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Rice Panicle Mite
Microscopic pest shows up in greenhouses out West

By Brenda Carol
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It’s certainly not a disaster at this point, but it’s a concern. The rice panicle mite (PRM) has been identified in greenhouses in California. Researchers and others in the industry are not taking any chances with this microscopic pest, which has wreaked havoc in many rice-producing areas around the world.

At low populations, the rice panicle mite feeds on the internal surfaces of leaf sheaths. However, when populations expand, the pest can be found on all parts of a rice plant. It causes deformation to the rice shoot, browned rice hulls and leaf damage. The microscopic mite can indirectly transmit sheath rot as well as other pathogens that can cause sterility.

The rice panicle mite has already been identified in several ricegrowing regions of the United States. It has been detected in Louisiana, Arkansas, Texas, Ohio and New York. Even with California’s temperate climate, the state is not necessarily immune to this tropical pest. At present, it has not been detected in commercial production, but researchers aren’t ruling out the possibility. It was first detected at UC-Davis greenhouses in 2007, and immediate measures were taken to eradicate it.

According to LSU AgCenter, the mite has reportedly caused as much as five to 90 percent of crop losses in the Caribbean region. Researchers believe that it originated in Asia, then spread to India, Africa, Central America, the Caribbean and Mexico. It is particularly a concern in field and greenhouse crops in Asia, but the pest has also moved into other rice-producing countries. USDA’s Plant Protection Quarantine (PPQ) under the auspices of Animal and Plant Health Inspection Service (APHIS) has instigated a Federal Domestic Quarantine prohibiting the movement of rice plants and untreated seed from Puerto Rico to the United States.

Every precautionary measure being taken
It’s unknown at this point if the rice panicle mite is actually a serious threat for California rice production.

“It’s hard to know if the mite will become a problem in California,” says Luis Espino, University Cooperative Farm Advisor (UCCE), Colusa County. “Given that it is a tropical species, it is unlikely that it will prosper under our temperate conditions. However, one can never be 100 percent sure. The best thing to do for now is to take all the precautionary measures necessary to avoid further spread of the mite.”

Rice panicle mite is not airborne, but it can be spread by clothing, farm machinery or infested rice seed.

The threat is enough to get California’s attention. Larry Godfrey, Cooperative Extension specialist at University of California-Davis reported his concerns recently to the Rice Research Board.

“In January 2009, the panicle rice mite, Steneotarsonemus spinki, was found in California on the UC-Davis campus infesting rice growing in greenhouses,” according to Godfrey. “This invasive pest is a “Q” rated (quarantine) pest in California.

“UC-Davis, in cooperation with representatives of regulatory agencies at the federal (Animal and Plant Health Inspection Service) and state (California Department of Food and Agriculture) levels, worked quickly to eradicate this pest,” he adds. “A rice-free period on campus, treatment of existing rice in greenhouses and new procedures were enacted to deal with the infestation and eradicate this pest. To date, the infestation in the UC-Davis greenhouses is non-existent; however, keeping this rice pest out of California is going to be an ongoing process.”

Resistance Management Matters

As tougher weeds loom on the horizon of California rice, PCAs and dealers are encouraging their growers to take a programmed approach to their weed management strategies with a strong eye to keeping resistant weeds in check.

Mike Griffith, general manager of AgriSource, encourages his growers to look to previous years’ weed pressure to know what weeds are likely to hit them this season.

“Too many growers fail to incorporate the weed spectrum they’ve seen in past years into their weed management plans,” Griffith says. “This includes the escapes or tough weeds.”

A programmed approach
“Gone are the days of making a single herbicide application early and thinking it would cover a field for the entire season,” says J.R. Gallagher, California rice specialist for Valent. “Today, growers need to use a more strategic, season-long program and understand the capabilities their herbicides offer, including the ability to work in either continuous flood or pin-point programs.”

“You need to start out with a good foundation herbicide – one that will get your watergrass, smallflower and sprangletop right from the beginning,” Griffith explains. “There are only a few herbicides that take care of sprangletop, and that’s a weed that is in a majority of California rice fields.”

Griffith uses a foundation application of Bolero UltraMax or Abolish, followed by a foliar application of Regiment CA and propanil. “I always encourage growers to be thinking of a ‘1- 2 punch’ for their program. Start out with Bolero UltraMax which, since you fly it on, you don’t have to worry about ground rates, and that is always a plus. Then come back with Regiment CA. That’s a good second punch. It gets your hard-to-get watergrass, especially that late watergrass.”

Bolero UltraMax offers foundation control of sprangletop and smallflower umbrellaplant with a 2010 recommended rate of 23.3 lb/A. “This single lower rate is new in 2010, an adjustment we made to the label after our field experience showed solid weed control offered with this improved granule’s release rate,” explains Gallagher.

Todd Miller, crop consultant with Agriform says, “There has been a lot of talk about rotating chemistries in recent years, and many rice growers are making that a part of their overall management plans. But there are still many who don’t alternate their modes of action. They keep doing what worked last year, just increasing the rates. It’s only a matter of time until they will be facing resistant weeds.”

Miller makes sure his growers understand the differences among the various chemistries that are within a single recommended program. “For example, Bolero UltraMax and Regiment CA are two different modes of action, which make for a strong program,” he says. “Where if you were to go Granite and Regiment CA, which have the same mode of action, you would not have as good an approach.”

Archer Malmo, which represents Valent, contributed information to this article.

Watch for and report any symptoms or injury
It’s not the first time California’s rice industry has faced an invasive pest challenge.
One example that comes to mind is a perennial weed, labeled “red rice,” that was identified in Glenn County in 2003 and subsequently spread to other rice-producing counties. Quick action on the part of the industry has kept that threat at a minimum.

California’s rice industry is constantly on guard for potential problems and elicits all stakeholders to keep a close watch in the field, whether it is for a suspicious weed or insect pest.

“Growers and PCAs need to be aware of the problem,” Espino says. “We at UC Cooperative Extension have been updating growers and have presented information in our annual winter meetings. If growers find any symptoms of injury that resemble those of the mite, they should contact us immediately.”

Since rice panicle mite is microscopic and not easily identified in the field, the only practical approach for a grower or pest control advisor to detect a problem is to maintain awareness of general crop health. If something is out of kilter and can’t be narrowed down to another pest, it may be time to consult a laboratory or a UC farm advisor.

A ‘wait and watch’ game for now
“Sampling for this mite is very tedious, and the identification has to be made by an expert,” Espino says. “We hope that with the measures taken so far we won’t see this mite in California rice fields.”

There are a variety of cultural and chemical controls that have proven effective against the pest. Natural enemies, such as predatory mites, may provide some level of protection. In addition, parathion, dicofol, dimethoate, isoprocarb, dichlorvos and thiophante have been fairly successful in research trials in several other areas.

For now in California, it’s a “wait and watch” game. Even though California’s rice-growing area enjoys a temperate climate not normally associated with this pest, no one should ever underestimate the capability of a pest to evolve and thrive.

Brenda Carol is a freelance writer currently based in California. Contact her at or (805) 226-9896

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