Keeping Bees Safe From Pesticides ‒ Do Current Methods Work?

Many of the most common methods for reducing the impact of pesticides on bee populations are rooted in minimal scientific evidence, according to scientists at University College Dublin. Their research was published today in the Journal of Economic Entomology.

Protecting vulnerable pollinators

While pesticides help to prevent disease carriers and harmful insects from damaging crops, these chemicals must be applied responsibly in order to avoid any detrimental effects on the environment.

One key consideration is the need to protect bees and other pollinators who might visit a treated crop. Pollination is crucial to both human-managed and natural ecosystems, and so cultivators want to lower the risk of bees coming into contact with high concentrations of pesticide. In a worst-case scenario, this bee could carry some amount of pesticide back to its hive and inadvertently kill off a large segment of the colony’s population.

Cultivators have adopted a number of “mitigation measures” that they believe will lower the risk of such colony collapse. For example, they might choose to only spray pesticide over their crops at night or to use specific nozzles on their sprayers that they believe will limit untargeted spread of the chemical.

Despite these practices having become a sort of conventional wisdom among cultivators, it is still unclear exactly how effective these approaches might be at protecting bee populations.

"It takes time, money, and effort to follow these rules, so if they are not actually helpful, they are a waste of time," said lead study author Edward Straw, a postdoctoral researcher in the School of Agriculture and Food Science at University College Dublin (UCD). "If they are helpful, though, they could be applied more widely, to protect bees further."

Scientific studies seem to be scarce

To establish the scientific grounding behind these various measures, Straw and his colleagues searched through published, peer-reviewed research for studies evaluating any kind of mitigation practices aimed at reducing the impact of pesticides on bees.

In total, the researchers found just 34 relevant scientific studies. Few mitigation measures had more than one or two studies dedicated to testing their effectiveness. The benchmarks for effectiveness also varied significantly across these studies; some tested for direct overspray while others looked for longer-term pesticide residues. Additionally, only three studies that were captured by this literature review evaluated measures that were frequently reported on pesticide labels.

The researchers also noted that of these 34 studies, 12 were dedicated to researching the potential of a fairly new method. This method relies on the inclusion of repellant additives in pesticide spray, with the idea that this will encourage bees to stay away from a recently sprayed crop.

"It is an interesting idea, but it is not yet ready to be used," said Straw. "It would need to be tested on a diversity of bee and insect species, as if it is only repellent to one or two species, all the other bees would still be exposed to the pesticide."

The other remaining studies assessed various methods for how pesticides are applied (i.e., alternatives to traditional spraying), the use of buffer zones, the removal of flowering weeds before spraying, the use of pesticides in certain weather conditions or crop stages and the effects of direct interventions such as covering or relocating local bee colonies.

Research still not focused on key areas

While it is clear from the paucity of studies that this field is under-researched, the UCD researchers also feel that a lot of this research is not being focused on the right areas. Or at least, in the areas which currently appear to hold the most promise.

"Least researched was testing on how you time a pesticide spray, be that time of day or time of year," Straw said. "There's good reason to believe that if you change when you spray, you could avoid peaks in bee activity. Yet surprisingly no one has really researched if this idea works. This is odd, as it's a very common mitigation measure and not overly hard to test."

Straw also noted that, even for the most researched method with its 12 related studies, current research is overwhelmingly weighted towards the study of honey bees. 

"Almost all research was centered around protecting honey bees. However, honey bees are a managed species that is not endangered," Straw said. "When we try to protect bees, we really want to be protecting wild, unmanaged bee species, as these are the species which are in decline."

This isn’t to say that the mitigation methods currently being used aren’t effective. Rather, the researchers argue that too much hinges on these protective measures for them to not have been thoroughly researched.

"We know that these mitigation measures are being followed," said Straw. "We just do not know if they are helpful yet."

"The main limitation is that these studies need to be big, well-funded pieces of research. To test changes to how a pesticide is applied to a crop, you need to have a crop, a pesticide sprayer, and someone licensed to spray,” he continued. “All of that is expensive and time-consuming, making it out of reach for most scientists.”

Reference: Straw EA, Stanley DA. Weak evidence base for bee protective pesticide mitigation measures. J Econ Entomol. 2023. doi: 10.1093/jee/toad118

This article is a rework of a press release issued by the Entomological Society of America. Material has been edited for length and content.