Bees exposed to climate change are more sensitive to pesticides

Bee populations are exposed to multiple stressors, including land-use
change, biological invasions, climate change and pesticide exposure, that
may interact synergistically. We analyze the combined effects of climate
warming and sublethal insecticide exposure in the solitary bee Osmia
cornuta. Previous Osmia studies show that warm wintering temperatures
cause body weight loss, lipid consumption and fat body depletion. Because
the fat body plays a key role in xenobiotic detoxification, we expected
that bees exposed to climate warming scenarios would be more sensitive to
pesticides. We exposed O. cornuta females to three wintering
treatments: current scenario (2007-2012 temperatures), near-future
(2021-2050 projected temperatures), and distant-future (2051-2080). Upon
emergence in spring, bees were orally exposed to three sublethal doses of
an insecticide (Closer, a.i. sulfoxaflor; 0, 4.55 and 11.64 ng a.i./bee).
We measured the combined effects of wintering and insecticide exposure on
phototactic response, syrup consumption, and longevity. Wintering
treatment by itself did not affect winter mortality, but body weight loss
increased with increasing wintering temperatures. Similarly, wintering
treatment by itself hardly influenced phototactic response or syrup
consumption. However, bees wintered at the warmest temperatures had
shorter longevity, a strong fecundity predictor in Osmia. Insecticide
exposure, especially at the high dose, impaired the ability of bees to
respond to light, and resulted in reduced syrup consumption and longevity.
The combination of the warmest winter and the high insecticide dose
resulted in a 70% longevity decrease. Smaller bees, resulting from smaller
pollen-nectar provisions, had shorter longevity suggesting nutritional
stress may further compromise fecundity in O. cornuta. Our results show a
synergistic interaction between two major drivers of bee declines, and
indicate that bees will become more sensitive to pesticides under the
current global warming scenario. Our findings have important implications
for pesticide regulation and underscore the need to consider multiple
stressors to understand bee declines.