Four pesticides commonly used on crops to kill insects and fungi also kill honeybee larvae within their hives, according to researchers from Penn State and University of Florida.

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The scientists also found that N-methyl-2-pyrrolidone (NMP) – an inert, or inactive, chemical commonly used as a pesticide additive – is highly toxic to honeybee larvae. Regulators in California and the EU have expressed concern about NMP's uses, and its potential effects on human health, and as a result companies are beginning to seek alternatives.  

Teams from the two universities chose to examine the effects of four common pesticides on bee larvae – fluvalinate (a pyrethroid used to control varroa mites in bee colonies), coumaphos (also used to control varroa), chlorothalonil (a broad-spectrum fungicide, often applied to crops in bloom when honeybees are foraging) and chlorpyrifos (an organophosphate and active ingredient in Lorsban). In the field, bees are exposed to combinations of several different chemicals. However, current EU and U.S. Environmental Protection Agency risk assessments do not account for the combinatory effects of chemicals, which some bee experts warn makes them inadequate protection for bees and other insects.

In 2012, researchers at Royal Holloway University in London conducted a study which found that, although coming into contact with certain pesticides did not kill bees outright, exposure to a combination of pesticides is more harmful to bees than exposure to any chemical on its own. Dr Nigel Raine, of Royal Holloway University said, "We are looking at the effects of multiple pesticides, not just neonicotinoids, but also pyrethroids, which is the sort of situation that bees are faced with in the field, they visit multiple crop species which may have different pesticides applied to them."

U.S. studies show combinations of pesticides kill bee larvae

The researchers reared honeybee larvae in their laboratory. They then applied the pesticides alone and in all combinations to the beebread to determine whether these insecticides and fungicides act alone or in concert to create a toxic environment for honeybee growth and development.

"We found that four of the pesticides most commonly found in beehives kill bee larvae," said Jim Frazier, professor of entomology, Penn State. "We also found that the negative effects of these pesticides are sometimes greater when the pesticides occur in combinations within the hive. Since pesticide safety is judged almost entirely on adult honeybee sensitivity to individual pesticides and also does not consider mixtures of pesticides, the risk assessment process that the Environmental Protection Agency uses should be changed."

According to Frazier, the team's previous research demonstrated that forager bees bring back to the hive an average of six different pesticides on the pollen they collect. Nurse bees use this pollen to make beebread, which they then feed to honeybee larvae.

Chris Mullin, a professor of entomology at Penn State, said the tested pesticides may directly poison honeybee larvae or they may indirectly kill them by disrupting the beneficial fungi that are essential for nurse bees to process pollen into beebread. "Chronic exposure to pesticides during the early life stage of honeybees may contribute to their inadequate nutrition or direct poisoning with a resulting impact on the survival and development of the entire bee brood," he explained.

Worryingly, two of the chemicals tested are often used by beekeepers to control Varroa mites, and are found to persist within beehives for about five years.

Elaborating on the significance of the teams' findings, Professor Mullin said, "This is the first study to report serious toxic effects on developing honeybee larvae of dietary pesticides at concentrations that currently occur in hives." The team also found that increasing amounts of NMP corresponded to increased larval mortality, even at the lowest concentration tested.

Mullin continued, "There is a growing body of research that has reported a wide range of adverse effects of inactive ingredients to human health, including enhancing pesticide toxicities across the nervous, cardiovascular, respiratory and hormone systems. The bulk of synthetic organic chemicals used and released into U.S. environments are formulation ingredients like NMP, which are generally recognized as safe. They have no mandated limits on their use and their residues remain unmonitored.

"Multi-billion pounds of these inactive ingredients overwhelm the total chemical burden from the active pesticide, drug and personal-care ingredients with which they are formulated. Among these co-formulants are surfactants and solvents of known high toxicity to fish, amphibians, honey bees and other non-target organisms. While we have found that NMP contributes to honeybee larvae mortality, the overall role of these inactive ingredients in pollinator decline remains to be determined."

UK research shows restricted pesticide affect bees' ability to gather food

Coinciding with the release of the Penn State/ Florida study, further research from the Universities of Sussex and Stirling suggests that, even at small doses, exposure to controversial neonicotinoid pesticide imidacloprid can reduce the amount of pollen collected by bumblebee colonies by 57 per cent, and that the effects last for at least a month after exposure.  

Imidacloprid was one of three neonicotinoid pesticides subjected to tight restrictions by the EU Commission last year. A 'partial ban' on the pesticide's use came into force in December.

Researchers from the Universities of Stirling and Sussex fitted bees with electronic tags so that their movements could be tracked, as part of their study. They found that many pesticide-treated bees failed to collect any pollen at all, and those that did tended to collect less than untreated bees. 

Professor Dave Goulson, the study’s senior author, commented, "It is unclear what will happen when the moratorium expires, as the agrochemical companies that produce them are in a legal dispute with the EU over [the Commission's] decision. Our new study adds to the weight of evidence for making the ban permanent."

Although honeybees, kept commercially, are often taken into account in risk assessments, populations of wild bees and other pollinating insects are examined less frequently; Professor Goulson suggested that bumblebees are particularly sensitive to the chemical agents in neonicotinoids, that their nests grow more slowly when they have been exposed to them, and that they produce fewer new queens.

The Sussex University Professor added, "Pollen is the only source of protein that bees have, and it is vital for rearing their young. Collecting it is fiddly, slow work for the bees and intoxicated bees become much worse at it. Without much pollen, nests will inevitably struggle.”"

Over the course of the past year, the EU has introduced bans or restrictions on use of four chemicals; three neonicotinoids (imidacloprid, alongside clothianidin and thiamethoxam, or TMX) and fipronil, part of the phenylpyrazole family of chemicals, over concerns that the chemicals are affecting non-target wildlife including bees.  

The UK government has backed pesticide manufacturers and said it rejects neonicotinoid science. A study commissioned by two neonicotinoid manufacturers and released while debates on the chemicals were ongoing in the EU suggested loss of neonicotinoids would cost British farming £630 million each year, though experience from France and Italy, where the chemicals were banned before December 2013, suggest there has been no discernible financial impact.

Although the practice of placing a financial value on 'ecosystem services' remains extremely controversial, a May 2012 study by the EU STEP pollinator project suggested that wild insect pollinators are worth £1.8 billion annually to farmers.