Andrew T. Palrang - Post-Doctoral Researcher, Department of Entomology, UC Davis

Albert A. Grigarick - Professor Emeritus, Department of Entomology, UC Davis

1995 RWW Flight

The 1995 flight of the rice water weevil was characterized as one of the most intense in 30 years of monitoring. A total of 3,340 weevils were captured at the Rice Experiment Station, compared with 1,832 the previous' year. One large peak occurred May 20, followed by two secondary peaks on April 22 and May 16. The date for completion of 95 percent of the flight was May 28. Use of delayed planting date as a weevil management strategy was not practical due to the late weevil flight.

Chemical Controls

Researchers examined the active ingredients of six chemical insecticides in three studies at the Rice Experiment Station and four studies in growers' fields. The chemicals included carbofuran, Dimilin, fipronil, imidacloprid, lambdacyhalothrin and V-71639. Numerous formulations and application timings were used in ring plots and in replicated field plot studies. To gauge the effectiveness of these various com- pounds on the rice water weevil, researchers compared feeding scars, larval densities, plant growth and grain yield. They also watched for any signs of phytotoxicity to the rice plants.

Rice water weevil does most of its damage in the larval stage. The com- pounds found most effective against RWW larvae were Furadan 5G, preflood Fipronil (1.5G and 8OWDG) and the Karate treatments. However, researchers noted several other treatments provided excellent larvae control. These included Furadan 4F, postflood Fipronil (1.5G and 8OWDG) and postflood Dimilin.

In additional studies researchers found RWW larval control by Dimilin to be more effective under pinpoint flood conditions than under continuous flood conditions. Dimilin must be consumed by adult weevils to exert an effect. Since more rice foliage is exposed with pinpoint conditions, this may place more of the Dimilin on the plant leaves and is thus more available to adult rice water weevils.

The most promising ring plot treatments from 1994 were stepped up to more realistic, small basin tests in 1995. The formulations are applied with commercial methods to natural RWW infestation levels. The compounds found to be most effective against RWW larvae were Furadan 5G (preplant and postflood), Fipronil 1.5G (preplant and postflood), Fipronil BOWDG (preplant) and Dimilin (postflood). All were comparable in their control and had fewer larvae than the untreated plots.

In a comparison of the effectiveness of Dimilin and Furadan 5G at four grower field sites, grain yields were similar, both yielding in the 7,400 pounds/acre range — about 500 pounds/acre better than the control.

M-202 Response to RWW

Researchers continued their examination of rice water weevil injury on M-202 growth, development and yield. Yield losses have occurred in each of the three years of the study, though the magnitude of these losses has varied. This tells researchers that there are multiple factors at work. A number of environmental factors, such as weather, can affect plant vigor and therefore RWW's impact to rice productivity.

Larval populations ranged from an average of just under two per plant to 18. Researchers noted that larval injury reduced tillering in the three highest infestations. The greatest reduction was nearly 46 percent. Rice maturity was not significantly affected by RWW larval feeding, which is consistent with data from the previous two years. However, larval damage clearly reduced grain yields. Uninfested and low infestation treatments yielded the equivalent of about 8700 pounds per/acre, while high infestation treatments reduced yield by about one-third to 5700 pounds/acre.

Researchers also examined the relation- ship between leaf scarring by adult RWW, larval density, plant growth and grain yield. Of the compounds that hold promise for replacing Furadan 5G, two and possibly three could be applied as a post-flood, as-needed treatment, rather than a preventative treatment. Therefore, clear economic thresh- olds for weevil damage to rice will be critical in management decisions. Observations of these variables under grower conditions will continue.

Overwintering Biology

Entomologists continued a study of the overwintering biology of RWW. The pest is in an inactive state during the winter. Samples taken during the fall of 1994 and winter of 1995 showed that the majority of RWW adults were concentrated in the top 1.5 inches of the soil/ root crown area. Low numbers of adults were found between 1.5 to 3.0 inches.

Researchers found most adult weevils on north-facing levees and field margins. However, a surprisingly high number of weevils were observed in field basins (even though standing water was present from January through March). Adult weevil density declined from November to March, probably from natural mortality during the overwintering period.

Researchers are investigating how to wake weevils from the winter- time inactive period or 'diapause.' This ability would permit additional greenhouse and laboratory study periods.

Adult weevils were field- collected in December, January, February and March, then exposed to variations of temperature, light, and plant substrate in laboratory conditions. Sample evaluation and data analysis are continuing.