Sharma, Charu2025-08-292025-08-292025-08-162025-08-16http://hdl.handle.net/1993/39274Honey bee colonies (Apis mellifera) continue to suffer high overwintering losses, with Varroa destructor mites and viruses like deformed wing virus (DWV) recognized as major contributing factors. Effective and timely fall treatments are necessary to reduce Varroa populations before winter without compromising colony health. This study examined the efficacy of oxalic acid (OA) treatments for controlling Varroa mites and their impacts on colony performance and virus dynamics. In the first set of experiments, colonies were treated once in early fall with 3.5% OA trickle, and monitored for changes in mite levels, virus titers, protein content, colony weight, and survival. Although daily mite mortality increased post-treatment, final fall mite abundance was not significantly reduced. However, surviving colonies treated with OA showed lower levels of DWV-A and sac brood virus (SBV) viruses in the spring. Cage experiments revealed that OA alone increased DWV-B levels in the absence of mites. Although it may have had some suppressing impact on block queen cell virus (BQCV), the buffer used for virus inoculation also showed independent virus-suppressing effects. In the second set of field experiments, we evaluated the frequency, timing and interval between OA vapor applications in late fall and colony performance under different wintering environments. Colonies received zero, one, or four treatments at 2-5 day variable intervals in November in one experiment, and were treated four times either at 5-day or 12-day intervals spanning from September to November or concentrated at the end of the treatment window. Colonies were wintered either indoors or outdoors, and assessed for mite levels, cluster size, colony weight, survival, total protein, and viral load. Colonies treated at 5-day intervals showed better survival then controls for the colonies with low initial mites. While both one and four applications caused mite mortality, only four applications significantly reduced Varroa populations by spring as measured by alcohol wash. Treatments did not directly affect protein or virus levels, but colonies with low initial mite loads had reduced DWV-B levels when treated. Indoor wintered colonies had consistently lower DWV levels than outdoor-wintered colonies. Overall, repeated OA vapor treatments in late fall helped reduce Varroa loads and supported colony survival, but their effectiveness depended heavily on initial infestation levels and treatment timing. These findings highlight both the potential and limitations of OA as a late-season Varroa control strategy.engOxalic AcidVarroa MitesDWV