Rapid decline of male honey bees in summer: the potential impact of Japanese barn swallows on high male loss

Given the population size of eusocial hymenopteran insects, particular emphasis is placed on evaluating the number of reproductive female individuals (queen). However, male production also accounts for a substantial part of the reproductive output of a colony. The number of breeding males in the mating population plays a vital role in the health of a colony and, ultimately, a population size by influencing the mating status of the queen. The lifespan of male honey bees (Apis mellifera) varies considerably, influencing their dynamics in a population. However, studies on the factors that cause male mortality are limited. This study evaluated the cohort survival of male honey bees and the relationship between the activity of males and Japanese barn swallows (Hirundo rustica gutturalis), a potential predator that significantly influences male honey bee survival. The majority of male loss in colonies was observed around the ages when flight was first initiated, particularly in the summer. Swallows frequently visited the apiary in the afternoon and preyed on the bees. The daily activity of swallows was closely linked to the activity of male honey bees, which was high in the summer. A hazard model analysis revealed that the risk of male honey bee loss increased by approximately 40% with an increasing number of swallows. The observed significant relationship between swallows and males presents the possibility that swallows are key factors causing the high loss of males during summer.