Integration of management tactics for the rice water weevil Lissorhoptrus oryzophilus Kuschel with particular emphasis on use of plant resistance (tolerance)

Studies were conducted to test the hypothesis, significant benefits can be achieved for the management of rice water weevil, Lissorhoptrus oryzophilus Kuschel, by the use of rice varieties with low levels of host plant resistance coupled with the judicious use of cultural practices and insecticides. Tolerant traits of a current commercial variety of rice, "Bengal", was manifested consistently over multiple years. Study on rice growth as effected by weevil injury showed that pruning of root systems by larvae of weevil resulted in a decrease in tiller number and shoot biomass of rice plants in the vegetative stage of growth. Yield losses were due to a combination of decreases in panicle densities, numbers of grains per panicle, and grain weights. Experiments on density-yield relationships documented that larval densities during earlier stages of rice growth were more strongly correlated with yield losses than were larval densities later in the growing season. Slopes of yield loss were greater negative for early-flood than for delayed-flood plots, and greater negative for susceptible varieties. A phenology model predicted that emergence of weevils occurred after the accumulation of 139.2 degree-days (°C * Day). Using a temperature threshold of 10°C, the total degree-days required for development of one generation is about 623.4 degree-days (from egg to pupation was 359.1 ± 19.4; pupal development was 264.3). Larval development did not differ between varieties with different resistant levels. In conclusion, this study showed that host-plant resistance, delayed-flooding and reduced insecticide can be integrated together. Although use of high rate of insecticide currently gives the best economic return, integrated control provides a combination of adequate control and environmental friendliness. The same economic return can be achieved using a more tolerant variety and delayed-flood without insecticide treatment.