A Tale of Two Pollinators: More Evidence of Neonicotinoids’ Effect on Wild Bees

Entomology Today September 17, 2021 1 Comment

eparate studies on bumble bees—such as the common eastern bumble bee (Bombus impatiens), shown here—and mason bees exposed to imidacloprid add to the body of evidence that wild bees may be particularly vulnerable to neonicotinoid insecticides. (Photo by David Cappaert, Bugwood.org)

By Melissa Mayer

Melissa Mayer

Neonicotinoid pesticides kill insect pests by acting on their nervous systems, eventually causing paralysis and death. That makes them effective pesticides, but it’s not great news for pollinators that come into contact with neonicotinoids while foraging or nesting.

A pair of studies published in August and September in Environmental Entomology look at how exposure to the neonicotinoid imidacloprid affects common eastern bumble bees (Bombus impatiens) and a species of mason bee, Osmia lignaria. In the first study, exposed bumble bees were less efficient foragers, flying farther between flowers rather than visiting the closest rewarding flower. In the other, mason bees given nesting choices that included contaminated soil showed no avoidance response at all.

Inefficient Foraging

Harry Siviter, Ph.D.

Researchers at the Muth lab at the University of Texas had noticed that bumble bees fed imidacloprid seemed to be less efficient foragers. So, Harry Siviter, Ph.D., a postdoctoral researcher at the lab, tasked undergraduate student Anthony Johnson with reviewing video data of foraging bumble bees (from a previous experiment) to quantify their efficiency between flowers.

“We know that bumble bees get better at foraging as their life goes on, as they get more experience,” says Siviter. “They learn the different floral traits and floral scents. They learn how to navigate their environment better. … And it’s really important that they do forage effectively, because, if they don’t, then the colony won’t grow as well.”

Time spent foraging trains bumble bees to make good foraging decisions like visiting the closest rewarding flower. Siviter says that’s like humans shopping at the grocery store down the street instead of driving across town.

It turned out the bees exposed to imidacloprid did make worse foraging decisions, flying to flowers that were farther away instead of foraging the flowers nearby—to the tune of a 7.8 percent increase in distance traveled. That might not be a big deal for one flower, but bumble bees forage thousands of flowers each day, so the inefficiencies add up.

Previous research has shown that neonicotinoids can mess with bumble bees’ flight velocity and endurance, impair their motor abilities, and even cause hyperactivity, and the researchers at the Muth Lab observed that exposed bumble bees appeared frantic and as if they were having trouble flying and landing.

“We know that neonicotinoids influence all sorts of other things,” Siviter says. “They can influence flights. They can influence the ability to learn about flowers. They can influence thermoregulation. … And what we’ve really done is shown another thing that it influences. If you take all that stuff together, you are basically getting bumble bees that are less good at their job.”

Soil Dilemma

Christine Cairns Fortuin, Ph.D.

Soil is another route of neonicotinoid exposure for wild bees who nest underground or build brood cells with mud. Take mason bees. A mother mason bee selects a hollow space to nest, packs a portion of it with pollen, and lays an egg on the little food store. Then, she collects mud to cap off the cell before starting another layer.

But won’t mason bees just pick clean soil? That’s what Christine Cairns Fortuin, Ph.D., postdoctoral researcher at the Forest Entomology Lab at the University of Georgia, wondered while doing fieldwork in 2020.

Separate studies on bumble bees and mason bees—such as Osmia lignaria, shown here—exposed to imidacloprid add to the body of evidence that wild bees may be particularly vulnerable to neonicotinoid insecticides. (Photo by Heather Holm, via iNaturalist, CC BY-NC 4.0)

“I kept thinking: In a realistic environment—in a field scenario or in an orchard or something like that—these kind of high residue concentrations are only going to be in limited areas within the landscape,” says Cairns Fortuin. “We really need to understand, from a risk perspective, how likely are they … to interact with those areas of high contamination when they have an entire landscape of soil that they can choose from.”

To figure that out, Cairns Fortuin first offered mason bees the choice between two chambers with various pairings of untreated and treated soil (up to 780 parts per billion [ppb]). Over 5 minutes, the research team logged how often each bee entered each chamber and how long it spent there.

Then, they looked at nesting choices. Getting mason bees to nest in the lab is super challenging; in fact, some people have said it couldn’t be done. So, the team provided nesting bees with specialized 3D-printed well plates (designed by Natalie Boyle, Ph.D., at Pennsylvania State University) provisioned with pollen. Then they offered the bees two boxes of mud, one untreated and one treated (either 100 ppb or 1,000 ppb).

None of the mason bees in any of the experiments avoided the treated soil—not even at the very highest level of contamination. That could mean that the risk of wild bees picking up contaminated soil would scale with how much of the landscape is treated.

Getting mason bees to nest in the lab is challenging. Researchers at the University of Georgia were successful in studying soil preference of Osmia lignaria mason bees by providing nesting bees with specialized 3D-printed well plates (designed by Natalie Boyle, Ph.D., at Pennsylvania State University) provisioned with pollen. (Photo by Christine Cairns Fortuin, Ph.D.)

Those well plates worked well, so they could be a flexible tool for rearing mason bees and studying their reproductive behaviors in the lab. And the research team noted an interesting similarity among the bees that reproduced successfully during the experiment.

“The two cages where they actually did produce offspring were also cages where they selected the untreated soil more heavily,” says Cairns Fortuin. “I feel like we need to do more replications to really decode what’s going on with regard to that.”

The issue of neonicotinoid pesticides and the risk of exposure among pollinators—especially native bees—is a complex one. Studies like those undertaken by Siviter and Cairns Fortuin add to the data helping scientists figure out the full risk profile for wild bees.