They’re small and operate behind the scenes, but they’re critical to agriculture — and Congress is starting to notice.

Honeybees are responsible for pollinating everything from apple orchards to vegetable patches, contributing as much as $30 billion a year to U.S. agriculture. About a third of U.S. crop production relies on bees for pollination.

But these workhorses of agriculture are dying at unsustainable rates, triggering concerns in farming circles and beyond. Bees are considered an indicator species, and their decline signals broader systemic problems, environmental scientists say.

Over the past five years, roughly a third of commercial bee colonies in the United States have died each winter, according to Department of Agriculture surveys.

And while the most recent survey offered some good news — with only about 23 percent of bee colonies dying over the winter — critics say that number is questionable, because just more than 20 percent of beekeepers responded.

These over-winter die-offs have been exacerbated by, or may be related to, the puzzling syndrome known as Colony Collapse Disorder, in which bees leave their hives and never return. The disorder first captured lawmakers’ attention in 2007, prompting three subcommittee hearings in 2007 and 2008. Since then, congressional interest in the topic has waned — until recently.

A bill (HR 2692) to temporarily ban certain pesticides linked to bee deaths appears to be gaining some momentum with lawmakers, now bearing 65 co-sponsors.

In addition, the farm bill (PL 113-79) signed into law in February contains language directing the Agriculture Department to find strategies addressing bee declines, and orders the department to work with the Environmental Protection Agency to publish “guidance on enhancing pollinator health.”

In April, a House Agriculture subcommittee held a hearing on pollinators.

“The issue of pollinator protection is one that has significantly grown in its momentum on the Hill,” said Larissa Walker, policy and campaign director for the Center for Food Safety, “especially in the last year.”

The White House is also getting involved. In April, it convened a meeting on the issue, with about 60 attendees from academia to the chemical industry. Bee experts say they expect a new White House pollinator initiative to be released this month, timed to coincide with National Pollinator Week, which begins June 16.

The renewed attention on bees can be traced back to a headline-making event last June, when roughly 25,000 bees dropped dead in an Oregon parking lot. State officials attributed the mass die-off to a pesticide called Safari, which was sprayed on nearby linden trees.

The state temporarily banned Safari’s main ingredient, dinotefuran, which belongs to a pesticide class called neonicotinoids, or neonics.

The deaths prompted the Save America’s Pollinators Act, introduced by Reps. John Conyers Jr., D-Mich., and Earl Blumenauer, D-Ore., which would ban four neonicotinoids, including dinotefuran, until the EPA determines they don’t harm pollinators.

“It’s requiring what EPA should have done from the start,” Walker said. “We still don’t have the data showing there won’t be adverse impacts.”

Researchers, however, blame a range of factors for bee declines and say isolating one variable ignores the complexity of the problem.

The Agriculture Department’s lead bee researcher, Jeff Pettis, has said consensus is building that a complex set of stressors, pesticides among them, is to blame. The stressors include a lack of diversity in the agricultural landscape, leading to less forage for bees, and modern weed control, which has meant fewer weeds for bee nutrition.

While neonics seem to be linked to a bee gut pathogen, called Nosema, another pesticide class, called pyrethroids, is a larger problem, Pettis said. He maintains that a parasitic mite, Varroa destructor, is the greatest threat.

Still, to the chagrin of neonic makers, recent research makes a strong link between bee health and that class of pesticide. Most recently, a study by the Harvard School of Public Health said the likely culprit in honey bee die-offs is imidacloprid, a neonic produced by Bayer.

The European Union issued a temporary two-year ban restricting the use of three neonics last year, including imidaclropid, based on regulators’ determinations that exposure poses a “high acute” risk to bees. The European Food Safety Authority said science on the subject came mostly from industry, and urged regulators to impose a temporary ban to enable further study.

The EPA has said its own conclusions on risks posed by neonics are similar to those of EU regulators, but its mandate requires the agency to address risk management. “The EPA bases its pesticide regulatory decisions on the entire body of scientific literature,” the agency notes on its website, “including studies submitted by the registrant, journal articles and other sources of peer-reviewed data.”

The agency has taken a defensive posture, underscoring its attempts to address the issue, including new labeling requirements on some neonics and letters to the pesticide industry, sent last year.

The agency has also issued best management practices, and notes it is currently reviewing six neonics, including dinotefuran and imidacloprid

The industry has taken its own approach. Three companies — Bayer, Syngenta and Monsanto — have said they’re all working on a solution. Monsanto, which recently bought the bee research company Beeologics, held a first-ever bee summit last year, while Bayer recently opened its North American Bee Care Center in North Carolina.

“Bayer CropScience is working with state, federal and agricultural stakeholders to develop a comprehensive effort to promote bee health,” Christopher Loder, a company spokesman, said in an email. “These efforts include expanding available forage for pollinators, adoption of best management practices for agriculture and apiculture, and more focused research on protecting bees from the invasive Varroa mite.”

Critics say these efforts are an attempt to deflect attention from pesticides, and say they’re worried Congress will be swayed by the industry.

At a hearing in April held by the House Agriculture Subcommittee on Horticulture, Research, Biotechnology and Foreign Agriculture, lawmakers heard from the Agriculture Department and the pesticide industry. Testimony focused either on the Varroa mite, best management practices or on the need for neonicotinoids. “The chemical class, when used properly, is vital to the success of our industry,” said Jeff Stone, executive director of the Oregon Association of Nurseries, in his testimony.

David L. Fischer, manager of Bayer’s bee care center, underscored the complexity of the problem, saying no one factor was to blame. “Contrary to the opinion of some anti-pesticide groups, extensive research has shown these products do not represent a long-term threat to bee colonies,” Fischer said.

Whether lawmakers agree depends on whose science they believe.

“If they want to be truly informed about the bee crisis,” said Tiffany Finck-Haynes, director of the food and technology program at Friends of the Earth U.S., “they need to rely on independent scientists, not scientists from Bayer.”