How do pesticides that are used in plant nurseries impact wild bees?

Very little research has been done so far on this topic, so University of California (UC), Riverside entomologists Jacob Cecala and Erin E. Wilson Rankin decided to figure it out. They counted over 150 species of wild bees at California nurseries alone.

They conducted an experiment to see how use of a common neonicotinoid that is often sprayed on ornamental plants would impact the solitary alfalfa leafcutter bee (Megachile rotundata). The pesticide used was imidacloprid, a neonicotinoid sold as Marathon® that is on the market since 1994 and was designed for nursery and greenhouse use.

The results were shocking. When applied at only 30% of the recommended dose, it still reduced bees’ reproduction by 90%.

Ornamental plants are not detrimental to solitary bees. They are critical resources for them. The problem is how we manage these plants, and what chemicals we apply to them, according to Cecala.

This research was published last month in Proceedings of the Royal Society B. Cecala and Rankin wanted to see if the amount of water the plants received helped or harmed the bees. Did varying nursery pesticide management practices make a difference?

According to Cecala, to understand the impacts of neonicotinoids on solitary bee and other pollinator reproduction, it is also important to know if and how the plants are affected by the chemicals.

Neonicotinoids are water soluble, so he thought that watering the plants more may reduce the impact on bees, as outlined in a UC Riverside press release. Bees were introduced to ornamental plants. Some plants were treated with 30% of the label dose, and some were untreated, with different watering amounts.  

They were surprised by the results. The pesticide-treated plants that were watered more had less imidacloprid in their nectar but were just as harmful to bee foraging and reproduction as the pesticide-treated plants that were watered less.

They observed highly detrimental impacts on solitary bees' reproduction at application levels of only 30% of the recommended insecticide dose. Most people follow the label's recommended dose, according to Cecala. When applied to a flowering or soon-to-flower plant, there would be a similar negative impact on bees.

This 6:24-minute video by citizenscampaign is called Pollinator Protection Forum Highlights:



Daniel Raichel, acting director of the Natural Resources Defense Council (NRDC)'s pollinator initiative, was not surprised by these results. The study adds to growing evidence that neonicotinoids are very harmful for bees and other animals.

Raichel says these pesticides are "phenomenally toxic." One study found that agriculture became 48 times more toxic to insects in the U.S. between 1992 and 2014, largely due to neonicotinoid use.

He says that just one neonic-treated corn seed has enough active ingredient to kill a quarter million bees or more and one square foot of neonic-treated lawn, at the EPA approved label rate, has enough active ingredient to kill a million bees.

Neonicotinoids were designed to be absorbed by the entire plant, so the plant becomes the pesticide. These poisons are easily absorbed by the environment and spread through the soil, contaminating wild plants and bodies of water. Ever more research shows that this spread has vast negative consequences like delayed songbird migration, birth defects in white-tailed deer and collapsed fisheries.

Raichel says these neonics are popular nationwide and are used year in and year out. They are building up in the soil and spreading out into water, food, and the ecosystem. He likens it to what happened due to the dangers of DDT, as described in the book Silent Spring by Rachel Carson.

The new research points out some solutions, including improved pesticide labelling with more explicit language on product labels about the dangers to bees, especially solitary bees, according to Cecala. The label states it is 'highly toxic' to bees (though not on the first page), but it only warns the user not to apply it if bees are foraging. Study results stress that very real threats are posed to bees, weeks, and months later, even if the plant was not blooming at the time the pesticide was applied.

Raichel advocated for a targeted sale and use ban of neonicotinoid-coated seeds where they are not truly needed.

Cecala says home gardeners can do things to protect native pollinators. The same active ingredient (imidacloprid) is in many products designed for residential gardens and lawns. Home gardeners wanting to 'save the bees’ should closely inspect the labels of chemical products and try to avoid neonicotinoids entirely.