Honey bee hygienic behavior in relation to pathogen and parasite resistance

Abstract

Hygienic behavior is an important trait of honey bees (Apis mellifera L.) contributing to pathogen and parasite resistance, and thus social immunity. This behavior has been studied since the 1930s with an aim of understanding how mechanisms of pathogen and parasite resistance contribute to colony health. Since the parasitic mite, Varroa destructor, became the main driver of colony mortality in the United States beginning in 1987, honey bee breeding programs have focused on ways to select for mite resistance stocks. Two traits that have been selected for are general hygiene and Varroa Sensitive Hygiene (VSH). General hygiene is defined as the detection, uncapping, and removal of dead, diseased, and some mite infested brood; VSH is defined as the detection, uncapping and removal of only mite infested brood. While the behavioral patterns performed by bees selected for these traits are the same, it is unclear if the neural and genetic underpinnings of these traits are the same. One approach to untangling potential differences between VSH and general hygienic behavior is to test their behavioral responses to both pathogens and mites in the field, which is the basis of my thesis. The main question my thesis addresses is if honey bee colonies bred for the VSH-trait are specifically resistant to mites, or whether the VSH-trait extends to other brood diseases. I hypothesized that bees bred for VSH would be resistant to both pathogens and mites, while bees bred for general hygiene would be resistant only to pathogens. I challenged colonies with spores of Ascosphaera apis, a common fungal pathogen that causes chalkbrood disease in honey bee larvae. Over two years in Minnesota and one year at USDA-ARS Honey Bee Breeding, Genetics, and Physiology Research Lab in Baton Rouge, Louisiana, I found that VSH colonies demonstrate the same resistance to chalkbrood as colonies bred for general hygiene, but the VSH colonies were more resistant to Varroa mites. The findings support my hypothesis and indicate that the bees bred for the VSH-trait can detect and removed Varroa infested brood and at least one other brood disease, chalkbrood. During my experiments, some problems arose that warranted further investigation. In 2023, many of the experimental colonies had signs of idiopathic brood syndrome before the chalkbrood challenge. Using real-time RT-qPCR and Oxford Nanopore Technologies (ONT) sequencing, it was elucidated that all colonies, even ones without overt signs, were infected with moderate to high levels of sacbrood virus. In the following year, all colonies were still positive for sacbrood but did not show signs of disease in the field. This finding suggests that sacbrood may be becoming a more serious threat to honey bees in the United States. The colonies also had high rates of queen failures, mostly due to supersedures (queen replacements by the worker bees), which could not be explained by viruses or mites. Honey bee health is under threat, largely from parasites and pathogens. Increasing colony health is a top priority to the agricultural and ecological systems in the United States. Incorporating stocks that demonstrate resistance to pathogens and parasites (i.e. bees bred for the VSH-trait) help improve the health of bees, and reduce the economic burdens faced by beekeepers. For beekeepers, this study supports the use of VSH bees in operations as the bee’s ability to resist mites and diseases increases their resilience and vitality, which would help curb colony loss. Overall, it is imperative to continue researching the mechanisms underlying honey bee resistance to pathogens and parasites, as well as the changing viral landscape. It would be valuable to confirm my finding that colonies selected for the VSH-trait are resistance to other pathogens by challenging VSH colonies with another pathogen such as Paenibacillus larvae the bacterium that causes American Foulbrood. Furthermore, continued research on other biological hurdles beekeepers face such as queen supersedure would help improve colony stability and our understanding of colony dynamics.