Editor’s Note: Michigan State University Extension recently published this consumer article by Dr. David Smitley on why the discovery of neonicotinoid insecticides in leaves and flowers of some garden center plants should not stop consumers from buying and planting flowers, because the benefit to bees far outweighs the potential risk. It’s a good resource to share with your customers and consumers. Dr. Smitley’s work is funded in part by MSU’s AgBioResearch.
Recently, some Internet and media sources have suggested that buying and planting flowers from your local garden center could be harmful to bees because traces of neonicotinoid insecticides were found in the leaves and flowers of plants randomly purchased from garden centers around the country. Although it is true that concentrations of more than 100 ppb of imidacloprid in nectar or pollen are toxic to honey bees, and lower concentrations (10 to 100 ppb) could affect their foraging behavior and immune response, the potential harm to pollinators in the yard and garden from buying and purchasing flowers from a garden center has been exaggerated. In fact, planting annual and perennial flowers and flowering trees and shrubs is expected to be beneficial for bees and other beneficial insects.
Greenhouse and nursery growers started using alternatives to neonicotinoid insecticides this year, and the amount used is less than in previous years. Michigan State University began working with growers in March of 2014 to identify pest control strategies where neonicotinoids have been used so that alternative strategies could be adopted. Also, experiments were initiated to determine the risk to bees after treated plants are sold and planted in the yard and garden. Several experiments are in progress now, where treated plants are put into screened cages with colonies of bumble bees.
People should not become too alarmed by the detection of neonicotinoids found in the leaves and flowers of some garden center plants. This does not mean that the plants will be toxic to bees. Here are several reasons why:
- Michigan is home to some of the largest greenhouse flower growers in the country. In a recent survey we found most growers do not make soil applications of neonicotinoid applications to petunias, impatiens, marigolds, New Guinea impatiens, verbena, ageratum, celosia, dianthus, portulaca, salvia, snapdragons, vinca and other bedding plants grown in flats. They usually do not spray flowers with any insecticide in the last two weeks of production. So flowers sold by the flat should be safe for bees.
- Many trees and shrubs, including all conifers, and many broadleaf trees like maple and oak are wind pollinated and are not usually visited by bees.
- Perennial flowers, roses, flowering shrubs and flowering trees will be a valuable resource for bees and other pollinators for many years after they are planted, while the risk to bees will be limited to those plants that were treated with a soil drench, and only when they are flowering in the first year.
- Bees feed on a large variety of flowering plants, and often forage as far away as a mile from their colony. Because they are feeding on many different plants, the presence of a neonicotinoid insecticide in one plant will be diluted when they feed on untreated plants.
Flowers bought in flats should be completely safe to bees. If you are worried about some of the perennials or flowering trees and shrubs that you purchase, the flowers could be removed during the first summer after planting. Also, if you buy trees or shrubs in a container, you can reduce the amount of imidacloprid or any neonicotinoid insecticide that may be present in the soil by watering them until you see water emerging from the bottom of the container, and then continue to run the water for another 10 minutes. This will flush any neonicotinoid insecticide residue that is not tightly bound to the organic matter in the soil.
Another way to encourage bees and beneficial insects is to avoid spraying insecticides in the yard and garden as much as possible, and never spray flower blossoms. If you have a problem with caterpillars chewing too many holes in the leaves of some plants, you can use a product containing Bacillus thuringensis or Bt, without harming bees and other beneficial insects.
Another bee-friendly option is to use horticultural oil or an insecticidal soap. They are effective on most soft-bodied insects and can be used on cool mornings, less than 50 degrees Fahrenheit, after sunset, or at any time that bees are not present. The soap and oil residue is not harmful to bees, but spraying them directly is. Soap and oil can cause some plant injury, especially to open flowers, so do not exceed the rate given on the product label. In rare cases where a plant needs to be protected against damaging insects by using a broad-spectrum insecticide, it should be sprayed after petal-fall (after the plant is done blooming).
What about neonicotinoid insecticides?
As with all insecticides, do not spray neonicotinoids on flower blossoms. Also, because they are systemic when absorbed by the roots, avoid using products that contain imidacloprid or clothianidin as a soil drench around plants that are attractive to bees. Using them as a basal (soil) drench around the base of wind-pollinated trees and shrubs, like most evergreens and trees like oaks and ash, is not likely to harm bees because bees rarely visit wind-pollinated plants.
What about the pesticides used on flowers, trees and shrubs sold at garden centers?
Nursery growers and greenhouse growers have been working closely with Michigan State University on how to grow plants that are safe for bees and other pollinators. They are following best management practices that include using alternatives to neonicotinoid insecticides, and they avoid spraying flowers close to when they are shipped to garden centers, to make plants as safe for pollinators as possible.
Could pesticide residue in the soil of garden center plants be contributing to the decline of commercial honey bee colonies?
Although it is desirable to grow flowers in the yard and garden for bees and other beneficial insects, our garden plants are not usually a primary food source for commercial honey bee colonies, and therefore have very little to do with the problems that beekeepers have had.
For more information on identifying and encouraging pollinators, see the Michigan State University Extension bulletin, “Native Bees & Their Conservation on Farmland,” by Rufus Isaacs and Julianna Tuell.
Source: Michigan State University Extension.