Rice Farming

Pests Back On
California’s Radar

Get familiar with cultural practices/insecticide options

By Brenda Carol

Rice water weevil and other invertebrate pests are usually only sporadic pests in California rice production, but populations have been a little more problematic than usual the past two or three years, according to Larry Godfrey, Extension entomologist with the University of California.

“Populations of rice water weevil in 2007 were about 20 percent higher than they were in 2006,” he says. “We’ve seen them trending higher over the past several years. They have not reached the levels that we saw in the mid-1990s, but still they are increasing.”

It’s a concerning trend for the industry due to limited control options and the damage the pest can cause. Adult rice water weevils feed on rice leaves, but the damage during this stage is relatively minor.

The larvae (root maggots) that feed on the roots of the rice plant after hatching cause the most significant damage. This feeding prunes the root system, reduces the development of tillers and panicles, slows the uptake of nutrients and delays maturity. The overall impact ultimately reduces yield by reducing plant vigor.

Control options for rice water weevil
Today, pyrethroids are the primary defense against rice water weevil, and those materials are in jeopardy, according to Godfrey. Pyrethroids are under scrutiny, and re-registration issues are threatening to limit use, particularly in production systems where runoff could contaminate nearby waterways.

While pyrethroids have not been found freely in water, the molecules bind to organic matter, which can be moved through water. Widely publicized research conducted by Don Weston, California adjunct professor, U.C. Berkeley, has shown that pyrethroids pose a potential toxicity problem in streams, lakes and other water bodies. Although that research is still ongoing, Godfrey says the industry needs to be looking for alternatives.

“About 95 percent of the insecticides used in California rice production are pyrethroids,” Godfrey says. “That’s concerning not only because those materials are under scrutiny, but also due to the increased potential for developing resistance with over-reliance on one class of chemistry.”

Fortunately, there are new chemistries in the pipeline. One product, clothianidan, from Valent is being evaluated at pre-flood, in the water and as a seed treatment.

“I do not see it as a seed treatment, but we are more interested in using it pre-flood or in a water application,” he says.

“I have used it at pre-flood at lower and lower rates, and it is a very active product. It has a lot of sustainability features, and the University is very interested in getting the product registered. Valent is facilitating that.”

A product with the trade name Trebon (etofenprox) is manufactured by Mitsui Chemicals Inc. and distributed by Landis International. The third possibility is Steward (indoxacarb) from DuPont. It has activity on rice water weevil, but also kills crayfish. While the product is unlikely to be registered in other rice-growing areas in the United States, the company could pursue an IR-4 registration in California for Steward.

In the meantime, researchers continue to fine tune cultural practices to better manage rice water weevil and reduce reliance on chemical control. Varietal resistance is one possibility although probably not a complete replacement for chemical control.

“We’re looking at varieties to see if they have some resistance,” Godfrey says. “The varieties may look the same to us, but larvae and adult weevils look at them differently.”

Other cultural practices that can reduce weevil populations include removing weedy vegetation on rice levees in the spring and drill seeding. Winter flooding can help reduce weevil populations by approximately 50 percent. Additionally, winter flooding enhances straw breakdown and provides habitat conducive to waterfowl.

If treatment is required, it is often only necessary to treat 30 to 50 feet adjacent to the levees. This is the area with the most rice water weevil infestation, according to research by the University of California.

Tadpole shrimp/rice seed midges
Rice water weevil is not the only pest making a comeback in recent years.

“We’ve had more tadpole shrimp and rice seed midges than usual,” Godfrey says. “These pests are sporadic in rice, but they can cause substantial damage at times.”

Tadpole shrimp reduce seedling rice stands by chewing off the coleoptile, roots and leaves of the rice seedlings, which can kill the rice plants, according to University of California research. Their digging activities associated with egg laying can reduce light penetration and slow the growth of submerged seedlings. Tadpole shrimp cause no injury to rice once leaves have reached the water surface, and roots are well established in the soil.

Rapid seeding after flooding is one way to help minimize problems. If control measures are warranted, carbaryl or copper treatments can be used.

Rice seed midges are damaging to germinating seeds and very young seedlings. The larvae feed on the emerging shoot, leaves, roots, or may hollow out the embryo and kill the plant. There are no registered chemicals available for control in rice; however, cultural controls such as draining the field and seeding as soon as possible after flooding can help keep damage to a minimum.

Unpredictable pests
“The ‘hit and miss’ nature of all these pests makes research difficult due to the unpredictability,” Godfrey says. “In addition, management with insecticides is also challenging because infestations are not consistent.

Cultural controls are important for these pests and represent some of the most cost effective control measures.”

The sporadic nature of rice water weevil in particular was clearly evident in 2007 at Godfrey’s research trials at the Biggs, Calif., Rice Experiment Station. Throughout most of the season, he caught nothing in the traps.

However, during two days in late April and three days in early May, he caught the entire season’s infestation of rice water weevils in traps.

“That obviously makes it very hard to predict and treat for rice water weevil,” he says.

Godfrey will continue to research invertebrate pests in rice and develop more refined strategies for effective management.

In the realm of chemical control, University researchers will also continue to look at insecticide use and how compounds may potentially affect other insects, birds, fish and animals in California’s extremely diverse rice ecosystem.

“We want to make sure we’re not having any negative long-term effects on the environment,” he says.

Brenda Carol is a freelance writer based in California. Contact her at (209) 728-9226 or brenda@brendacarol.net.