Limnology (2006) 7:41–43 © The Japanese Society of Limnology 2006 DOI 10.1007/s10201-006-0161-5 NOTE Shin-ya Ohba · Fusao Nakasuji Dietary items of predacious aquatic bugs (Nepoidea: Heteroptera) in Japanese wetlands Received: January 4, 2006 / Accepted: January 25, 2006 / Published online: April 7, 2006 Abstract Some species of predatory aquatic bugs (Nepoidea, including Belostomatidae and Nepidae) are designated as threatened-vulnerable species in the Red Data Book of Japan and are regarded as effective predators of freshwater snails and mosquito larvae. To reveal the feeding habits of Nepoidea, we investigated their dietary items in wetlands and obtained data from the published literature. Lethocerus deyrolli (Vuillefroy) preys mainly upon frogs, and the major diet of the nymphs is tadpoles. Laccotrephes japonensis (Scott) adults prey mainly upon tadpoles. Lethocerus deyrolli nymphs and L. japonensis adults compete against each other as same-guild members because they are sympatric in wetlands. Both nymphs and adults of Appasus japonicus (Vuillefroy) feed on snails, and L. japonensis nymphs feed on aquatic insects, including mosquito larvae (39.3% of aquatic insects). These species are effective predators of medically important pests such as snails, and mosquito larvae. Key words Nepoidea · Belostomatidae · Nepidae · Aquatic insect · Predator Introduction In temporary water pools devoid of large fishes, aquatic hemipterans play a significant role as the major component among aquatic fauna (Runck and Blinn 1994; Blaustein 1998). In particular, large heteropterans, such as Belos- tomatidae, Nepidae, and Notonectidae, are often at the top of the food chain in aquatic communities (Waters 1977; Runck and Blinn 1990). The bugs of Nepoidea, including Belostomatidae and Nepidae, prey upon a variety of aquatic animals such as insects, cladocerans, amphipods, S. Ohba (*) · F. Nakasuji Laboratory of Insect Ecology, Graduate School of Natural Science and Technology, Okayama University, Okayama 700-8530, Japan e-mail: oobah8ag@yahoo.co.jp tadpoles, and small fish (Hoffman 1924; Rankin 1935; Cullen 1969; Tawfic 1969; Menke 1979a,b,c; Okada and Nakasuji 1993; Smith 1997; Hirai and Hidaka 2002; Toledo 2003; Mori and Ohba 2004). Abundance of some of these taxa has decreased rapidly during recent years, and they are designated as threatened– vulnerable species in the Red Data Book of Japan (Japan Environment Agency 2000). Inadequate knowledge of nutritional physiology (Scriber and Slansky Jr 1981) has hampered an understanding of insect ecology. Generally, the population size of a predacious invertebrate is limited by its food resources (Lenki 1984; Pearson and Knisley 1985; Juliano 1986). In addition, these bugs are regarded as effec- tive predators of the freshwater vector snail or mosquito in developing countries (Roy and Raut 1991, 1994; Aditya and Raut 2001, 2002a,b). Understanding the feeding habits of these taxa in their natural environment is significant both for conservation and for pest management. To reveal the eco- logical status and feeding habits of Nepoidea in Japanese wetland ecosystems, we compared their dietary items based upon field data and the published literature. Methods To investigate the dietary items of Lethocerus deyrolli and Appasus japonicus in Belostomatidae, and of Laccotrephes japonensis in Nepidae, we conducted field censuses at inter- vals of 1 to 3 days in rice fields at Uekidani (35°1¢ N, 134°19¢ E) in Hyogo from June to mid-August in 2004 and 2005 and at Chuoh (34°58¢ N, 133°58¢ E) in Okayama from June to mid-August in 2004 and 2005. Censuses were conducted along the ridges around paddy fields. Nepoidea individuals were directly observed in the paddy fields and ditches using a flashlight (11 000 lx) from 2000 to 0100. Nepoidea is prima- rily nocturnal and ambushes its prey at the water surface after sunset; therefore, they are easier to observe at night. The flashlight did not interfere with their foraging behavior as Nepoidea did not stop feeding and ambushing prey (S. Ohba, unpublished data). When we found Nepoidea hold-