Life history of the endangered Japanese aquatic beetle Helophorus auriculatus (Coleoptera: Helophoridae) and implications for its conservation

Aquatic insect diversity in Japan is declining due to water pollution, invasive species, and habitat loss and modification. As a consequence, many Japanese aquatic insect species have been placed on the Japanese Red List. Helophorus auriculatus Sharp, 1884 (Coleoptera: Helophoridae), one of the ‘Endangered species’ on the Japanese Red List, is a small water beetle that inhabits shallow wetlands, such as small pools in riverbeds and paddy fields. Although it is important to elucidate the life history of endangered species to assist in the recovery of their populations, such ecological knowledge has been scarce for H. auriculatus. Therefore, to clarify the basic life history of this species, especially its phenology and reproductive ecology, we conducted field surveys in paddy field water systems and a rearing experiment in the laboratory. Our results showed that adult H. auriculatus mainly inhabit paddy fields and ditches during the fallow season, and reproduce from autumn to spring. The period from the egg stage to adult emergence required about four months to be completed in the laboratory. Survival from larval hatching to adult emergence was significantly higher at 15 °C than at 20 °C, and only 4.4% of larvae could develop into adults at 20 °C, while 22.7% could do so at 15 °C. We concluded that it is important to flood paddy fields from autumn to spring during the fallow season for the conservation of H. auriculatus.