Investigation of insect-weed interactions in the rice agroecosystem

Interactions between graminaceous weed and insect pests of rice and between management practices for these pests were investigated. Studies were conducted to examine preference and performance of rice water weevil, Lissorhoptrus oryzophilus Kuschel, on several weeds commonly found in rice fields. Several weeds were more preferred than rice, and larvae were present on several dicotyledonous weeds, suggesting the host range of L. oryzophilus is broader than previously thought. Effects of the presence of barnyardgrass, (Echinochloa crus-galli Beauv.), on rice stink bugs, Oebalus pugnax F., and L. oryzophilus populations in rice fields were investigated. Presence of barnyardgrass and the synchrony of barnyardgrass seed heads and rice panicles influenced O. pugnax densities on rice. Barnyardgrass served as a trap crop or as a source of infestation of O. pugnax depending on the developmental stage of barnyardgrass relative that of rice. Presence of barnyardgrass had little impact on L. oryzophilus . Similar studies conducted with borers and Amazon sprangletop, Leptochloa panicoides (Presl.) Hitchc. revealed that injury to rice was greater in weedy plots of rice than in pure plots of rice. Studies were conducted to determine how density of weeds affected O. pugnax populations and how weeds and O. pugnax combined to reduce grain yield and quality. Numbers of O. pugnax and percentages of filled seeds, pecky rice, and broken kernels increased as weed density increased. Increases were less severe in insecticide-treated than non-treated plots. However, yield losses from weeds and insects were not significantly greater than from weeds alone. Preference and performance of L. oryzophilus on herbicide-treated and non-treated glufosinate-tolerant rice were investigated. Glufosinate applications on glufosinate-tolerant rice reduced weevil oviposition by 30% and reduced larval densities by 20% compared to non-treated glufosinate-tolerant rice. Glufosinate was not toxic to L. oryzophilus at rates used in these experiments, and feeding was not deterred by glufosinate, suggesting that glufosinate application induced resistance. Larval densities on glufosinate-treated and non-treated glufosinate-tolerant rice in field experiments did not differ; however, delayed floods reduced numbers of larvae on rice compared to those on early flooded rice