Are “Safe” Pesticides Harming Bees?

Use of neonic pesticides in crops has dramatically increased in the past few decades. But a growing body of research suggests these treatments have a detrimental effect on bee populations. A recent study by Anson Main, Elisabeth Webb, Keith Goyne, and Doreen Mengel, published in Agriculture, Ecosystems, and Environment, found that the presence of neonics in soil significantly decreased the number of wild bee species in the vegetative areas surrounding crop fields. These findings indicate that neonics in soil may have an effect on the survival ability of at least some species of wild bees.

Neonicotinoids (or “neonics”) are a class of pesticides that work by overstimulating the nerve cell receptors of insects, eventually causing death. These pesticides are sprayed on fields or, more commonly, coated onto crop seeds. Because neonics are water soluble, they disperse throughout the body of the plant as it grows, making the plant itself toxic to insects that attempt to ingest it. As of 2011, around 44 percent of soybean crops and 79 to 100 percent of corn crops received neonic treatments of some form.

In their study, Main et al. test the effect of neonics on both the total number of bees of any species present (bee “abundance”) and the number of bee species present (bee “richness”). To do so, they collected soil samples in neonic-treated crop fields, neonic-untreated crop fields, and the vegetative areas surrounding crop fields, which are also known as “field margins”. These samples were collected during four time periods before and after planting and crop field pesticide treatment. They also collected vegetation samples from the field margins and from bees located in and around the fields and field margins. The authors controlled for variables such as time of year of sample collection, vegetative abundance in the field margins, presence of another form of pesticide known as fungicide, and the level of neonics present in crop field soil.

In field margins of treated crop fields, 53 percent of soil samples contained neonics before pesticide treatment and 93 percent of samples contained neonics after treatment. In the field margins of untreated fields, 33 percent of soil samples contained neonics before the crop fields were treated and 56 percent of samples contained neonics after they were treated. The mean concentration of neonics in treated crop field margin soil was up to four times higher than concentration in the untreated field margins. Although neonics were found in field margin soil, they were not found in high concentrations in field margin vegetation.

The authors ran regression models on neonics’ effect on bee abundance and richness in the field margins. They found that although the presence of neonics in soil did not significantly affect bee abundance, it did significantly affect bee richness field margins. In addition, as the presence of fungicides increased, the negative effect of neonics on bee richness increased. The authors hypothesized from this result that the presence of neonics in soil negatively affects bee richness because many wild bee species nest and breed in soil.

Since about 75 percent of our food depends on insect pollination, understanding the effect of widespread neonic usage on bee and other pollinator populations is critical to protecting our food supply. It is also important that we better understand the relationship between neonics and other pesticides on pollinator health and work to design policy that accounts for possible interactional effects. Although the EPA and states like California have taken some steps to study and regulate neonic usage and effects, much remains to be done to protect pollinator health.

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Main, Anson, Elisabeth Webb, Keith Goyne, and Doreen Mengel. 2020. “Reduced Species Richness of Native Bees in Field Margins Associated with Neonicotinoid Concentrations in Non-Target Soils.” Agriculture, Ecosystems, and Environment 287, no. 1 (January): 1–10. https://doi.org/10.1016/j.agee.2019.106693.