Losing an important tool that controls "the most important insect pest of rice in California" is significant, but there are alternatives.
"Before Furadan was beginning to be phased out, about 30 percent to 40 percent of the acres were treated annually for rice water weevil. Yield losses as high as 30 percent can occur with severe infestations--although they are usually spotty. However, losses in the 10 percent range are more common."
Rice water weevil larvae feed on roots, resulting in yield loss. In addition, root damage may result in stunted plants, reduced tillers and panicles and delayed maturity. This in turn, can create problems with weeds.
According to the University of California's Pest Management Guidelines, the heaviest infestations of rice water weevil and the most serious damage usually occur between late May and July within 15 to 20 feet of the margins of the fields and levees, where weevils are concentrated; moderate damage can occur in areas 20 to 35 feet from levees.
There are two excellent alternatives to Furadan, according to Godfrey. They are Dimilin and Warrior. However, these products differ from Furadan. The mode of action is significantly different, and recommended application techniques and timing reflect these differences.
According to Godfrey, although Dimilin does not kill adults, the females can pass it on to their eggs. "Upon exposure by consumption of treated foliage and/or swimming in treated water, the females are affected for about five days," he says. "Dimilin also has an effect on the egg, which is deposited in the leaf sheaths. It takes the eggs five to seven days to hatch. The effect of Dimilin is substantial early on and lessens as the egg ages. Dimilin has no effect on rice water larvae. As such we recommend application at two to five days after rice emergence above the water, which usually corresponds to the two to four leaf stage. Ideally, one application should do the job.
"The application timing with Warrior is comparable to Dimilin, perhaps emphasizing the early (two- to three-leaf) timing window slightly more. Warrior has no effect on the egg; it controls rice water weevil by killing the females before they can deposit eggs."
The scouting techniques and application strategies developed for use with Furadan are not viable for use with Dimilin or Warrior. "We really do not have an established treatment threshold for Dimilin or Warrior at this time," Godfrey says. "With Furadan, if 20 percent of the plants had adult feeding scars, an application was needed. This refers to a post-emergence application, where a grower would drain the field, apply Furadan, and reflood. The preplant application of Furadan was most common. The post-emergence threshold that we used for Furadan is too late for Dimilin or Warrior. At 20 percent scarred plants, the eggs are likely already deposited."
To address the issue of threshold levels and help growers determine when Dimilin or Warrior should be applied, Godfrey and other Extension personnel have been monitoring adult flights with light traps. The trapping program is part of the Rice Pest Management Alliance funded by the Department of Pesticide Regulation. The Rice Research Board is also involved in the monitoring project.
Rice water weevils overwinter as adults in crop debris on levees, roadsides and grasses. As the temperatures warm in the spring, they emerge from diapause and begin feeding on grassy weeds in preparation to fly to rice fields.
These flights occur during evenings from about 6 p.m. until midnight when temperatures are warm (about 70 degrees), somewhat humid, and calm. Light traps are used to monitor adult weevils that can fly several miles. Upon infesting rice fields, the adults, which are all females, begin feeding on rice foliage in preparation to lay eggs. Within a few days, the eggs are deposited into the rice leaf sheaths just below the water line. One female can lay up to 200 eggs over the period of several weeks.
"The timing of invasion of rice fields by rice water weevil adults and subsequent egg laying has taken on increased importance with the registration of Dimilin and Warrior in 1999," Godfrey says. "These materials, properly applied and timed, provide rice water weevil control by preventing the deposition of viable eggs. Once viable eggs are present -- or more importantly -- once larvae are present, no control can be achieved with these products. As a result, information on rice water weevil flight and biology may assist us in timing applications."
However, Godfrey cautions that rice water weevil infestations also result from adults crawling off nearby levees. "These short range movements appear to be becoming more important and common in the last 10 years," he says. "We need to take this into consideration, along with long-range flight monitoring."
Until a treatment threshold is established, growers should rely, at least in part, on past experience. "Growers generally know which fields or parts of fields have had historical problems with rice water weevil," Godfrey says. "That allowed them to effectively use preplant Furadan for 20-plus years and can help with applications of Dimilin or Warrior.
"Both products are very effective in controlling rice water weevil," Godfrey says. "It's just a matter of timing the application correctly."