Feeding habits of the endangered Japanese diving beetle Hydaticus bowringii (Coleoptera: Dytiscidae) larvae in paddy fields and implications for its conservation

摘要

The diving beetle Hydaticus bowringii Clark, 1864 (Coleoptera: Dytiscidae) is on the Red List of Japan as ‘Near Threatened’. However, there is no quantitative information on the feeding habits of its larvae, which could be used to aid its conservation. In order to determine the prey that are important for the survival and growth of larvae of H. bowringii, we combined the results of fi eld surveys of paddy fi elds and rearing experiments. In the fi eld, H. bowringii larvae predominantly feed on tadpoles of fi ve species of frogs and occasionally also on insects, loaches and worms. The phenology of the tadpoles was similar to that of larvae of H. bowringii, as their abundances increased from May to June and decreased in July. Experimentally reared larvae of H. bowringii grew faster when fed tadpoles than when fed Sigara nymphs or a mixture of both prey, and more emerged as adults when tadpoles were included in their diet. Adults were larger in the tadpole treatment than in the Sigara treatment. Based on these results, we conclude that tadpoles are more suitable prey for the survival and growth of larvae of H. bowringii than insects. The decline in the abundance of frogs could be one of the factors determining the decrease in the local abundances of H. bowringii. In conclusion, we affi rm that in order to conserve populations of H. bowringii it is crucial to maintain paddy fi eld environments in which frogs are abundant.