Larry D. Godfrey - Extension Entomologist Associate Entomologist Department of Entomology UC Davis

A.T. Palrang, postdoctoral researcher, Dept. of Entomology, UC Davis

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

1994 RWW Flight

Scientists monitored the 1994 spring migratory flight of rice water weevil with light traps located at the Thompson Ranch near Glenn, the Dennis Ranch near Maxwell, the Scheidel Ranch near Nicolaus, and at the Rice Experiment Station. The traps at all three ranch locations were redesigned in 1994 to improve their efficiency.

Trap counts revealed a "slightly above average" infestation that began with the first of three peaks about April 4 after the accumulation of 83 degree-days. A second spike occurred between April 15 and April 22, depending upon location. The remaining peaks occurred throughout a two-week time frame ending May 15. Fields planted after this date were more likely to escape substantial infestation from migrating adults. Scientists point out that the variability in these peaks between locations emphasizes the need for local monitoring to more effectively plan management strategies.

Chemical Controls

Several experimental RWW insecticides utilizing new chemistry entered the research arena last year. Entomologists examined these compounds - Fipronil, Karate®, Admire® and an experimental from Rohm & Haas - along with continuing investigations of Furadan® and Dimilin.®

All materials were tested in aluminum ring experiments set up in a plot on the Rice Experiment Station. Twenty-one treatments involving different rates, timing and methods of application were examined. The scientists then evaluated the effects of each treatment on levels of adult weevil feeding, larval populations, and the effects of weevil injury on plant growth and yield.

Furadan® 5G, Furadan® 4F and Karate® 1E all reduced leaf scarring, an indication of adult feeding activity, by more than 90 percent. The remaining treatments reduced leaf scarring between 83 percent and 8 percent.

One of the more encouraging findings with two of the Fipronil treatments was a 94 percent reduction in larval density Altogether seven of the 21 treatments resulted in lower larval densities than resulted with Furadan 5G. Generally, the Furadan®, Fipronil and Karate® treatments fared best against RWW larvae, This is significant because RWW does most of its damage by feeding in the larval stage on the roots of young rice plants. Ironically, several of the chemical treatments had larval densities greater than in the untreated plots. Researchers believe the larvae in the untreated rings devoured the majority of the rice plant roots and eventually many of the larvae starved to death. Even in the less effective treatments, enough control was provided to minimize this result.

Chemical treatments resulted in taller rice plants with more tillers, greater dry leaf and heftier root weights. All treatments, with the exception of the Robin & Haas experimental, increased grain yield. The Furadan® treatment produced the highest grain yield, with several of the other treatments producing within 95 percent of the top yield.

Plant Response Monitored

Researchers continued work from 1993 monitoring the effects of varying levels of RWW on flooded M-202 rice, flooded PI 506230 (an experimental line with weevil tolerance), and drill-seeded M-202. These tests were also done in aluminum rings at the Rice Experiment Station.

Adult feeding scars peaked at 73 percent in the flooded M-202 and at 64 percent in the PI 506230. However, feeding scars peaked at only 41 percent in the drilled M-202. Larval densities peaked at nine per plant in the water-seeded M-202 and at eight in the PI 506230. Surprisingly, larval densities reached nine per plant in the drilled M-202, which contrasted with the previous year's findings. Researchers speculate that the cause of this disparity may have resulted from the high number of adults introduced June 10 into the drilled rings after they had been permanently flooded.

Other findings from this experiment showed that RWW reduced tillering by 27 percent in water-seeded M-202, 8 percent in PI 506230 and 6 percent in drilled M-202. Infestation level had a near negligible effect on the timing of panicle emergence. Similarly, seeding method had no significant effect on heading date; drilled rice is initially delayed as it emerges through saturated soil, so it is difficult to interpret the slight difference that occurred in this study. Grain yield, however, was significantly reduced by RWW in water-seeded M-202 - a yield reduction of 25 percent was observed at both moderate and high infestation levels.

In the case of PI 506230, the weevil-tolerant experimental line, the greatest yield reduction was 12 percent. This line did not yield as high as M-202 in plots without RWW. Finally, in spite of the large larval densities observed earlier in the season, yield was not reduced in the drill-seeded plots.

Overwintering Biology

Entomologists initiated a study of the overwintering biology of RWW in the fall of 1994. Soil and root crown cores were collected from a rice basin and levees at two depths - zero to 1.5 inches and 1.5 to 3 inches deep. Adult weevils were concentrated in the top 1-inch of the soil/root crown area. Surprisingly, a large number of weevils were found in cores removed from a water-saturated rice basin.

Biocontrol

Biological control may offer yet another means of managing RWW. Many other serious insect pests have a group of predators and parasites that help keep their populations in check. Entomologists surveyed a spring generation of RWW adults and monitored them in a laboratory for parasite emergence. None was observed.