College of Food, Agricultural and Natural Resource Sciences
Killer in a Bottle?
Household insecticides may play a role in declining bee populationsBy Becky Beyers
Vera Krischik and her lab in the Department of Entomology are exploring a different avenue: how neonicotinoids, a group of insecticides commonly used in urban gardens and forests as well as in agricultural fields, might be making bees less resistant to the parasites and pathogens that researchers now believe are likely causes of Colony Collapse Disorder.
Infesting the systemAbout two-thirds of the world’s crops rely on bees and other pollinators; if the bees are gone, so are the fruits, vegetables and other plant-based foods they help create.
Other insects are less beneficial, however, so farmers and gardeners turn to insecticides to protect their crops. The first neonicotinyl insecticide, imidacloprid, came on the market in the 1990s. Since then, three more—thiamethoxam, clothianidin and dinotefuran—have been registered for use. All have similar toxicity to bees and all are commonly used in Minnesota: in 2009, more than 8 tons of insecticides primarily using imidacloprid and nearly 10 tons of clothianidin were used on farms and nearly another ton of imidacloprid was used on the state’s landscapes. When it was first introduced, imidacloprid was considered a breakthrough because it wasn’t harmful to humans or mammals, and they’re now extremely popular. “Most people didn’t realize that this is a systemic insecticide,” Krischik says.
Imidacloprid is commonly sold to farmers as Admire, Provado and Gaucho and to businesses and home gardeners under names Merit, Marathon, Bayer Advanced Flower and Shrub, Bayer Tree and Shrub Protect, Bayer Complete Insect Killer for Turf, Krischik says. When neonicotinoids are applied to a field or garden, the chemicals are absorbed through the plants’ vascular system, which makes the entire plant toxic to insects. That works well for unwanted leaf-feeding pests, but the chemicals that kill unwanted insects also go into the nectar and pollen that pollinators need. The toxic effects to bees can last for several months to years in pollen and nectar from just one application. When the insecticide is applied to soil it can last for years, adding to the imidacloprid reservoir in the plant.
Honeybees, bumblebees and solitary bees all respond to the insecticides, which are also commonly used for controlling emerald ash borer and Japanese beetles. Less-than-lethal exposures can cause honeybees to have problems flying and finding their way back to the hive, lose their sense of taste and have more difficulty learning new tasks, according to a group of scientists called the Xerces Society for Invertebrate Conservation, who summarized some of the existing research on bees and neonicotinoids this year.
Neonicotinoids are banned from use on corn and canola seed in both France and Germany, and after two studies last spring that made a strong connection between their use and declining bee health, support is growing in the United States for stronger regulation both in agricultural and home use. Early this year, beekeepers from Minnesota and California petitioned the Environmental Protection Agency to immediately suspend sales of neonicotinoid insecticides, but in July the EPA denied the request and said it will review the insecticides’ effects, a process that could take until 2018. More recently, members of Congress have asked the agency to speed up the process. Krischik says more research is needed.
Digging in to home useFor years, beekeepers, scientists and chemical companies have argued about the amount of insecticide used as a seed treatment. The imidacloprid seed treatment Gaucho permits 0.675 mg AI (active ingredient) imidacloprid/seed for corn and 0.11 mg/seed for canola, Krischik says. But the greenhouse rate used on perennial landscape plants allows for 300 mg AI/ per 3 gallons—a 444 times higher rate on landscape plants than on field corn. Consequently, greenhouse and urban landscapes use higher concentrations of imidacloprid, and to make matters worse, gardeners often disregard the products’ instructions and reapply the insecticide or use it at peak flowering time, giving home use much greater potential to affect bees and other beneficial insects.
It gets even worse with trees, Krischik says. A surface soil application of imidacloprid in agriculture is limited to about 4mg/sq ft. But when used to stop emerald ash borers or Japanese beetles, the allowable application for a similar area is 1,675 times greater. These higher application rates in urban areas have huge implications for movement of imidacloprid into flowers and effects on bees and beneficial insects.
Addressing the controversySo how much imidacloprid kills a bee? Bayer says 20 parts per billion will alter behavior and 170 ppb in food will kill a bee while it is drinking.
So how much imidacloprid ends up in nectar and pollen from a standard application dose in agriculture or landscapes? Is it enough to kill pollinators?
In landscapes, a standard 300mg dose to a 3 gallon pot results in 1,600 ppb in milkweed nectar, around 800 times more than from a sunflower seed treatment, according to Krischik. That concentration can cause high mortality in beneficial insects other than bees such as lady beetles or lacewings. Krischik’s lab is the first to study how the higher concentrations affect bees.
By partnering with a local golf course that had applied a surface drench of imidacloprid to linden trees, Krischik found that 1 month after application the linden leaves had around 100 ppb imidacloprid—which had the desired effect of killing Japanese beetles—but 12,865 ppb imidacloprid remained in the soil under the linden trees. Any flowering plant growing under the linden will pick up the imidacloprid and move it to pollen and nectar.
“Our data demonstrates that a homeowner application to rose bushes results in 812 ppb, which will kill any insect eating the pollen,” Krischik says. “We have seen in our trials that bees die on the flowers while feeding on a mint treated with the standard dose or a second application of imidacloprid, which is permitted. We showed that a rose bush will kill leaf-feeding Japanese beetles for 3 years after one standard consumer application of imidacloprid. However, you can apply it many times a summer.”
Because the neonicotinyl insecticides (imidacloprid, thiamethoxam, clothianidin and dinotefuran) have a similar lethal effect on bees and other beneficial insects, the problem is enormous, Krischik says. Her lab showed that 4 species of lady beetles, a parasitic wasp, a predatory green lacewing and bumblebees all die at the standard application rate of imidacloprid. In bumblebee colonies, queen mortality, colony weight and stored nectar all are affected in direct proportion to daily dosages. Imidacloprid also reduces bumblebee memory in her lab studies and stops bee foraging.
Unfortunately, she says, research about neonicotinoids’ effects and consumer education on how to use them safely aren’t getting the funding they need from federal agencies. Krischik was awarded a 2009 state grant to study the effects of imidacloprid on bees; a second grant in 2010 that would have allowed her to investigate the uptake of ash and linden trees of imidacloprid and potential nontarget effects on bees and beneficial insects was awarded, but revoked six months later by state legislators who didn’t see the value in understanding consequences of the high amount of imidacloprid used in urban landscapes. “Wow, it is frustrating,” Krischik says.
The Xerces Society report published earlier this year also made the case for continued long-term independent research, noting a long list of questions about bees and neonicotinoids that no one has yet studied in depth.
Bees are too important to be ignored, Krischik says. “If you don’t support managing them in the appropriate way, you’ll lose part of your diet, a lot of the anti-oxidants found in fruits… People rally for polar bears 3,000 miles away and that is great. We need people to rally for bees in their own back yard.”