# Japanese honey bees (Apis cerana japonica) have swarmed more often over the last two decades

## TL;DR

Japanese honey bees have increased their swarming frequency over 22 years, possibly due to warmer temperatures in early spring.

## Contribution

Long-term data shows a moderate increase in swarming events per cycle in Japanese honey bees linked to earlier spring temperatures.

## Key findings

- Swarming events per cycle increased from 2000 to 2022 with a positive trend of 0.03 per year.
- Colonies swarmed more often when the swarming process began in early spring, especially in March.
- Warmer March temperatures in Japan may be causing earlier reproduction and more swarming events.

## Abstract

The impacts of temperature increase are a concern for honey bees, which are major pollinators of crops and wild plants. Swarming is the reproductive behavior of honey bees that increases colony numbers. Honey bee colonies sometimes swarm multiple times, with each swarming termed a “swarming event” and a series of these events called a “swarming cycle.” The number of swarming events per swarming cycle varies widely depending on climatic conditions and subspecies, and the recent temperature increase due to global warming might be affecting the number of swarming events per swarming cycle of native honey bees. We clarified long-term changes in the number of swarming events per swarming cycle of Japanese honey bees (Apis cerana japonica) by collecting beekeepers’ swarming logbooks. The survey showed that between 2000 and 2022, Japanese honey bees swarmed 1 to 8 times per swarming cycle. Generalized linear model analysis indicated that year had a significant positive effect (coefficient, 0.03; 95% CI, 0.01–0.04); that is, the number of swarming events per swarming cycle showed a moderate increase over time. In addition, we found that colonies swarmed more often in a cycle when the swarming process began in early spring, especially in March. Considering the notably strong trend in Japan of warmer temperatures in March, the number of swarming events per swarming cycle may be increasing because reproduction is beginning earlier in the year. Further analyses are needed to verify the causal relationship of temperature increase on the number of swarming events per swarming cycle.

The online version contains supplementary material available at 10.1007/s00114-024-01902-y.