Blue orchard bees pollinate fruit trees in orchards across the United States. However, multiple farming practices pose a threat to the bees’ survival. Though both pesticides and changing landscapes can affect survival of bees in the lab, little is known about their combined effects in the wild. Recent work by Clara Stuligross and Neal Williams from the University of California Davis, USA, suggests that the harmful effects of pesticides are worsened when bee food supplies are limited.

Stuligross and Williams first tested how the number of flowers available for foraging affects the bees’ reproduction. The team built flight cages in a North Central California field for the bees to live in. Some cages had access to more flowers than the bees needed, while other cages had just enough flowers for the bees’ survival.

The duo noticed that female bees in flight cages with fewer flowers not only took longer to build their nests, but also spent less time at them and their brood were also a little smaller. These effects might be because there was only enough food available to keep the adults alive and not enough to feed the larvae or to give the bees the energy needed to tend to their young. This suggests that farming practices that reduce the number of flowers could harm bee reproduction.

Next, the researchers examined the effects of an insecticide – imidacloprid, commonly used in California fruit orchards – on the female bees’ nesting behaviours. Stuligross and Williams drenched the soil in the flight cages with the maximum amount of imidacloprid allowed in Californian orchards before letting the bees inside. Similar to bees with less food, bees exposed to imidacloprid took longer to make their nests and spent less time there. This might be because the chemical is known to slow the development of females’ ovaries, which could affect the hormones that trigger nesting behaviours and egg laying. Yet, larval bees exposed to imidacloprid were larger than those not endangered by the chemical. The researchers think that this difference in size might be because insecticide-affected bees had fewer offspring and thus had the opportunity to give each of their surviving progeny more food than their pesticide-free counterparts.

Stuligross and Williams then tested how pesticide use and the number of flowering plants might work together to affect the bees’ reproduction, and found that pesticide use and food limitation combined reduce the number of larvae by nearly 60%. If there are fewer offspring, then the number of adult bees in a colony will shrink over time and this decline is worsened by the fact that pesticides result in a population of almost all male larvae. Without females, blue orchard bees cannot reproduce, worsening the decline of the adult bee population. Taken together, these results suggest that combining harmful pesticides with fewer flowering plants could cause bee populations to decline and, eventually, go extinct.

However, the researchers only detected imidacloprid in the pollen that the bees brought home in two out of the eight flight cages. Imidacloprid might have occurred at such low levels in the pollen that the bees consumed that it was not detected in the researchers’ analyses; however, the scientists are concerned that even tiny doses could have a significant impact on bee health.

Farming practices should include ways to protect the bees that pollinate farm crops because multiple stressors can combine to have catastrophic effects on bee populations. If there are fewer blue orchard bees pollinating farmers’ trees, there will be less fruit to go round for all of us.

N. M.
Pesticide and resource stressors additively impair wild bee reproduction
Proc. R. Soc. B