Chemicals to combat rice weevil effective with management change

The rice water weevil, which ranges from the Sacramento Valley to the upper San Joaquin Valley, is one of the most economically damaging rice pests in California.

Larry Godfrey, Extension entomologist with University of California, Davis, has some tips for management of the weevil. He has been doing research on it since 1992 at test plots at Maxwell and Biggs and in grower fields.

The adult weevil is approximately 4 millimeters long, has a prominent beak, and is gray with dark marking on the back from the base of the head to the middle of the wing covers. It hibernates in weeds along the levees and ditch banks of rice fields during the winter.

In the spring, during calm and humid evenings, adult weevils, which can fly several miles, leave their resting place to feed on emerged rice or grasses in water along levee banks.

After feeding in the fields for a few days, adult females lay eggs under water in the leaf sheath tissue above the plant crown. A female reproduces without mating and may lay 200 or more eggs during a period of several weeks.

"The adult weevils move into the field in the spring right when the rice is starting to break through the water," Godfrey says. The adults feed on the leaves, but this doesn't harm the crop. Emerged larvae, however, feed on the roots, causing the primary damage: reduced yields.

From late May to July when larvae begin feeding, plants may become stunted and yield loss occurs because maturity is delayed. The heaviest concentration of weevils is 15 to 20 feet from the margins of the fields and levees.

Lost Furadan For the past 20 years growers have used Furadan 5G (carbofuran) to combat the rice water weevil, but 2000 was the final year of availability in California. The phasing out of the chemical spurred Godfrey to research alternatives, including both insecticides and cultural control measures.

"Once the industry knew in the late 1990s Furadan was going to be taken off the market, it was very anxious to find alternative products," Godfrey says.

Warrior (lambda-Cyhalothrin) and Dimilin (diflubenzuron) were both registered in 1999 for use against the weevil in California. Both are very effective, Godfrey says, but require a management strategy different from Furadan.

Furadan is applied pre-plant and incorporated into the soil, killing the eggs before they hatch into larvae. Warrior and Dimilin are applied after the rice is planted. Warrior kills adults before they lay eggs, and Dimilin sterilizes the adults.

Keith Davis, a grower in Yuba City, fights the rice water weevil every year, and he has been using Warrior since 1999. "I tried some because I knew they were phasing out Furadan," he says. "I think Warrior is effective. It's a little more costly, but it seems to do a good job."

The chemical and its application method make it more expensive than using Furadan. Davis was able to apply Furadan himself, but Warrior is applied through aerial spraying just before the rice breaks through the water.

With the assistance of the California Rice Commission in obtaining registration for Furadan, California growers were allowed to use it in 2000 to deplete the stock on hand.

Growers in Texas, Louisiana, Arkansas, and Mississippi have been using Icon (fipronil), registered with the Federal EPA in April of 1999, but it is still under review by the California Department of Pesticide Regulation (DPR).

Icon is applied the same as Furadan, and Godfrey found it is easier to use than Warrior and Dimilin. Icon doesn't have the waterfowl toxicity of Furadan, but it does have problems with toxicity to aquatic organisms.

"Basically, some additional testing needs to be done, but the company thus far hasn't been willing to do it, mainly because of the expense," Godfrey says.

Cultural practices During his research, Godfrey also looked at cultural management practices to reduce the weevil population.

Cultural practices aren't stand-alone tools, he says, but when used in conjunction with chemical applications, they can be effective. In certain areas of the valley having lower infestations of the rice water weevil, growers might manage with cultural controls alone.

Delaying the seeding date of rice helps reduce the weevil population. However, delayed planting can also reduce yields.

Keeping the levees free of weeds helps reduce the infestation of the rice water weevil. Large, laser-leveled fields have fewer levees and field edges and are less susceptible to the pest.

Godfrey recommends removing weeds from levees about two weeks after seedling emergence to reduce adult infestations in fields and larval populations later in the season. Because the larvae attack the roots and reduce tillering, it is important for growers to have early and effective weed control.

Davis waits to remove weeds from levees and roads until after the hatch of pheasants. "Rice growers as a whole are very conscious of wildlife. We try, if we can, to leave some cover for them on levees and on the road and canals."

Leaving weeds does encourage the rice water weevil, Davis admits, but he says he feels it's equally important to protect upland game. "It's a delicate balance."

Madera County, Calif., Cooperative Extension county director and farm advisor Ron Vargas and Frank Zalom, entomologist and director of the University of California Statewide Integrated Pest Management program, have been selected to receive outstanding achievement awards from Friends of Agricultural Extension.

They will be honored Feb. 28 at TorNino's in Fresno, Calif.

Each year Friends of Extension, a group of farmers and ranchers honor a UC Cooperative Extension farm advisor and a specialist for their contribution to California production agriculture in a 12-county area in the central part of the state.

Vargas and Zalom will receive awards of $5,000 each.

Vargas is being honored for his work in cotton weed management, including the use of metham sodium to cut nightshade hand hoeing costs by 50 to 75 percent; developing a program to facilitate the registration of the over-the-top herbicide Staple, and more recently his efforts evaluating transgenic herbicide-tolerant cotton varieties.

Vargas is now evaluating these transgenic cottons in conservation tillage farming.

Zalom has served as state IPM director since 1986, but is being honored specifically for his work in developing the California almond integrated pest management program, which reduced the reliance on organophosphate dormant sprays and still controlled the peach twig borer.

He has also conducted applied research in a wide array of other crops, including grapes, strawberries, processing tomatoes, stone fruit, applies, cotton, alfalfa, rice and cole crops.

Information on the awards program is available by calling (559) 665-4187 or (559) 787-2122.

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