Life histories and population dysamics of two aquatic pyrarid moths, Elophila interruptalis (Pryer) and Neoshoenobia decoloralis Hampson, were surveyed in 1980 at Midorogaike Pond, Kyoto, where these two species coexisted. E. interruptalis was trivoltine, feeding on various kinds of water plants, Nupar subintegerrimum, Brasenia schreberi, Trapa bispinosa var. iimurae, etc. From the first to mid instar, the larvae lived on the undersides of leaves, spun between two leaves, or made portable cases of host leaf flagments ; and the mature ones usually spun the leaves or made the cases in order to feed and move. Pupation took place between two leaves or in the cases, attached to the hosts. The adults laid eggs in masses. Egg density was not very fluctuated among generations (within 2 times), and severe consumption of the host was not seen in any quadrat. N. decoloralis was bivoltine, and the life cycle was completed only on the N. subintegerrimum. From the first to mid instar, the larvae mined the leaves, later the larvae became petiole borers directly from the leaves or indirectly from the other leaves by moving on the surface of the water. Pupation took place in the petiole near the root. The adults laid eggs in masses and egg density increased in the second (overwintered) generation, 30-40 times that of the first one. In some quadrats a very high mortality during the younger larval stage was observed because of the defoliation of the leaves. Considering these results, E. interruptalis seems to be an ecologically generalist and N. decoloralis to be a specialist. They seem to coexist, even when they depend on the same host, by feeding on different portions of the host.
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