Seasonal prevalence and migration of aquatic insects in paddies and an irrigation pond in Shimane prefecture [Japan]

Faunal composition and seasonal changes in the number of aquatic Coleoptera and Hemiptera were investigated in paddies and an adjacent irrigation pond in Shimane Prefecture, Japan. The paddies held water from mid April to mid June, and then were irrigated intermittently until the end of August. The irrigation pond was filled with water throughout the year. Twenty-two species of Coleoptera and nine species of Hemiptera were collected from both the paddies and the pond. The number of species of adult insects in the paddies increased from May to June, then gradually decreased toward autumn. In contrast, the number in the pond was highest during autumn. The number of species of larval insects was high in paddies especially from May to July, but low in the pond throughout the year. Thirteen of the aquatic insect species observed were classified into four types on the basis of their habitat utilization pattern: A) those using the pond for both non-reproductive and reproductive purposes, B) those using the pond as the main habitat and the paddies for reproduction, C) those using both the pond and paddies as a living habitat and also the paddies for reproduction, and D) those using the paddies for both non-reproductive and reproductive purposes. Paraplea indistinguenda was the only species belonging to type A, and stayed in the irrigation pond throughout the year. Hyphydrus japonicus, Laccophilus difficilis, Cybister brevis, Cybister japonicus, Hydrophilus acuminatus, Ranatra chinensis and Notonecta triguttata were type B insects. Their adults immigrated (probably from the pond) into the paddies and reproduced from May to July. The new adults then migrated to the irrigation pond before the watered area of the paddies dried up. Type C included Agabus conspicuus, Rhantus suturalis and Appasus major. Laccotrephes japonensis and Sigara sp. were classified as type D, and they were seldom found in the pond. My observations indicate that paddies play an important role in the reproduction of aquatic Coleoptera and Hemiptera, whereas the pond provides stable habitat for non-reproductive stages, as well as a place for reproduction of some species. This means that the coexistence of paddies and ponds is important for the species richness of aquatic insects in this region.