Virginia Tech study finds sublethal glyphosate exposure reduces honeybee foraging and alters brain chemistry

A study led by researchers at Virginia Tech has found that sublethal exposure to glyphosate, the world’s most widely used herbicide, reduces honeybee foraging activity and alters the balance of biogenic amines in their brains. The research, published in the Journal of Experimental Biology, shows effects after just three days of exposure (Journal of Experimental Biology; PubMed).

Glyphosate, the active ingredient in many commercial weedkillers, kills plants by inhibiting an enzyme in the shikimate pathway required for photosynthesis. Honeybees (Apis mellifera) do not possess this enzyme, leading to long-held assumptions that the chemical poses little direct risk to them (Virginia Tech News). Previous work has linked glyphosate to impairments in learning, memory, navigation, and gut microbiota in bees, but its impact on foraging behavior and underlying brain chemistry remained less clear.

In the experiments, led by Associate Professor Margaret Couvillon and then-Ph.D. student Laura McHenry of Virginia Tech’s Department of Entomology, cohorts of honeybees from the same hives were trained to forage at artificial feeders. One feeder offered a 1 mol l⁻¹ sucrose solution; the other contained the same solution plus glyphosate at a concentration of 5 mg acid equivalent per liter. This dose is considered sublethal and environmentally relevant. Researchers monitored foraging behaviors and, in a subset of bees, recruitment behaviors. After three days, they also quantified levels of the biogenic amines octopamine, tyramine, and dopamine, along with the amino acid precursor tyrosine, in the bees’ brains (PubMed abstract).

Bees exposed to glyphosate reduced their foraging by 13.4% (P=0.022) compared with controls. Brain content of tyramine was modulated by an interaction between glyphosate treatment and the number of feeder visits (P=0.004). In treated bees, levels of octopamine correlated significantly with its precursors tyramine and tyrosine, a pattern not observed in control bees. These neurochemical shifts provide a potential proximate mechanism linking glyphosate exposure to reduced foraging performance (Journal of Experimental Biology; ScienceDaily).

“For a colony, a 13 percent reduction in foraging can be consequential,” Couvillon said in a university release. “If the entire colony was exposed, this could lead to decreased pollination effectiveness and reduced honey production, risking colony survival and long-term stability” (Virginia Tech News).

McHenry, now a postdoctoral researcher at Penn State, noted that understanding these effects can inform more strategic regulatory decisions on herbicide use to minimize harm to beneficial insects while retaining agricultural benefits (Virginia Tech News; SciTechDaily). The work was supported by the National Institute of Food and Agriculture and a Virginia Tech Department of Entomology graduate student grant.

Honeybees are major pollinators of crops and wild plants, and glyphosate is applied extensively in agricultural fields, gardens, and landscapes where bees forage. The authors emphasize the need for further research into the non-target mechanisms of action of glyphosate in insects to support better-informed pollinator protection strategies. The study did not find acute lethality at the tested dose but highlighted sublethal behavioral and physiological impacts that could accumulate at the colony level under real-world exposure conditions.