Insecticide Residue in Soil Harms Wild Bees
Mason bees have no queens or worker bees. They are a solitary, wild species of bees that belong to the Osmia bee family. Blue orchard mason bees are one of few native North American bees that can be rounded up like honeybees as managed pollinators for orchards. Although these dark, metallic blue pollinators are about the same size as honeybees, they don’t have much else in common when it comes to lifecycle or environmental interaction.
The Southern Sustainable Agriculture Research and Education program funded new research which was conducted at the University of Georgia that shows that residue from imidacloprid harms wild bees. This study is the first of its kind. Christine Fortuin, now a post-doctorate researcher at UGA, developed a more accurate understanding of how blue orchard mason bees suffer from lethal and sublethal effects of neonicotinoid exposure by studying multiple pathways of imidacloprid exposure. This 2-year research project was conducted by Fortuin, working with UGA professor of forest entomology, Kamal Gandhi, through Southern SARE’s Graduate Student Grant program.
Imidacloprid is one of a group of neonicotinoid insecticides. These are highly toxic and deadly to bees. According to Fortuin, it can be used in several ways, but her research focused on the soil-drench application method. This refers to when the pesticide is applied to the soil directly and soaks down into the earth and through the roots of trees and vegetation to prevent beetles and other pests.
Little is known about the impact of soil-drench application of neonicotinoid insecticides on blue orchard mason bees even though their activities in orchards are very important.
Neonicotinoid insecticides mimic nicotine. They disrupt and stop the insect’s central nervous system from functioning correctly. Since they are water-soluble, they can be applied to the soil, onto seeds or sprayed onto plants, as reported by the National Pesticide Information Center.
This unrelated 1:47-minute video by Science Magazine discusses how pesticides are now found in honey worldwide:
The USDA Forestry Service states that these bees prefer fruit trees. They enjoy pollinating fruits like apples, cherries, and blueberries. They are efficient pollinators and will even continue to pollinate in colder weather than that in which honeybees are comfortable. Mason bees carry dry, loose pollen instead of gathering it into balls like honeybees do, so they have a much higher pollination rate.
Another difference is that honeybees stay focused on an area until it is depleted of pollen and nectar, whereas orchard bees collect pollen from a variety of sources. According to Fortuin, this allows one female mason bee to pollinate as much as 120 honeybees would pollinate during her lifetime. Considering how efficient honeybees are, that is beyond incredible.
These adaptable blue orchard mason bees build nests in existing holes using mud and don’t mind if the holes are natural or were created by other insects. This allows farmers to create habitats for these bees on their farms.
The study had three major objectives. First, experiments assessed the impact of imidacloprid-treated soil on adult female mason bees and their nests. Second, they studied the developmental differences in offspring raised within nests built from treated soil. Third, experiments were conducted to determine if nesting mason bees showed a preference for soils with lower residual concentrations of imidacloprid.
This study was the first attempt to explore soil-based imidacloprid exposure in wild solitary bees. The majority of wild bees either nest in the soil or use it as building material for their nests. As wild bees become more popular in managed pollinator systems, accidental exposure risk increases.
Experimental results showed both lethal and non-lethal effects caused by imidacloprid residue in the soil which could have significant short-term and long-term impacts on bee populations.
Testing showed that exposure to treated soil for one hour reduced nesting activity. For bees exposed to soil containing 390 ppb, nesting activity was reduced by 42%. In bees exposed at 780 ppb, nesting activity was reduced by 66% and they produced 40% fewer nest cells per day.
Data suggests that larval bees may see limited effects from exposure, but adult bees can be seriously impacted by soil-based exposure. These findings support the idea that the soil pathway is an important point of consideration in risk assessments for imidacloprid and other neonicotinoids. This pathway is not used in risk assessments by regulatory bodies currently, which set the safety guidelines for pesticide use.
Fortuin suggests a simple solution to limit adult bee exposure is to cover the soil with mulch or bedding after applying the pesticide. Although this isn’t always practical on a farm it works and limits the contact between bees and soil. She says bees have a limited forage range and usually stay within 100 meters of their nests but may forage up to 500 meters away.
Covering the soil around the base of treated trees in areas close to bee nests limits the risk of exposure. Mulch coverings like pine straw or wood chips can act as barriers between bees and treated soil while helping to maintain moisture and prevent weeds around the base of the trees.
More work is needed to determine the most effective risk management strategy. This research justifies the need to include the soil pathway as a consideration in risk assessments for imidacloprid and other neonicotinoids applied directly to soil for the wellbeing of blue orchard mason bees.
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