A new study released this week in the Journal of Insect Science shows that neonicotinoid exposure worsens varroa mite infestations in honeybees. The study, conducted by researchers at the University of Georgia, the University of Delaware, and Auburn University, is believed to be the first to illustrate in the field how sublethal exposure to neonics increase varroa mites in honeybee colonies.
At three apiary sites in Georgia, Delaware, and Alabama, each containing 24 experimental colonies of honeybees, the bees were given controlled food intake. The colonies were “force-fed” pollen-- provided with pollen patties by the researchers and prevented from bringing foraged pollen into the hive. Colonies were fed either “clean” pollen patties or patties containing sublethal doses of clothianidin or thiamethoxam insecticide. The researchers looked at about 20 different indicators of bee colony strength and behavior, including brood size, queen survival, pollen gathering, aggression of guard bees, Varroa parasitism, mite drop, honeycomb construction, brood survival, and others.
Colonies exposed to neonicotinoids showed an average of 5.3 more mites drop in 24 hours than non-exposed colonies. Mite wash counts (also indicating heightened parasitism) were higher in exposed colonies. The research states:"There is no shortage of observational and experimental studies emphasizing the synergistic impacts of pesticide and parasite exposure on honey bee health (Goulson et al. 2015, Zee et al. 2015, Dolezal et al. 2016, Sánchez-Bayo et al. 2016, Bartlett et al. 2021a), including neonicotinoid and Varroa pressure (Straub et al. 2019, Tesovnik et al. 2019, Morfin et al. 2020). Moreover, field observations have explicitly linked neonicotinoid exposure to vulnerability to Varroa (Alburaki et al. 2015, Dively et al. 2015), with crucial laboratory studies by Annoscia et al. (2020) demonstrating how the neonicotinoid clothianidin increases Varroa reproduction rates due to reducing honey bee hemolytic immune response. Our work completes this literature body by showing that the individual-level findings of Annoscia et al. (2020) are mirrored at the colony level with contaminated pollen (see Fig. 3) and is the first (to our knowledge) demonstration of this link between neonicotinoid and Varroa parasitism using a manipulated field experiment. We emphasize that neonicotinoid exposure increasing vulnerability to Varroa parasitism and subsequent increased mite abundance is a subtly different, possibly supplementary, mechanism to simply demonstrating whether the combination of pesticide exposure and parasitism has antagonistic, additive, or synergistic impacts on honeybee health compared to either effect in isolation—possibly explaining the mixed range of outcomes reported for these interactions Straub et al. 2019, Harwood and Dolezal 2020, Bird et al. 2021, Bruckner et al. 2021, Siviter et al. 2021).....It is a notable finding that we confirm in-field that neonicotinoids can exacerbate the abundance of the parasite Varroa, which is arguably the single most severe contributor to managed honey bee losses in the United States (Traynor et al. 2020)."
This groundbreaking research further strengthens the Pollinator Stewardship Council's long-held stance that sublethal pesticide exposure is overlooked and underestimated. The PSC has consistently argued that colonies often die from secondary or opportunistic issues because sublethal exposure to neonics causes damage including a weakened immune system, leaving bees too vulnerable to effectively combat varroa mites or other pests and pathogens. In 2019, the PSC published a pamphlet titled "It's the Mites Because" addressing this critical issue. PSC hopes that this new study will bolster pollinator protection efforts, leading to the removal of neonicotinoids and a shift toward a regenerative model of agriculture.
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