Effects of Temporary Drainage on Selected Life History Stages of the Rice Water Weevil (Coleoptera: Curculionidae) in California

Adults, eggs, first instars, and root-feeding larvae of the rice water weevil, Lissorhoptrus oryzophilus Kuschel, were experimentally confined to rice, Oryza sativa L., plants in flooded and drained fields during experiments conducted in 1989-1990. In both years, oviposition was significantly greater under flooded conditions (5.8-32 mean number of eggs per plant) than under drained conditions (0-1 mean number of eggs per plant). Scars from leaf-feeding by adults were more abundant on plants in flooded plots in both years. Lengths of feeding scars were greater on plants from the flooded plot in 1989, but scar lengths did not differ between treatments in 1990. In 1989, survival of eggs exposed to drained conditions for 7 d was greater than for eggs exposed to flooded conditions for the same period. Survival of weevil eggs in 1990 did not differ among flooded and drained treatments. In 1989, fewer larvae established on plants when drainage occurred 1 dafter transplanting than on plants that were transplanted at the same time but remained under flooded conditions. In 1990, water management, block, and the interaction (water management and block) influenced larval establishment. Survival of root-feeding larvae did not differ between flooded and drained treatments in 1989, whereas in 1990, survival declined with an increasing duration of drainage. Draining rice fields may be effective in controlling rice water weevil infestations, particularly when targeted against adults. However, earlyseason drainage conflicts with other management practices and appears to be more costly, and appropriate timing of this practice may be difficult to establish.