(AP Photo)

Government testing of pesticides should be expanded to ensure they don't impair the ability of insects to pollinate, causing lower crop yields, researchers at Simon Fraser University in British Columbia say in a report published this month.

Pesticides at levels previously thought to be safe impair the ability of wild bees to forage, which could result in weaker colonies and lower pollination of crop plants, said the study in the peer-reviewed journal Pest Management Science.

"Adult bees that have been exposed to a pesticide during larval development may display symptoms of poisoning that are not detected with current tests required by regulatory agencies," study leader Lora Morandin said.

Bees are important pollinators of crops. About one-third of human food in developed countries relies on pollination. Wild bees are thought to contribute significantly to this quantity.

Although many pesticides are known to be toxic to bees, toxicity testing is largely restricted to direct lethal effects on adult honeybees, if it includes bees at all.

"Testing new pesticides on some species of wild bees will aid in developing pesticides and use recommendations that minimize impact on wild bees, leading to healthier populations of bees and potentially better crop yields," Morandin said.

"In order to ensure sustainable food production, agricultural pesticides need to be safe for wild pollinators," she added.

The researchers said sub-lethal effects on honeybees could be going unnoticed, and that different bee species could also be affected.

The team tested the effects of different levels of spinosad, a natural pesticide derived from the bacteria actinomycetes, on bumble bee colony health and foraging ability. Spinosad is used in more than 30 countries including North America, Canada and the U.K. to combat common crop pests such as caterpillars and thrips.

Adult bumble bees exposed to the pesticide spinosad during larval development – at levels they could encounter in the environment – took longer to access flowers, resulting in longer handling times and lower foraging rates.

The bees also displayed trembling, which impaired their ability to land on the flowers and enter the flower tubes.