Farmers know not to apply pesticides on their fields during flowering season when bees are present. But wild bees can come into contact with pesticide residues, in some cases, for months after spraying.

Neelendra Joshi, associate professor of entomology for the Arkansas Agricultural Experiment Station, the research arm of the U of A System Division of Agriculture, said wild bees provide important pollination services for crops and native flowering plants.

Managed western honeybees, long the go-to pollinator for fruit and vegetable producers, are experiencing population declines in recent years. An Environmental Protection Agency report says about 30 percent of honeybees die off each year from colony collapse disorder.

Joshi said the decline of managed honeybee colonies makes native bees more important for agricultural production.

Rich diversity

Joshi said there are about 4,000 documented species of native bees in North America. His lab and other entomology labs in the state have documented more than 200 native species in Arkansas. He plans to conduct another survey to find as yet undocumented species.

"We may have more than 400 native bee species in Arkansas," he said.

Most people are familiar with honeybees, Joshi said, which are a European species commonly used for pollination services. "They are an excellent managed species," he said. 

People tend to think of social bees, like honeybees and various species of bumblebees, that live in colonies, Joshi said. But most bees in Arkansas are ground-nesting and tunnel-nesting species, and they are solitary bees. These include carpenter bees, leafcutting bees and mason bees. Some, like carpenter bees, dig their own tunnels for nests. Mason bees, like blue orchard bees, use existing tunnels cut into trees or structures by other insect species.

The Arkansas Cooperative Extension Service maintains information about native bees in the state.

Joshi said mason bees use soil and leaf particles to divide a tunnel into multiple chambers in which they lay eggs. Female eggs are placed in the deepest chambers and males in the chambers closest to the entrance because they hatch and emerge first. The chambers are stocked with pollen collected from flowers that the newly hatched bee larvae feed on before they pupate over winter. In the spring, they emerge as adults and dig their way through the chamber walls to leave the nest.

The males of tunneling species are usually smaller and hang around the tunnel exits after hatching, waiting for the females to emerge so they can mate. After emerging and mating, the females do the labor of collecting pollen, building chambers and laying eggs.

The pollen-collecting habits of these wild bees makes them valuable to fruit, vegetable and ornamental plant growers, Joshi said. It's also what makes them susceptible to residual pesticides.

Like solitary bees, Joshi said, honeybees are also exposed to pesticide residues, and the exposure level may vary depending on the crop and on how the pesticides are applied. Jon Zawislak, assistant professor and Arkansas Cooperative Extension Service apiculture specialist, examined pesticide risks to honeybee pollinators foraging in agricultural landscapes in Arkansas.

"Extensive sampling of bee-collected pollen throughout the growing season revealed that some insecticides were present, but at very low levels for most of the season," Zawislak said. "We were particularly interested in neonicotinoid chemicals, which have been widely blamed for honeybee losses, but found only traces of these early in the season, associated with planting, and none at all when crops were actually blooming.

"We did find residues of herbicides throughout the season in wildflower pollens, in every sample, and sometimes at very high concentrations," Zawislak said. "This can potentially have significant ecological effects on many pollinator species."

Native solution

Entomologists and beekeepers have documented declines in honeybee populations. The reasons for these declines are poorly understood, and the declines are thought to be a result of a combination of several interacting stressors affecting bee health, Joshi said. What is better understood is that the loss of honeybees threatens fruit and vegetable food sources that depend on insect pollination.

The good news, Joshi said, is that America's native bees can pick up the mantle of food pollinators. And some of them are easily managed, though with different methods than for honeybees.

Joshi said honeybees can easily be moved from site to site because when you move the queen, the colony goes with her. The hive boxes are carried from site to site as pollination season migrates from one agricultural region to another. Depending on the crop and the availability of honeybees, growers may pay as much as $200 per hive that is placed in their fields typically from one to two weeks.

As an alternative to recurring honeybee rentals, growers can place pre-cut tunnel habitats in their fields to encourage permanent native bee nesting. Farms can purchase unhatched native bees from breeders for 30 cents to $1 each and establish a permanent population on their property.

Once established, a native bee population becomes a permanent, self-sustaining presence on the farm or orchard, Joshi said.

It's unlikely that migrating honeybee pollination services will cease altogether, Joshi said. But this change in pollination practice can prove an economical alternative for growers who find honeybee pollination increasingly difficult or too expensive to obtain.

The minefield

Pesticides are an important tool for farmers who need to protect their crops from insect pests that can inflict severe damage on fruit and vegetables, Joshi said. Although growers time applications to miss flowering season when bees are present, pesticide residues present hidden dangers to bees, Joshi said.

Some pesticide residues can remain toxic in soils and organic ground covers for months after spraying, Joshi said. After application, systemic pesticides translocate from different parts of trees or other plants to blooms, where they contaminate pollen by the time bees are in the field. Bees pick up that contaminated pollen and carry it back to their nests, where they store it to feed their young upon hatching.

Pesticide residues in the soil and leaf litter can also be carried back to the nests by mason bees that are building chambers within the tunnels. Emerging adult bees are exposed to those toxins while digging out of the chambers.

Joshi said bees may also be exposed to toxic residues accumulated in water sources or while flying through adjacent fields that are being sprayed.

Health impacts

Even when the residual pesticides are too diminished to kill bees, they may have debilitating health effects. In research conducted in 2021, Joshi said some bees were reduced to crawling on the ground because they could not fly.

At a lower concentration of a systemic pesticide exposure, some bees were able to fly, but unable to locate the right flowers. Others that found their target flowers were unable to collect pollen. "They just crawled around on the bloom," Joshi said.

And still others, after collecting pollen, were unable to fly or navigate to other flowers.

Joshi's team is continuing research to determine how varying levels of exposure to residual pesticides affect wild bees. They are also studying ways to reduce or avoid exposure and how exposed bees can recover from the effects.

"A lot of research has been done for honeybees," he said. "And some has been done for bumblebees. But there has been almost no research on field-realistic residual pesticide effects on solitary bees."

The research

Ngoc Phan, a post-doctoral researcher and toxicologist in Joshi's lab, investigated the effects of pesticide exposure as a graduate student at Pennsylvania State University. There she established research protocols for conducting the research, with an eye toward changing pesticide management systems to protect bees.

"Risk is a product of exposure and hazard," Phan said. "The hazard (of pesticides) is the intrinsic toxicity, so it is unchangeable. But we can change the exposure by changing the spray program in terms of pesticide selection and spray timing."

At the Arkansas Agricultural Experiment Station, Phan is using those research protocols to expand her research to more native bee species under field-realistic conditions.

Phan looks not only at exposure to mature bees during pollination activity, but also to bee larvae exposed to pesticide residues in stored pollen and in the soil particles used to build the nest chambers. This is important for the native solitary bees because they only reproduce once during a year, and if one generation is lost, that line dies out.

For those larval bees, Phan will track the health effects of exposure through their entire life cycle, including its impact on their ability to procreate.

Phan's research has shown that larvae exposed to non-lethal levels of pesticides have shorter lifespans and are smaller than normal. "Body size matters," she said. "It is related to foraging efficiency, homing ability, dispersal and, most importantly, the ability to produce an abundance of offspring. Smaller female bees lay fewer eggs or may not be able to lay eggs at all."

Olivia Kline, a Ph.D. student in Joshi's lab, is also investigating how the bees' physiology may help them survive exposure, including how they detoxify the pesticides in their systems.

"I'm looking at how they might use enzymes or even their gut microbiome to help detoxify these pesticides," Kline said, "and then looking at how different diets might affect their health as well."

Kline said the gut microbiome refers to the communities of microorganisms, including bacteria, that exist in the intestinal systems of animals, including humans. Many of those microorganisms provide beneficial services to the gastro-intestinal system that have beneficial health effects for their hosts. These benefits include help with digestion, the immune system and with detoxification of poisons.

Scientists have investigated the gut microbiomes of honeybees and other social bee species, Kline said. But little is known about these microorganism communities in native solitary bees.

"We're doing some additional studies to look at if there are certain bacteria species found at times in solitary bees that can help them survive certain pesticides and whether these pesticides might be affecting those gut microbes."

Kline said that if they could identify bacteria that are particularly effective at detoxifying pesticides, they might be incorporated into foods that can be fed to managed bee populations.

"Especially with managed bees that are used in agriculture so that they're being used in fields that have been treated with pesticides, if they could be given a sort of probiotic treatment that could help them be more tolerant of that pesticide, and then keep up their pollination service even if they are being exposed to some potentially harmful insecticide."

Joshi said he plans to develop a solitary bee nesting system with radio frequency identification technology for his research on native bees. Micro-RFID tags will be glued to the bees to monitor how often they leave their nests, how long they stay out before returning, or if they do not return, or even if they do not leave the nest to collect pollen after exposure to different agricultural pesticides.

Education

Joshi also plans to conduct public education programs about Arkansas' native bees.

"The public knows about honeybees and bumblebees and carpenter bees," Joshi said. "But awareness about solitary wild and native bees is low."

In addition to live programs, Joshi wants to produce publications based on his ongoing surveys of native bee species in the state. "We want to raise awareness of the existence and importance of Arkansas' native bees."

To learn more about Division of Agriculture research, visit the Arkansas Agricultural Experiment Station website: aaes.uada.edu. Follow us on Twitter at @ArkAgResearch. To learn more about the Division of Agriculture, visit uada.edu. Follow us on Twitter at @AgInArk.

About the Division of Agriculture: The University of Arkansas System Division of Agriculture's mission is to strengthen agriculture, communities, and families by connecting trusted research to the adoption of best practices. Through the Agricultural Experiment Station and the Cooperative Extension Service, the Division of Agriculture conducts research and extension work within the nation's historic land grant education system. The Division of Agriculture is one of 20 entities within the University of Arkansas System. It has offices in all 75 counties in Arkansas and faculty on five system campuses. The University of Arkansas System Division of Agriculture offers all its Extension and Research programs and services without regard to race, color, sex, gender identity, sexual orientation, national origin, religion, age, disability, marital or veteran status, genetic information, or any other legally protected status, and is an Affirmative Action/Equal Opportunity Employer.