Popular pesticides linked to declines in bee populations appear to harm bumblebees by impairing their ability to learn how to gather food, new research shows.

Young adult bumblebees normally take some time to learn how to land on flowers, collect pollen and pack it onto their legs so they can carry it back to their hive, said Nigel Raine, a professor and bee researcher at the University of Guelph.

While bees usually come back to the hive with more and more pollen as they become more experienced foragers, a new study conducted by Raine and Imperial College biologist Richard Gill shows that isn't the case for bees exposed to neonicotinoid pesticides.

"They started off worse than control, unexposed individuals and they never got any better — and in some cases actually their performance deteriorated," Raine said in an interview. "This kind of suggests that something is not allowing them to continue the normal process of learning how to handle flowers and collect pollen."

The study, published today in the British Ecological Society's journal Functional Ecology, also found that the effects were amplified when colonies were exposed to a second kind of pesticide, pyrethroid, along with the neonicotinoid. Colonies exposed to both pesticides were more prone to fail and lose all their bees.

The results suggests bumblebees are quite sensitive to neonicotinoids, Raine said. He added that they are likely more sensitive than honeybees, since their colonies are so much smaller — hundreds of individuals rather than the tens of thousands seen in honeybee colonies.

bumblebee with RFID tag

A bumblebee wears an RFID tag glued to its back. The tag was scanned as the bee entered and left the hive, so the researchers could keep track of individual bees. (Oscar Ramos-Rodriguez)

Neonicotinoid pesticides are commonly coated on agricultural seeds for crops such as corn and canola to protect the plants from insect pests such as aphids. The chemicals act as a neurotoxin toward insects. According to the Canola Council of Canada, almost all of the 21 million acres of canola planted in Canada are treated with neonicotinoids. Bayer Crop Sciences, which makes the pesticide, reported that in 2012 that 90 per cent of U.S. corn was also treated.

A number of studies have shown that bees exposed to the pesticides have smaller colonies, often fail to return to their hives and may have trouble navigating.

That has led to a partial, temporary ban on the pesticides in Europe, and a recent call for a wider ban from some scientists. In Canada, Ontario announced this week that it is moving towards restricting the use of neonicotinoids.

The goal of Raine's and Gill's study was to see how the pesticides affected the foraging behaviour of individual bees in a situation that mimicked, as closely as possible, conditions in the wild.

In order to do that, the researchers allowed 40 new, young colonies of bees to forage in the wild for four weeks at a time, but later in the growing season when their exposure to pesticides from farm fields should normally be very low.

They offered different colonies of bees small amounts of pesticide-laced sugar water containing:

  • A neonicotinoid pesticide called imidacloprid, or
  • A pyrethroid pesticide called lambda cyhalothrin, or
  • A mixture of the two pesticides.

The pesticide levels were designed to be similar to what bees would be exposed to from pesticide-treated crops. A control group of 10 colonies were offered plain sugar water.

The researchers found that the bees chose to drink all the sugar water, regardless of whether it contained pesticides or not.

Different flowers

Bees were identified with RFID tags – the same technology used by many companies such as Amazon to track the movement of packages. The tags, which weigh two micrograms, and did not affect the bees' ability to fly, were glued to each bee. RFID readers placed at the entrance to the hive recorded when each bee passed by, allowing researchers to track each individual as it entered and left the hive.

Observers recorded the size of the load of pollen each was carrying, as well as the type of flower it was from, based on its colour. The observation was not double-blind, but Raine said any bias should be minimized by the fact that the three observers were constantly rotated from hive to hive.

Bees exposed to the pyrethroid pesticide had higher mortality, but didn't seem to experience any long-term effects.

Bees exposed to neonicotinoids, however, had changes in behaviour. They didn't just bring back less pollen compared to unexposed bees with the same level of experience. They also chose different flowers to collect pollen from.

"The effects were really striking," Raine said.

The researchers weren't sure whether the pesticides were influencing bees' ability to choose flowers or whether they were simply choosing flowers that were easier to land on and collect pollen from.

The study found neonicotinoid-exposed colonies initially sent out more foragers, apparently to compensate for the smaller amounts of pollen collected.  However, by the end of the experiment, their forager numbers had dropped,  the foragers were foraging for longer periods of time, and they were increasingly coming back with no pollen. 

"The acute and chronically impaired pollen foraging performance induced by neonicotinoid exposure shown in this study has implications for colony growth and survival," the researchers wrote.

A previously published analysis of the study, focused on the effects on the colony rather than the individuals, showed that colonies exposed to neonicotinoids had 25 per cent fewer workers and were significantly smaller at the end of the study period, Raine said.

"What this and previous work from my lab shows is that these kinds of small impacts on individuals are feeding up to colony level impacts," he added.

Raine thinks small, long-term effects of pesticides like this should be included in tests to determine whether they should be approved or re-approved.