New British research reignites concerns that pesticides are harming bees, writes Chelsea Harvey.
Acommon pesticide could hinder bumblebees’ ability to pollinate
plants, says a new study – and that could be a big problem for both
agriculture and the natural ecosystems that depend on the bees for
A paper published on Thursday in the journal Nature
tested the effects of a pesticide called thiamethoxam on the ability of
bumblebees to pollinate apple trees. The study suggested that, at
certain levels, the pesticide can have negative impacts on the bees’
pollination abilities, causing colonies to visit fewer flowers and
return with less pollen, and resulting in apples with fewer seeds. These
results could indicate poor fruit quality and a risk of decreased
agricultural output down the line, fuelling an ongoing debate over how
certain pesticides affect the environment and whether their continued
use should be permitted.
The researchers used three groups of
bees in their experiment. They exposed the first two groups to two
different “field-realistic” levels of the pesticide, or amounts that are
actually used by farmers to protect their crops – 2.4 parts per billion
and 10 parts per billion. They didn’t expose the third group to any
pesticides at all. Colonies exposed to the 10 parts per billion
experienced the strongest effects.
“One of the important things about our work is that it
highlights the importance of pollination services,” said lead author
Dara Stanley, who was a researcher at Royal Holloway University of
London at the time the study was conducted, and will soon be starting a
position at the National University of Ireland Galway.
to Stanley, this paper is among the first studies to examine the way
pesticides affect bumblebee pollination services, and not just bumblebee
biology or physiology.
“Most of the studies in the past have
focused on direct effects on the bees, both the adults and the larvae,”
said Robbin Thorp, professor emeritus and bee expert at the University
of California, Davis, who was not involved in this paper. These can
include effects on bee mortality or reproduction.
“This study now
clearly demonstrates that in addition to effects on the bees, both
direct effects and sublethal indirect effects, that these effects are
influencing their ability to pollinate plants,” Thorp said. “And they
used apple as an example of this, as an important crop.” Thiamethoxam
belongs to a class of pesticides known as neonicotinoids, which are
common in US agriculture and are used in New Zealand. Their use has been
widely debated in recent years, as research continues to emerge
suggesting that the chemicals may have negative effects on pollinators,
including bees – even as critics continue to dispute the findings. In
the light of such research, the European Union restricted the use of a
handful of neonicotinoid pesticides in recent years, including
thiamethoxam. However, its use is still permitted in the US.
was developed by agribusiness Syngenta and is an active ingredient in
several insecticide brands. The company says the insecticide has gone
through myriad safety tests and that “scientific evidence clearly shows
that bees and other pollinators can coexist safely with neonicotinoids”.
Campbell, senior environmental risk assessor at Syngenta, criticised
the new study’s conclusions in a statement to the UK Science Media
Centre, saying, “The conclusion reached in this study that thiamethoxam
impairs pollination services provided by bumblebees to apple trees is
not conclusive, it is premature, and only representative of a single
experiment conducted under artificial conditions for the apple trees
being pollinated and using unrealistically exposed bumble- bees.”
also made a number of other technical criticisms. For one thing, he
highlighted that certain other aspects of the bees’ pollination services
were not affected by the treatment. Although treated colonies were less
active pollinators and were associated with fruits that contained fewer
seeds, they did not seem to make a significant difference in whether
apple trees produced fruit or how many fruits they produced.
the authors maintain in the paper that it’s unclear whether
pesticide-exposed colonies’ altered behaviour may affect other plants
And they also noted that the reduced seed production
observed in this study could be a serious problem for agriculture,
indicating a lower fruit crop quality with the potential to reduce
So far, most studies on neonicotinoids and
pesticides have focused on honeybees, said Thorp. This is largely a
reaction to widespread honeybee losses to colony collapse disorder,
which causes colonies to suddenly abandon their hives. The phenomenon is
still poorly understood, but is thought to be the result of a complex
combination of factors, including habitat degradation, disease – and
But it’s important not to forget about bumblebees,
Thorp cautions. While honeybees are generally considered a managed
species in the US, “wild bees are kind of the under-appreciated resource
that we have out there,” he said.
“They’re extremely important
in pollination of our native ecosystems, and many of them … are
important contributors to crop pollination.”
said, if honeybees continue to decline, other pollinators – including
the bumblebee – will become increasingly important to the survival of
plants that rely on pollination for their reproduction.
study has some worrying implications for a valuable wild pollinator. But
Stanley cautions that the study “only looked at one pollinator species
and one crop species”, so it’s impossible to say for sure that the
effects would be the same with other bees or other plants – or other
pesticides, for that matter.
The study also doesn’t provide a
clear explanation for why, exactly, thiamethoxam caused the bees’
pollination services to suffer.
In a mysterious twist, the
researchers found that when bumblebees were released into the field one
by one – not as a whole colony – the bees exposed to pesticides were
actually more active than the control group, visiting more flowers,
spending longer periods of time foraging and switching between apple
trees more frequently.
“If you’re having bees visiting more
flowers and [being] more active, you would presume that they’re probably
going to deliver better pollination services,” Stanley said. It was
only when whole colonies were released at once that the bees’ collective
pollination services suffered.
“When we opened up the colonies,
less bees were actually coming out of the colony,” Stanley said. “The
colony overall was less active in sending out foragers.”
phenomenon raises questions about the effects of pesticides on
individual bee behaviour versus on colony behaviour as a whole – and the
reasons for the discrepancy are not completely clear. The study opens
the door for future research on the mechanisms that affect bee
behaviour, Thorp said.
Even if the study can’t be generalised to
all bees or all crops, it raises more questions in the ongoing debate
over pesticide use in the US.
“I think it’s kind of a wake-up
call to growers that they ought to be paying more attention to what
they’re putting on their crops,” Thorp said. “Because it’s coming right
out of their pocket as well if they’re damaging the ability of
pollinators that they rely on to pollinate their crops.”