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The potentially harmful impact of “inert” ingredients in pesticides on bees may be more significant than previously understood, according to scientists.

Bees, vital for pollinating over a third of global crops and contributing significantly to agriculture’s value, confront various challenges such as pathogens, habitat loss, and climate-induced extremes. A recent study sheds light on a less-explored threat to bees: the “inert” components in pesticides. Pesticides consist of active ingredients targeting specific pests, and inert ingredients encompass various substances like emulsifiers, solvents, and fragrances. The study exposed honeybees to isolated active ingredients and the complete formulation of the fungicide Pristine, revealing a surprising outcome – the whole formulation, including inert ingredients, impaired honeybees’ memory, unlike the active ingredients alone.

This implies that the toxicity of Pristine to bees might be attributed to the inert ingredients in the formula, either due to inherent toxicity in the inerts or their interaction with the active ingredients, enhancing their toxicity. As a social scientist specializing in bee decline, I posit that, regardless of the scenario, these discoveries hold significant implications for both pesticide regulations and the well-being of bees.

What do inert ingredients entail? Inert ingredients serve various purposes, such as prolonging a pesticide’s shelf life, mitigating risks for applicators, or enhancing the efficacy of a pesticide. Some inerts, known as adjuvants, assist in adhering pesticides to plant surfaces, minimizing drift, or facilitating improved penetration of active ingredients into a plant’s surface.

The term “inert” is colloquially misleading. According to the U.S. Environmental Protection Agency (EPA), inerts are not necessarily inactive or non-toxic. In fact, users of pesticides often possess limited knowledge of how inerts function within a pesticide formulation. This knowledge gap is partly due to the distinct regulatory treatment of inerts compared to active ingredients.

Assessing bee impacts The Environmental Protection Agency (EPA) administers pesticide regulation under the Federal Insecticide, Fungicide, and Rodenticide Act (FIFRA) in the United States. To register a pesticide product for outdoor use, chemical companies must furnish reliable risk assessment data on the toxicity of active ingredients to bees. This includes results from an acute honeybee contact test.

The acute contact test monitors honeybee reactions to a pesticide application over a brief duration. It aims to determine the dose of a pesticide that will cause mortality in 50% of a honeybee group, known as the LD50. In assessing the LD50, scientists apply the pesticide to bees’ midsections and then observe the bees for signs of poisoning over a period of 48 to 96 hours.

In 2016, the EPA augmented its data prerequisites by mandating an acute honeybee oral toxicity test, involving the feeding of a chemical to adult bees. Additionally, a 21-day honeybee larval test was introduced, monitoring larval responses to an agrochemical from the egg stage until their emergence as adult bees.

These assessments aid the agency in gauging the potential risk an active ingredient might pose to honeybees, among other crucial data. Depending on the outcomes of these diverse tests, pesticides receive labels categorizing them as nontoxic, moderately toxic, or highly toxic.

Despite these rigorous evaluations, considerable uncertainties persist regarding the safety of pesticides for bees. This is especially true for pesticides exhibiting sublethal or chronic toxicities, meaning they do not cause immediate death or evident signs of poisoning but can have other significant impacts.

This knowledge gap about sublethal and chronic effects is concerning because bees can encounter repeated exposures to pesticides over extended periods through floral nectar or pollen, as well as pesticide residues accumulating in beehives. Exposure can also occur through miticides used by beekeepers to manage Varroa mites, a harmful bee parasite.

Complicating matters, symptoms of sublethal exposure are often more nuanced or take longer to manifest compared to acute or lethal toxicity. These symptoms may include altered foraging and learning abilities, reduced egg laying by the queen, wing deformities, stunted growth, or diminished colony survival. The EPA does not consistently mandate chemical companies to conduct tests that could identify these symptoms.

Inert ingredients introduce an additional layer of mystery. While the EPA reviews and approves all inert ingredients, it does not demand the same level of toxicity testing as required for active ingredients. This is because, under FIFRA, inert ingredients are shielded as trade secrets or confidential business information, with only the total percentage of inert ingredients mandated on the label, often presented collectively as “other ingredients.

Sublethal Armaments

An increasing body of evidence indicates that inert ingredients are not as benign as their name implies. For instance, exposure to two types of adjuvants – organosilicone and nonionic surfactants – can hinder honeybees’ learning capabilities. Since bees heavily depend on learning and memory functions for food gathering and hive navigation, the loss of these vital skills poses a threat to a colony’s survival.

Bumblebees are also susceptible to the effects of inerts. In a 2021 study, exposure to alcohol ethoxylates, a coformulant in the fungicide Amistar, resulted in the death of 30% of the exposed bees and induced various sublethal effects.

While certain inerts may be nontoxic when considered individually, predicting their effects when combined with active ingredients proves challenging. Research indicates that the combination of two or more agrochemicals can render them more toxic to bees than when applied independently, a phenomenon known as synergistic toxicity.

Synergism can also manifest when inerts are combined with pesticides. A separate study in 2021 demonstrated that adjuvants, which were individually nonhazardous, led to increased colony mortality when combined with insecticides.

An Enhanced Testing Approach

The accumulating evidence regarding the harmful effects of inert ingredients underscores the need for three fundamental changes to better safeguard bee health and reduce their exposure to potential stressors.

Firstly, environmental risk assessments for pesticides could encompass the complete pesticide formulation, incorporating inert ingredients, to offer a more comprehensive understanding of the pesticide’s toxicity to bees. Although this is currently practiced in some instances, making it a mandatory requirement for all outdoor applications where bees may encounter risks could enhance protection.

Secondly, product labels could list inert ingredients, allowing for independent research and risk assessment.

Thirdly, additional testing might be mandated to explore the long-term sublethal impacts of pesticides on bees, such as learning impairments. This research becomes particularly relevant for pesticides applied to blooming crops or flowers that attract bees.

Calls for changes of this nature have been ongoing since at least 2006, championed by researchers and environmental groups. However, implementing such changes requires congressional action due to federal regulations governing pesticide regulation. This poses a political challenge as it would increase the regulatory responsibilities on the chemical industry.

Nevertheless, the growing concerns surrounding bumblebee declines and the substantial annual losses experienced by beekeepers present a compelling argument for adopting a more precautionary approach to pesticide regulation. Given the mounting global population and the escalating stress on food supplies, supporting the invaluable contribution of bees to agriculture has become more crucial than ever.



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