The study found that 36 out of 71 plant samples - 51 percent - bought at top garden retailers in 18 metro areas across North America - including 1 in 4 plants from a Home Depot in Cockeysville - contained neonicotinoid pesticides, which research has linked to the loss of managed honeybee colonies and wild pollinators.
The pesticide levels detected in the stalks and leaves of some flowers were high enough to kill bees outright, the study said, assuming comparable concentrations were present in the flowers' pollen and nectar.
Timothy Brown, co-author of the report from the Pesticide Research Institute, said the data indicate that many plants sold in nurseries and garden stores across the United States and Canada have been pretreated with systemic neonicotinoid insecticides, rendering them potentially harmful to bees and other pollinators.
"Unfortunately, these pesticides don't break down quickly," Brown said, "so these flowers could be toxic to bees for years to come."
Maryland beekeepers reported losing nearly half their hives in the past year, while nationwide one in three hives were lost. Honey bee losses have been linked to multiple factors, especially Varroa mite infestations, but some research suggests that neonicotinoid pesticdes are a major contributor, weakening bees' ability to withstand other stressors.
One of four plants bought from the Cockeysville Home Depot had at least three different neonicotinoids in it, according to Ruth Berlin, executive director of the Maryland Pesticide Network, which participated in the study.