Research conducted on this project during 2001 included important aspects of rice water weevil biology, cultural control and insecticidal management. Significant progress was reported on all objectives.

Rice Water Weevil (RWW) is the most important invertebrate pest of California rice. In 2001, RWW was effectively managed with Dimilin® 2L and Warrior®, the first year of total reliance on these products since the phase-out of Furadan® 5G. Growers and pest control advisers have readily embraced these new products and the research on them.

Applied after flooding, these new products require a different management strategy from previous pre-plant materials. The goal of 2001 research was to optimize use for these new insecticides and to develop a better understanding of RWW biology. More detail is below.

Heavy RWW Flight

The 2001 adult RWW flight was the heaviest in five years. More than 8,000 RWW were captured during April, May and June in the blacklight trap at the Rice Experiment Station. That compares with 4,100 in 2000 and only 1,000 in 1999. The conditions for RWW flight in 2001 were met almost constantly from April 24 through May 11. During the remainder of May, some flight occurred but not nearly as intense. The flight was 90 percent complete by May 12.

The timing of RWW flight in the spring has been monitored for nearly 40 years at RES. Monitoring weevil flights is important to determine the levels and intervals of peak flight periods and to compare RWW trends over time. The switch to a control strategy targeting adult RWW with the use of post-flood insecticides places even greater emphasis on the need to understand RWW flight timing.

Chemical Controls

Studies continued in small ring plots at the Rice Experiment Station and in grower fields to compare experimental materials with registered standards and to improve management of the existing products. Here are some highlights from the ring plot work:

• The experimental compound Mustang (zeta-cypermethrin) provided good control. This product is similar to Warrior and is on track for registration.
• V10101 was tested for the first time last year and provided moderate control at three different rates.
• Dimilin and Warrior worked well with standard application parameters. Combined, they performed exceedingly well. Warrior may have given better residual control than Dimilin.
• Warrior applied preplant proved effective. Dimilin and Mustang applied preplant proved less effective, with erratic yields and weaker larval control.
• Messenger was evaluated for the first time. This product contains “harpin” proteins that simulate the natural defense of plants, thus repelling many insects. Messenger showed light activity against RWW in ring plots but not to a useable level.

Grower field studies examined how border applications of Dimilin compared with full basin treatments; how effective Warrior is in combination with Dimilin; what residual control is provided by Dimilin; and whether Warrior is effective in a preplant application.

Dimilin was compared at three locations—broadcast over the entire basin in one treatment and over the first 50 feet nearest border levees in the other. Larval populations were substantially reduced by the Dimilin application, with the border-only treatment slightly more effective than the entire basin treatment.

The Dimilin plus Warrior combination was compared against Dimilin only in a border treatment at three separate locations. The Dimilin-only treatment reduced RWW levels about 80 percent, compared to 93 percent for the combination. Grain yield also responded favorably in this experiment, with a 17.3 percent increase in the Dimilin-only treatment and an 18 percent increase in the combined treatment.

The duration of residual control provided by Dimilin is unknown. Entomologists have approached this question in ring plots, but “real world” information from grower field settings is needed. Although progress was made on establishing research protocols for future work, no conclusive data were collected last year.

Finally, ring plots may be excellent for screening numerous treatments in a small area, but they also introduce several constraints—reduced water movement, elevation of water temperature, altered RWW behavior etc. Therefore, experiments were conducted in grower fields last year to determine Warrior’s preplant effectiveness. Results were inconclusive, although low larval populations were observed in the two preplant Warrior-treated basins. Grain yield data was variable, with the highest yield in an untreated plot. Further work in this area is needed.

RWW Biology

Previously developed leaf-scarring thresholds for post-emergent Furadan treatments are not applicable to the new generation of post-flood products. A floating barrier trap developed at the University of Arkansas is showing merit as a new sampling tool for measuring in-field RWW populations. In an effort to determine their utility in California, traps were placed into 10 grower fields within two days of flooding/seeding.

RWW captures were monitored three times a week. RWW were captured in all fields. More than 50 percent of the adults were captured by the one-leaf stage, generally the first five days after seeding. By the three-leaf stage, more than 75 percent of the total RWW collected had been captured. This bodes well for the usefulness of these traps by growers and PCAs because this is the period when management decisions must be made.

In related research RWW immatures were sampled twice in each field to see how the adult capture related to the number of damaging larvae. The statistical relationship was weak. As more adult RWW were trapped in a field, there were more larvae in the samples a few weeks later; however, there was considerable variability in the data. For every adult capture in a trap, slightly less than one larva resulted.

One of the key questions regarding use of Dimilin and Warrior is, given these compounds’ relatively short residual power, whether multiple applications are necessary to effectively manage prolonged RWW flights and infestation. Season-long control is not practical but multiple applications, if economically justified, can provide significant residual control and may be cost-effective. Studies were conducted in ring plots with RWW infestations at the two-leaf stage and every seven days thereafter for five weeks.

Rice development was influenced by the severe and early RWW infestation and feeding. The high infestation at the two-leaf stage delayed panicle emergence by five to seven days. Grain yield was reduced by RWW infestation and yield loss was more severe than that seen in similar year 2000 studies. Higher infestation rates reduced grain yields with all infestation timings. The earliest infestation timing reduced grain yield by 52 percent. Even with an infestation at the eight- to nine-leaf stage, grain yield was still reduced by 16.8 percent.

Cultural Controls

Two aspects of cultural control were studied in 2001. RWW populations were monitored at a long-term straw management site near Maxwell, Colusa County. For the fifth year of the six-year evaluation, larval population was significantly higher in unflooded winter plots than winter-flooded plots.

The relationship between RWW damage and rice variety was also investigated. At present, there are no RWW-resistant varieties. However, there may be differences in the susceptibility of common rice varieties to RWW. Seven public varieties and one experimental line were grown in plots treated for RWW and in untreated plots. The naturally occurring RWW population was low in all plots. Yields ranged from 6,200 pounds/acre in the untreated M-205 plot to 3,250 pounds/acre in the untreated Calmati-201 plot. This study needs to be repeated with heavier RWW pressure.