to 1,000 parts per billion (ppb) can persist in the soil for several months. We chose to test a good range of residue levels to get a better understanding of the possible exposure levels,” said Fortuin.

This study represents the first attempt to explore soil-based exposure in wild solitary bees. The majority of wild bees either nest in the soil or use it as a material to build their nests. As wild bees become more popular in managed pollinator systems, the potential for accidental exposure becomes larger.

Research demonstrates that adult bees can be seriously impacted by soil-based exposure under the right conditions. These findings support the idea that the soil pathway is an important point of consideration in risk assessments for imidacloprid and other neonicotinoids. Currently, this pathway is not used in risk assessments by regulatory bodies in setting guidelines for pesticide use.

According to Fortuin, one simple solution to limit adult bee exposure is to cover the soil with mulch or bedding after applying the pesticide.

“It isn’t always practical on every farm but it does work. It limits the contact between bees and the soil," she said.

By covering the soil around the base of the treated trees in areas close to nests, the risk of exposure can be reduced. Completed research justifies including the soil pathway as a consideration in risk assessments for imidacloprid and other neonicotinoids applied directly to soil.

Roger Gates is the agricultural and natural resources agent for University of Georgia Extension, Whitfield County. Contact him at roger.gates@uga.edu.

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