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There is mounting evidence showing neonics impair bees’ abilities to forage, reproduce, and withstand pathogens and long winters.Frank Augstein/The Associated Press

Apple trees pollinated by bumblebees exposed to a pesticide linked to declines in pollinator populations produced fruit with fewer seeds, a sign of a lower-quality crop, according to a new study in the journal Nature.

The study, co-authored by University of Guelph professor Nigel Raine, found exposure to real-world doses of the widely used class of insecticide known as neonicotinoids caused bees to visit flowers less often and collect less pollen, and resulted in an apple crop that had up to 36-per-cent fewer seeds, a key measure of a crop's health. The trees visited by the 24 colonies of bumblebees in the British study were also more likely to drop their fruit prematurely, according to the study released on Wednesday.

While there is mounting evidence showing neonics impair bees' abilities to forage, reproduce, and withstand pathogens and long winters, the new study is the first to examine the pollination services provided by bees.

"We found there was an impact on individual behaviour and on the services provided by the colonies exposed to pesticides," Prof. Raine said in an interview.

The findings have broader implications for other crops and, in particular, wild plants, where seed counts are closely related to reproduction and species survival, Prof. Raine said.

The reduced crop quality also highlights the challenges faced by farmers who do not use neonics in their fields but are surrounded by those that do, said Sheila Colla, a York University professor who has studied bees for more than 10 years.

Prof. Colla, who was not involved in the study, called the results important but said it came as no shock that neonics are reducing crop yields and the effectiveness of pollinators, given the number of studies on the negative effects of neonics on the bees themselves.

The three most common types of neonics are nearing the end of a two-year ban in Europe. The Ontario government's ban on neonic usage for corn and soybeans goes into effect in 2017, unless farmers can show they cannot use other methods to control grubs and worms that feed on crops.

Previous studies, including some by Prof. Raine, have shown neonic exposure hampers bees' memories and abilities to navigate and learn, all important attributes for pollinators.

Studies have also found that neonics persist in groundwater and reduce bird populations by killing the insects they prey upon. The most comprehensive study of their impact was released last year by 29 scientists who reviewed more than 800 papers. The Task Force on Systemic Pesticides said its work led to the conclusion that even low-level exposure to neonics was having negative effects on pollinators, birds and earth worms.

The popularity of neonics has exploded in the past decade, replacing the older classes of chemicals that were found to be bad for humans and the environment.

However, neonics are believed to be a contributor to the decline in bees and other pollinators, compounding the effects of climate, viruses and lack of food over winters.

In Canada, neonics are used to grow everything from fruits and vegetables to ornamental flowers and turf. All grain corn, canola and more than half the soybean seeds are treated with neonics before planting. Growers and the chemical companies that sell the seeds and treatments say neonics are safer for humans than the compounds they replaced. The chemicals become present throughout the grain plant and protect it from hungry worms and bugs.

But exposure to the chemicals through pollen and airborne dust is believed to have contributed to the 58-per-cent bee losses suffered by Ontario beekeepers in the winter of 2013-14.