Introduction

In North America, antimicrobials have been used for decades as part of integrated pest management of infectious disease in honey bees. In contrast, in the European Union, it is illegal to treat honey bee colonies with antimicrobials. Since 2017 in the United States, and 2018 in Canada, veterinarians have been federally mandated to oversee prescribing of medically important antimicrobials to beekeepers for treatment of their colonies. Considering that many veterinarians do not receive education in honey bee medicine and disease, this chapter provides guidance for veterinarians who wish to better serve their beekeeper clientele.

Honey Bee Biology

A basic understanding of honey bee biology is essential to understanding pathogen transmission and disease control in honey bees. Honey bees are social insects. Under normal circumstances, a honey bee colony consists of three castes: a queen, drones, and worker bees. Both queen and workers are diploid females and have 32 chromosomes, while drones are haploid males with 16 chromosomes.

The honey bee queen is the only mated, egg‐laying female with a developed reproductive tract in the colony. The queen can lay ~1500 eggs per day that develop into workers, drones or queens (if needed). Besides egg laying, the other major role of the queen is production of multiple pheromones that maintain colony homeostasis and productivity (Bortolotti and Costa, 2014). It is common for beekeepers to replace the queen in their colonies every two years to ensure vigorous egg and pheromone production, which in turn promotes colony health and vitality.

Drones are male bees whose main function is to mate with virgin queens, after which the drones die. During the summer, drones often drift from colony to colony, providing an opportunity for pathogen transmission among colonies (Table 40.1). During the fall, surviving drones are expelled from the colony in preparation for winter.

Worker honey bees are sterile females that have hypoplastic ovaries. Worker honey bees are responsible for all activities in the colony except reproduction. The honey bee workers are divided into two major types based on seasonality and tasks: summer and winter workers.

Summer workers live approximately six weeks. The first half of the life of an adult summer worker (~first three weeks) will be dedicated to housekeeping tasks in the hive, including cleaning brood cells; nursing brood; attending to and feeding the queen; building wax comb and sealing brood and honey; processing nectar into honey; processing pollen into bee bread; removing dead brood and adults from the hive; and ventilating and defending the colony. Many of these behaviors facilitate horizontal transmission of pathogens within a colony (Table 40.1). The second half of an adult summer worker’s life (~lasts three weeks) is dedicated to field work which consists of collecting nectar, pollen, water, and propolis. The most intense foraging occurs within two miles, but the forager bees can fly twice as far if nectar or pollen are not available closer. Drifting of honey bee workers from hive to hive is not as common as in drones but it does occur.

Winter workers emerge at the end of summer and the beginning of fall. Their life span is 150 days on average (Currie et al., 2015). The main role of winter worker bees is to ensure that the queen survives until the spring season begins and a new population of bees is generated. The winter bees usually do not defecate within the hive; instead, they accumulate waste within the rectum which can distend and occupy a substantial portion of the abdominal cavity. On sunny winter days when the temperature is above 0° C, bees will fly out of the hive for a short period to empty the waste from their rectum (cleansing flights).

There are seasonal variations in the total number of worker bees per colony. To maximize honey production, the highest number of worker bees in a colony (50,000–70,000 bees) should coincide with the major blooming season of nectar‐producing plants. In contrast, there are only 15,000–30,000 bees in a colony during the late winter.

Developmental stages of brood are the same for all three castes, even though the total development time is different for each caste. From fertilized eggs, diploid larvae emerge three days after oviposition and will develop into workers or queens depending on the larval food they are provided by nursing bees. Nonfertilized eggs will develop into haploid drones. After hatching, larvae are attended and nursed by many young nursing bees visiting each larva many times per day. After completion of the larval stage (5–6 days), the comb cells containing larvae are sealed with wax (capped) and prepupal and pupal development occurs for 8–15 days, depending on caste. Eclosion (emergence) of the imago (adult) stage occurs at day 16, 21, and 24 post oviposition for queen, worker, and drone, respectively. Larvae and pupae that die during the development process are removed and their brood cells are cleaned by housekeeping worker bees. In addition, worker bees have the ability to detect diseased pupae and remove them; this is called “hygienic behavior,” which interferes with disease progression and improves colony resistance to disease. Hygienic behavior is a part of social immunity and contributes significantly to the overall health of a colony. This is an inherited trait and there are various queen‐breeding programs that selectively enhance this behavior.