Monsanto Co is hosting a "Bee Summit." Bayer AG is breaking ground on a "Bee Care Center." And Sygenta AG is funding grants for research into the accelerating demise of honeybees in the United States, where the insects pollinate fruits and vegetables that make up roughly a quarter of the American diet.

The agrichemical companies are taking these initiatives at a time when their best-selling pesticides are under fire from environmental and food activists who say the chemicals are killing off millions of bees. The companies say their pesticides are not the problem, but critics say science shows the opposite.

Die-offs of bee populations have accelerated over the last few years to a rate the U.S. government calls unsustainable. Honeybees pollinate plants that produce roughly 25 percent of the foods Americans consume, including apples, almonds, watermelons and beans, according to government reports.

Scientists, consumer groups, beekeepers and others blame the devastating rate of bee deaths on the growing use of pesticides sold by agrichemical companies to boost yields of staple crops such as corn. Monsanto, Syngenta, Bayer and other agrichemical companies say other factors such as mites are killing the bees.

"This is a difficult, high-stakes battle," said Peter Jenkins, a lawyer with the Center for Food Safety, which sued the U.S. Environmental Protection Agency (EPA) in March on behalf of a group of U.S. beekeepers and environmental and consumer groups over what they say is a lack of sound regulation of the pesticides in question. They may have a lot of money. But... we're going to win," Jenkins said.

The uproar worries officials at Bayer and Syngenta, who make the pesticides, as well as Monsanto, DuPont and other companies who used them as coatings for the seed they sell.

"Everybody is concerned by it," said Monsanto Chief Technology Officer Robert Fraley in an interview.

Monsanto plans to host a summit in June for experts from around the country to analyze the issue and discuss potential solutions. Bayer is breaking ground on a facility in North Carolina to study bee health.

The European Union said this month it would ban the class of pesticides known as neonicotinoids, or "neonics," used for corn and other crops as well as on home lawns and gardens. Similar constraints in the United States could cost manufacturers millions of dollars in sales.

"We are concerned... that the science sometimes gets trumped by the politics," said Dave Fischer, an ecotoxicologist at Bayer CropScience who is meeting with bee keepers and studying the bee deaths. He said critics "are searching for a culprit."

The companies point to a vicious insect mite as one of many factors harming the bees.

Corn Seed Treatments

But environmental scientists say evidence increasingly points to pesticides coating corn seeds as the problem, not mites. In recent years, U.S. corn seed suppliers have offered more corn seed pre-treated with types of neonic insecticides so that as the plant grows it repels harmful pests.

A study published last year by scientists at Purdue University in Indiana found evidence that planting the coated corn generates dust that contains very high levels of the neonics that can move beyond the fields where the seeds are planted. The researchers said they found the poison in the soil as well and in pollen collected by bees as food. The neonics were present on dead bees collected for study.

The study's co-author, Purdue University scientist Christian Krupke, said the issue needs more research.

Syngenta and Bayer say they are doing just that. This month both companies announced they were helping fund research grants awarded to Iowa State University and Ohio State University and a Canadian farm group to study the impact of insecticidal seed treatment dust on bee losses.

"This research will provide valuable information," Jay Overmeyer, an ecotoxicology expert at Syngenta, said in a statement.

Unsustainable Levels

A May 1 report funded by the U.S. Department of Agriculture found that nearly one in three managed honey bee colonies in the United States were lost over the winter of 2012-2013. The losses are 42 percent higher than losses seen the previous winter, the report found. Fewer bees spells higher food prices, according to the government.

U.S. officials say there is no conclusive proof that pesticides caused the bee deaths, and they cite many other factors, including the mites.

The U.S. Environmental Protection Agency said it is "working aggressively to protect bees and other pollinators from pesticide risks through regulatory, voluntary and research programs" but sees no need for a moratorium on pesticides. The EPA has said it will study the situation, but many experts say immediate action is needed.

"One third of the food supply depends on pollinators to be productive," said Doug Gurian-Sherman, a scientist at the Union of Concerned Scientists. "It's hard to say that these are definitively the cause of major bee declines. But there is a lot of data coming together that should be seriously examined."