Cold storage as part of a Varroa management strategy: Effects on honey bee colony performance, mite levels and stress biomarkers

Abstract

Placing honey bee colonies in cold storage has been proposed as a way to induce a pause in brood production as part of a Varroa mite treatment plan. Here, we exposed colonies to combinations of with or without a late summer-early fall cold storage period and with or without a subsequent miticide application, and then measured the effects of those treatments on colony-level variables (i.e. colony size, Varroa density, and hive temperature) and pooled individual-level variables that are associated with nutritional and stress responses. Colonies were assessed before and after cold storage, and post winter, and the experiment was conducted twice. The cold storage plus miticide treatment strategy did not have a major impact on colony health. Cold storage did induce a brood pause: little brood was present after cold storage, and hive temperature data indicated that most or all brood had emerged after about two weeks. However, Varroa levels in February after treatments were not significantly affected by treatment, and we found comparatively few effects on either the colony or individual level because of the cold storage or miticide application. The two experiments were very different: rainfall, and bee forage availability, were much higher the second year, and colonies were over 2.5 times larger on average the second year compared to the first, both in terms of adult bee mass and brood area. Expression levels of nutrition and stress response genes were also significantly higher the second year. Colony survivorship from September to February was 42% the first year and 60% the second year. The results indicate that limited cold storage itself would likely have little impact on most colony- and individual measures of health, but for such a strategy to succeed levels of stressors, such as Varroa, may also need to be low.