L.D. Godfrey, extension entomologist, Department of Entomology, UC Davis

A.T. Palrang, research assistant, Deparment of Entomology, UC Davis

A.A. Grigarick, professor emeritus, Department of Entomology, UC Davis

1993 RWW Flight

The 1993 spring migratory flight of the rice water weevil as measured by light traps placed in the fields of cooperating growers in Butte, Colusa and Sutter counties and the Rice Experiment Station in Biggs - was characterized as, "moderately low." Differences among the various sites point out the variability in flight patterns.

Approximately 90 percent of the flight was completed by May 1; 95 percent by May 18. The 1993 figures contrast sharply with 1992 results when a significantly greater RWW flight occurred. Data collected from the Rice Experiment Station showed three distinct peaks, occurring April 29 and 30, May 9 and 11, and May 16 and 18.

Information gathered from this annual monitoring is also being studied for the potential to develop a flight model that would allow predictions of flight timing.

Chemical Controls

Entomologists studied two formulations of the insecticide carbofuran (FuradanŽ) applied pre-flood and post-flood in "aluminum ring" tests at the Rice Experiment Station. Both products reduced adult feeding from 76.3 percent in the untreated controls to an average of 14.8 percent during a four-day period in June. Larval density of RWW dropped by an impressive 96 percent. Grain yields were higher in treated plots than in untreated plots, although researchers noted no consistent trends among the yields in the treated plots. Yield loss from RWW feeding averaged 28 percent.

Researchers also continued their examination of a biological insecticide, Beauveria bassiana. In greenhouse experiments, tests were done on potted, flooded rice plants containing newly hatched RWW larvae, intermediate-size RWW larvae and RWW adults. B. bassiana afforded no control of RWW larvae but excellent control of RWW adults. Ninety-one percent of the adults were dead 14 days after treatment.

Plant Response Monitored

Researchers set up another experiment to monitor RWW injury to rice plants grown under three different conditions flooded M-202, a flooded experimental line (PI506230) with RWW tolerance, and drill-seeded M-202. The experiment examined increasing levels of RWW infestations and a FuradanŽ treatment.

Adult feeding scars peaked at 77 percent on the flooded M-202 and at 67 percent on the experimental line. In contrast, the drill-seeded M-202 showed only 32 percent feeding scars. Similarly, larval densities peaked at 14 and 12 larvae per plant in the M-202 water-seeded and the experimental treatments, respectively. Within the drill- seeded treatment, the larval density averaged only four larvae per plant. Researchers believe the environmental conditions in drill-seeding are not as favorable to larval survival as they are in water-seeded rice. RWW infestations showed no effect on plant maturity.

Grain yield, however, was substantially affected by different treatments. The highest RWW infestation reduced grain yield by more than 45 percent in water-seeded M-202, compared with only 29 percent in the experimental line. Maximum yield decrease in the drill-seeded M-202 was 10 percent. However, uninfested, water-seeded M-202 yielded 20.7 percent more than the uninfested drill-seeded M-202 plot, a difference most likely caused by weed Competition. Obviously, there are tradeoffs involved that ultimately affect grain yield. Drill-seeding may limit damage from a rice water weevil infestation but at a cost of reduced yield from weed competition. Because of the year-to-year variations in yield response, researchers say the study nee& to be repeated to confirm their observations on how RWW affects rice yields.

Overwintering Biology

The overwintering habits of the rice water weevil were also the subject of investigation in last year's research. Scientists found "considerable densities" in the top one inch of soil on levees near rice fields; a few adults were found down to the three-inch level. On warm days in February and March, RWW adults were found feeding on leaves of annual bluegrass on levees.

Seeding Methods

Researchers again compared weevil damage in both drill-seeded and water-seeded rice with and without a treatment of the insecticide FuradanŽ (carbofuran).

Adult feeding was sharply reduced in the drilled plots during the early drained period compared to water-seeded plots that were continuously flooded. The untreated drilled plots showed adult feeding scars on 8 percent of the plants, while 73 percent of the plants in the untreated water-seeded plots showed signs of feeding activity. However, the differences dissipated two weeks after the drilled rice was permanently flooded. Apparently, the drained conditions involved in drill seeding inhibit adult feeding. The heaviest adult weevil feeding is thus delayed but not eliminated. Egg laying may also be delayed.

Application of FuradanŽ did little to limit damage in the drill-seeded plots because conditions were already less favorable to RWW. In the drill-seeded plots adult feeding with and without FuradanŽ averaged 17 percent and 20 percent, respectively. The insecticide does, however, show a marked effect on larval populations in water-seed plots. RWW larval populations were more than three times as high in the water-seeded plots without FuradanŽ than they were on the drill-seeded plots. Larval density was reduced by 18 percent with FuradanŽ.

Drill seeding has inherent limitations, particularly with respect to weed management. However, some growers are successfully using drill seeding in rotation with fallowing to manage weeds and to get an early start with rice planting.