Internationally, bees are in decline. Why? There’s no obvious culprit but the finger’s been pointed at several candidates – including parasites (such as varroa), poor bee nutrition and insecticides.

International research

Insecticides – and specifically a group known as neonicotinoids – are a popular target in debates about bee health. Imidacloprid, the most widespread neonicotinoid, is licensed for use on 140 crops in 120 countries. Here, it's applied to seeds such as grass, potatoes, maize and sweetcorn before they're sowed.

Neonicotinoids work systemically to protect plants from within. Bees encounter these insecticides at trace levels in pollen and nectar. In these circumstances, the level of insecticide is "sub-lethal": it won't kill bees outright. But 2 high-profile studies released in 2012 indicated that sub-lethal levels of neonicotinoids may still harm bumblebees and honey bees.

In the first study, UK researchers fed neonicotinoid-laced sugar water to bumblebee colonies. The affected bees produced fewer queens (which means fewer new colonies in the next generation).

The second study comes from France. Researchers there found the tested neonicotinoid befuddled foraging honey bees, making it harder for them to find their way back to the hive. In unfamiliar territory, 32 percent of affected bees failed to make it home.

Reaction to the studies was mixed. Supporters of a ban on neonicotinoids said both papers backed their stance. Meanwhile, Bayer CropScience (a manufacturer of neonicotinoids) said the level of insecticide used in the studies was significantly higher than levels encountered in the field.

The European Food Safety Authority said the studies weren't necessarily representative of field conditions. But before drawing definite conclusions, it said further research was needed. It's now conducting an in-depth review of 5 neonicotinoids. The review will consider the impact of sub-lethal doses on bee survival and behaviour.

What's happening here?

5 neonicotinoids are approved for use here as pesticides or veterinary medicines. 2 of them – imidacloprid and thiamethoxam – were at the centre of the English and French studies.

Peter Fisher from Bayer New Zealand says that neonicotinoids have been used to protect Kiwi pastures and other crops for the past 20 years without any reported harmful effects on bees or bee colonies.

He says neonicotinoids are used primarily as a seed dressing on crops that aren't attractive to bees: "The highly targeted application reduces the amount used and minimises exposure to non-target organisms such as bees."

Associate Professor Peter Dearden, a genetics expert from the University of Otago, told us there was no evidence to suggest commercial pesticides were harming our bee population. But he says that's no reason to discount the possibility: “the science implies that sub-lethal neonicotinoid insecticides have detrimental impacts on bees."

Barry Foster, President of the National Beekeepers' Association (now Apiculture New Zealand), says there's a lack of hard data about the level of insecticides in hives. He argues that further research into the impact of insecticides (and fungicides) is necessary if we're to get a good handle on the health of our bees. "If we allow bee death-rates to reach levels found in other parts of the world, we're essentially playing Russian roulette with some of the biggest industries on which this country relies."

The Environmental Protection Authority hasn’t scheduled a formal reassessment of neonicotinoids. However, it told us it was "having ongoing discussions with the National Beekeepers' Association about the possibility of reviewing some or all neonicotinoids."

Garden sprays

Imidacloprid is also an active ingredient in domestic garden products such as Yates Confidor and Rose Gun Advanced. Bayer's Peter Fisher says the neonicotinoid provides "fast-acting, long-lasting" control of common pests such as aphids, thrips and scale.

Professor Dearden is surprised neonicotinoids are available in garden sprays: "These are chemicals designed for systemic treatment of plants in commercial situations, and yet I can buy them and spray them on my roses."

He believes we're too reckless with insecticides in general: "We think of insects as bad things that we need to get rid of. But many of them are essential. Is it sensible to be spraying these products when they kill all insects?"

The National Beekeepers' Association wants gardeners to avoid using garden products that contain neonicotinoids or other pesticides that harm bees. As Barry Foster points out, "insecticides by nature are not bee-friendly".

Products classified as toxic to bees under the Hazardous Substances and New Organisms Act must not be applied to areas where bees are foraging. They can't be applied to any plant or tree that's likely to be visited by bees, or, for instance, plants in bloom.

Neonicotinoids aren't alone here. Other bee-toxic substances include chlorpyrifos and diazinon (organophosphates) and carbaryl (a carbamate). The EPA is currently reassessing these substances. According to the authority, chlorpyrifos and diazinon are not approved for home garden use in either the EU or the US; carbaryl is also not approved for use in the EU.

Spray free

Many pest problems can be prevented without garden sprays:

• Keep down weeds by using a mulch and weeding frequently.
• Grow a mixture of plants to attract a balanced mixture of insects.
• Destroy heavily infested plants before the pests spread.

You can also attract pest predators (such as ladybirds) to your garden by planting flowers such as Michaelmas daisies, calendulas, Californian bluebells and sea hollies. “Ladybirds love aphids,” Professor Dearden says. “There's nothing more effective if you want to protect your roses.”

For more advice see Gardening without insecticides.

Our view

• Instead of spraying insecticides on your garden, experiment with other measures to control pests.
• If you do resort to an insecticide, make sure you follow the instructions on the label. Spray only in the evening after bees have returned to their hives and don't spray on plants that are flowering or are visited by bees.
• More research is needed to assess the impact of commercial pesticides on bumblebees and honey bees. Gaps in our knowledge could prove costly.

Report by Luke Harrison.