Solitary bees such as alfalfa leafcutting bee (Megachile rotundata) face different—and less well-understood—challenges from pesticide exposure than their colony-dwelling honey bee cousins. Here, an alfalfa leafcutting bee visits an alfalfa flower. Pollen grains are visible behind the bee, as the bee has “tripped” the flower, causing pollen to be released outward. This facilitates pollination for alfalfa. (Photo credit: Theresa Pitts-Singer, Ph.D.)

By Meredith Swett Walker

Humans and honey bees go way back. We’ve been raiding their hives for honey for at least 10,000 years, and we domesticated them almost 5,000 years ago. Not only do they provide sticky sweet goodness, but we also use our tiniest livestock to produce our food: Honey bees are the most commonly used pollinator for commercial crops in the United States. They are managed by commercial beekeepers to provide pollination services for more than 100 crops.

Meredith Swett Walker

Though honey bees may be our BFFs, they are not the only bee species that pollinate the food we eat. They are also something of an anomaly in the bee world. The vast majority of the more than 4,000 bee species in North America do not live in a colony with all their closest relatives. In fact, most native bees are solitary and have a very different lifestyle than do honey bees. These bees often nest in cavities, like holes in plant stems or burrows in the ground. And, while we know a whole lot about honey bees, we know comparatively less about these solitary bees.

In the last decade, concern has mounted over the decline in pollinator populations. Much of the research and media attention has focused on honey bees, but solitary bees face many of the same challenges: loss of habitat, parasites, disease, nutritional deficiencies, and exposure to pesticides. Unfortunately, we lack the baseline data necessary to understand how wild bee populations are affected by these factors. And, when it comes to pesticides, we typically only check their effects on honey bees before authorizing their use—even though solitary bees may interact with these chemicals in very different ways.

In a report published last week in Environmental Entomology, Utah State University graduate student Andi Kopi and Theresa Pitts-Singer, Ph.D., of the U.S. Department of Agriculture’s Agricultural Research Service make the case that we need to look beyond honey bees when assessing how pesticides affect pollinators. Their paper describes how managed populations of solitary cavity-nesting bees are exposed to pesticides, how these routes of exposure are different than those experienced by honey bees, and what this might tell us about how wild solitary bees are exposed to pesticides.

The authors focus on bees in two genera that are managed by commercial beekeepers to provide pollination services: Osmia (mason bees and the blue orchard bee Osmia lignaria) and Megachile (primarily the alfalfa leaf cutting bee, Megachile rotundata). Both of these are solitary cavity nesters that share many lifestyle traits with thousands of species of wild native bees. But, because these species are also kept by beekeepers, they are easier to study than wild bees. We know more about them, but not nearly as much as we do about honey bees.

Many wild bee species like blue orchard bees (Osmia lignaria) are solitary and make their nests in natural cavities or, as shown here, at an artificial nest. Note the nest tunnels capped with mud. (Photo credit: Derek Artz, Ph.D.)

This cardboard nest tube has been opened to show an egg of the blue orchard bee (Osmia lignaria) sitting on a provision made the purple pollen of the flower Phacelia tanacetifolia. (Photo credit: Derek Artz, Ph.D.)

Solitary cavity nesting bees like Osmia or Megachile have a very different lifestyle than the social honey bee. Each female—not just a queen—makes her own nest in naturally occurring holes or artificial tunnels provided by beekeepers. She deposits an egg and a “mass provision” (a ball of pollen and nectar) to feed the larva in each brood cell. The female uses cut leaves, soil, chewed-up plant material, or plant resin to wall off each brood cell down the length of the nest tunnel. The mass provision is the only food source for the larva until it reaches adulthood. Adult bees only live four to six weeks, while larvae pupate, go into diapause over winter, and emerge as adults in the spring, ready to mate and build nests of their own.

Like honey bees, adult and larval solitary bees are exposed to pesticides when they ingest contaminated nectar or pollen. But, while honey bee larvae are fed continually by workers, cavity nesting bees get one large meal. If that meal is contaminated, there won’t be anything uncontaminated on the menu tomorrow—they have no choice but to eat it. Larval solitary bees may also be exposed to pesticides through their nest structure. If the plant material or soil used to wall off their brood cells is contaminated, the larva faces long-term exposure to the pesticide.

Adult solitary bees may also come into more contact with pesticides than honey bees because of the way they build their nests. They must cut or chew leaves and dig into or carry soil. If the soil or leaves are treated with pesticides, the adult bee will have a lot of direct contact with the chemical. Different types or formulations of pesticides may persist in soil or leaves for different lengths of time, which will greatly affect solitary bee exposure.

So, the routes of exposure to pesticides can be very different for solitary bees, but Pitts-Singer says there is not yet enough research to say if individual pesticides are more or less toxic to solitary bees than to honey bees. Nevertheless, the consequences of pesticide exposure can be much higher for solitary bees.

“A nesting season, as well as pollination service, ends for a cavity-nesting bee if she is killed or lost due to pesticide effects, while negative pesticide effects on only some of the worker bees from a honey bee colony would not end the life of the hive nor the complete function of honey bees as pollinators,” says Pitts-Singer. “In this context, the cavity-nesting bees—or any solitary bees—are more vulnerable to negative effects of pesticides.”

And Pitts-Singer points out that many of these solitary bee species are some of our best pollinators. “Honey bees are not always the best pollinator for a given crop. Bumble bees and solitary bees do better pollination jobs … for such crops as blueberries, alfalfa, cucurbits [melons, squash, and cucumber], orchard fruits, and nuts,” she says. This is due to their behavior on the flowers and the fact that, while honey bees wet pollen before stowing it for transport, many of these species carry their pollen dry. This allows it to fall off their bodies more easily and thus more efficiently fertilize flowers.

According to Pitts-Singer, the scientific community has a lot more work to do exploring the routes of exposure and toxicity of pesticides to all bees, not just honey bees. This paper starts that conversation and points researchers in the right direction.

Meredith Swett Walker is a former avian endocrinologist and wannabe entomologist. She now studies the development and behavior of two juvenile humans in the high desert of western Colorado. When she is not handling her research subjects, she writes about science and nature. Find a sampling of her work at www.magpiescicomm.com.