It's tempting to reach for quick-fix insecticides when aphids cut a swathe through your roses. But you may also be harming insects that are beneficial to your garden – particularly bees.
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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.
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.
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."
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.
Many pest problems can be prevented without garden sprays:
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.
Report by Luke Harrison.
Honey bees were introduced here in the mid-1800s. They've created a honey export industry worth $102 million in 2011.
Even more importantly they're a major cog in our agricultural industry, pollinating about a third of our food sources.
Federated Farmers is encouraging farmers to plant bee-friendly trees and shrubs in waterway margins, windbreaks and field edges. The federation recommends different plants for different regions.
Apiculture New Zealand also provides information on beneficial plants for bees in urban areas. Its recommendations include herbs such as basil, sage and borage, and plants and trees such as wisteria, pohutukawa and cabbage trees.
The varroa mite is considered to be the biggest threat to our honey bees. It was discovered in Auckland in 2000 but has now thought to have spread throughout the country.
Varroa feeds on both adult and developing bees. Infested colonies are weakened by a decline in the number of bees produced. Varroa may also act as a vehicle for bee viruses.
The mite has ripped into our honey bee population, reducing the number of bees in managed hives as well as wild colonies. In managed hives, it's controlled by using miticides – an example of insecticides helping rather than harming bees.
Overseas, varroa has developed resistance to many of the chemicals used to control it here. Professor Dearden says: “We desperately need to develop new chemicals that kill mites, but not bees.”