Farmers and ranchers must balance the use of land to produce food with maintaining natural environmental processes. This is evident in how insecticides, needed to maintain or increase crop yield, affect bee populations, which are required for crop pollination. Insecticides are a crucial tool for ag producers. Knowing this, researchers continue to investigate methods to control pests while protecting pollinators.
In North America, there are two types of bees: solitary and social. Honeybees are social, with the colony containing a queen and nonreproductive workers. A colony will last for years, with adults storing food for use during the cold season for progressive feeding. With solitary bees — alfalfa leaf-cutting and blue orchard — each female collects nest-building material and food, and lays eggs. She dies at the end of the season, and her progeny hatch to feed on the stored mass provisions before emerging from the nest.
With the honeybee population decline in the United States, ag producers are looking to leaf-cutting and blue orchard bees to fill the pollinator gap. Leaf-cutting bees were brought to the U.S. accidentally from Eurasia in the late 1930s, and later they were intentionally propagated in response to a drastic drop in alfalfa seed production (the alfalfa plant was imported from Europe).
TWO SPECIES: This carrot blossom has two kinds of bees at work: two leaf-cutting bees and a honeybee. Leaf-cutting bees are becoming more popular for pollination in ag crops.
“Honeybees don’t really like alfalfa flowers,” explains Theresa Pitts Singer, a Utah-based research entomologist with the USDA Agriculture Research Service, “because the flower mechanism pops open — called ‘tripping the flower.’ Honeybees are like, ‘I don’t like being slapped around when I visit this flower.’ To avoid being hit, honeybees go to the side of the flower and just suck the nectar out of it. But they don’t pop it open, because the bee needs to approach from the front to do so.
“Leaf-cutting bees don't mind the mechanism at all. They want to pop it open, because they will collect both nectar and pollen. The way leaf-cutting bees handle the flowers is way more effective than a honeybee in producing seed, because it increases pollination.”
Blue orchard bees are native to North America, and nest and forage in a similar fashion to leaf-cutting bees.
“Leaf-cutting and blue orchard bees are great pollinators,” Pitts Singer says. “But they are solitary bees, so if a female dies from insecticides, disease or predators, her reproduction will end. Whatever nest cells she’s already made might survive, but cells she had the potential to create will, of course, never occur. If many reproducing solitary females in a locality are simultaneously killed and, therefore, leave behind few or no offspring, then the local population may have a reduced number of bees the following season.
“Whereas for honeybees, they will produce more queens as necessary to start a new colony, or to replace a queen who has died or is not effective.”
The insecticide effect
There are several types of insecticides in use today. Neonicotinoids are a class of insecticides introduced in the mid-1990s. These are less toxic to humans and mammals than previously used chemicals. Unfortunately, neonicotinoids are highly toxic to all insects, including bees.
Ramesh Sagili, an associate professor of apiculture at Oregon State University, conducts research on honeybee health and nutrition. In one of his studies, he looks at the effects of insecticides and fungicides on honeybee colony growth and survival. In this field study, colonies were fed field-realistic concentrations of a neonicotinoid insecticide and a commonly used fungicide.
Preliminary results suggest the field-realistic concentrations used did not significantly affect colony health. Effects of pesticides can have different level of effects on different types of bees. Honeybee colonies are relatively resilient to loss of individual bees due to pesticide exposure because of their colony structure (the greater number of worker bees living together may act as a buffer), but pesticide exposure could seriously affect the reproduction of solitary bees.
Sagili and his team also measure the amount of neonicotinoids in the pollen and nectar collected by honeybee hives.
“While foraging in the field, if bees encounter higher concentrations of neonicotinoids or are directly exposed to neonicotinoids, it could be lethal to them,” Sagili says. “We are still working on the realistic exposure of bees to insecticides, As bees in nature, outside of our controlled experiments, pick up more than just one pesticide as they forage across large areas of crops, it is important to understand the combination of exposures.”
Pitts Singer finds in her research that blue orchard bees, which pack soil into cavities for nesting, may be affected by pesticide residue in soil. “When the blue orchard bee visits in the springtime,” she explains, “the female could collect and ingest the chemical from moist soil. It could be something sprayed to protect developing fruits or nuts at non-bloom times, when bees were not present. It was still sitting there in the soil from earlier in the year.”
In addition to insecticides, fungicides also affect bees. “We’ve found that fungicides make blue orchard and leaf-cutting bees confused,” Pitts Singer explains. “After exposure, the bees aren’t able to recognize which nest cavity is their own. There’s something weird that happens to them, which physically we don't yet understand.”
Some studies show that certain insecticides used with a fungicide are worse for bees because of a yet-unidentified synergism. The interaction is more lethal than the single application of insecticide or fungicide.
MAKING MORE BEES: These blue orchard bees, shown here mating, are a solitary variety. They can be effective pollinators, but their habits can make them susceptible to insecticide treatments.
Insecticides are an easy scapegoat for the decline in bee populations. While it is a controllable effect on bees, pests and pathogens also affect bee health. Unfortunately, disease-carrying mites develop resistance to chemical controls quickly, and beekeepers currently have only a few tools to control mites and maintain healthy bee populations. Ag producers can help pollinators by controlling the type of insecticides and timing of their application on crops.
“Reducing the exposure of insecticides to bees is more important than just focusing on totally restricting insecticide sprays,” Sagili says. “Bees have to be put in the field for pollination, and farmers must deal with the pest problems to grow their crops. There should be a balance.”
To inform applicators, the U.S. EPA requires companies to report the length of residual toxicities on the product labels: how long the chemical residues will stay on the crop and remain lethal to pollinators. But for many years, the EPA didn’t enforce this.
“The EPA is now paying more attention to this,” Sagili says. “A label might say, ‘The residual toxicity of compound A is eight hours.’ So the beekeepers are not supposed to put their bees near those fields until at least eight hours after the insecticide is sprayed.”
Moving bees away from a field for spraying isn’t advised for bees that nest in cavities, like leaf-cutting and blue orchard. “After they have emerged and begun to nest,” Pitts Singer says, “it’s a big disturbance. If their nesting site is moved, the bees get lost and don’t return or don’t continue to nest. You may lose the bees by spraying while they’re there, but probably don’t gain much by moving them.”
Insecticide labels also contain a warning if the insecticide shouldn’t be sprayed while bees forage. “These insecticides should be applied in the evening or late at night, when all the bees have returned home,” Sagili advises. “Or if that’s not possible, then the applications should be done early in the morning, before the bees return to forage on the crops.”
Honeybees will begin to forage at 55 degrees F, and bumblebees will fly at lower temperatures of 45 degrees F. Leaf-cutting bees prefer summer temperatures above 70 degrees F, while blue orchard bees will fly at temperatures lower than 55 degrees F in the spring.
“Honeybee foraging is dependent on the sunlight,” Sagili says. “Sometimes in summer, they forage even until 8 p.m. That can be the tricky part, when farmers want to spray at more convenient times.”
Producers can also select insecticides that have fewer toxic effects on bees. “All of these measures will help keep bee populations robust,” Sagili says. “If a farmer rents bees or is totally dependent on bees for pollination, they want healthy and robust hives that will do a good job of pollination.”
Researchers also find that monoculture cropping systems affect bee health because of poor nutrition. Additionally, if bees do not have a diverse diet, it is harder for them to survive when exposed to insecticides. If a farmer has a hundred acres of one crop, even a half-acre of flowering plants that are good for bees is very valuable.
For recommended plants by state and region, the Xerces Society has lists of native plants valuable to pollinators.
Hemken writes from Lander, Wyo.