Bees Are Being Killed in Greater Numbers by Pesticide Cocktails

Bees and other pollinators are essential to food production and the function of many ecological systems. The UN estimates that 75% of the world's crops that produce fruits and seeds for human consumption rely on pollinators. There are some 20,000 species that aid in plant reproduction and form key links in healthy ecosystems.

But these pollinators are under threat. In 2019, scientists determined that nearly half of all insect species globally are in decline, and a third could become extinct by the end of the century. One in six bee species have already become regionally extinct in parts of the world. 

Stresses on Bees

It has long been understood that the multiple stressors of intensive agriculture have put pressure on pollinator populations. Intensive farming has reduced food availability for pollinators due to a reduction in pollen and nectar-rich wildflowers, as well as less biodiversity. Large-scale use of managed bees increases the threat of parasites and diseases, as does the use of pesticides, herbicides, and fungicides.

Agrochemical Cocktails Amplify Stresses

A new meta-analysis of 90 studies has now revealed that the dangers of pesticides used in combination, as opposed to individually, may be greater than previously understood. When used together, cocktails of multiple pesticides significantly increase the threat to pollinators.

The synergistic interactions between the various threats substantially amplifies the environmental effect. The results clearly showed strong evidence that pesticide cocktails using multiple agrochemicals lead to higher death rates among bees. These findings could have important implications for policy-making relating to pollinator health.

"If you have a honeybee colony exposed to one pesticide that kills 10% of the bees and another pesticide that kills another 10%, you would expect, if those effects were additive, for 20% of the bees to be killed. But a 'synergistic effect' could produce 30-40% mortality. And that's exactly what we found when we looked at the interactions," said Dr. Harry Silviter of the University of Texas, who led the study.

This analysis is notable since it covered such a large breadth of bee responses, such as foraging behavior, memory, colony reproduction, and mortality. It also compares interactions between multiple classes of stress—looking at interactions between lack of nutrition, parasites, and agrochemical stressors, as well as interactions within each class of stressor. 

The scientists looked at almost 15,000 studies, and whittled these down using strict criteria and rigorous focus to a final set of 90 studies which were used for further analysis. The results confirmed that the cocktail of agrochemicals that bees encounter in an intensively farmed environment create a greater risk than each stressor on its own. 

Implications and Recommendations

Dr. Silviter urges consideration of the interactions between chemicals, not just each chemical in isolation, when making licensing decisions and when licensing commercial formulas. He also argued that post-licensing observation is essential so that if those pesticides used in combination kill bees, that harm is recorded. 

This meta-analysis shows that environmental risk assessment schemes that assume cumulative effects of agrochemical exposure may underestimate the interactive effect of stressors on bee mortality and fail to protect the pollinators that provide key ecosystem services in sustainable agriculture. As the study concludes:

"A failure to address this and to continue to expose bees to multiple anthropogenic stressors within agriculture will result in the continued decline in bees and their pollination services, to the detriment of human and ecosystem health."

Though the synergistic effects of agrochemicals on bee mortality are clear, how exactly these arise remains to be established. More work is needed to identify the mechanism that links exposure to behavior changes or physiological changes and mortality. 

There has been a general focus on the impacts on honey bees, but there is an urgent need for more research on other pollinators, which might react differently to different stressors. Further studies must look beyond nutrition, parasites, and agrochemical interactions to examine the effects of climate change, land-use changes, pollution, and the spread of invasive species on pollinators.

It is essential that we understand and map the risks to pollinators and pollination that come from multiple combinations of pressures relating to global human-driven changes. It is crucial not just for pollinator survival, but for our own survival on this planet.