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Most Wanted: Mexican Rice Borer

Sneaky Texas pest seeks succulent hideout in Louisiana rice fields


By Carroll Smith
Editor
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Although American newspaper editor Horace Greeley made famous the quote, “Go West, young man,” the Mexican rice borer (MRB), a troublesome pest for Texas sugarcane and rice producers, obviously did not get the memo. Instead, the MRB – a type of stem borer – headed east, and on Dec. 15, 2008, was discovered near Vinton in southwest Louisiana in a pheromone trap.

At the time, the Louisiana Department of Agriculture and Forestry (LDAF) had been working with LSU AgCenter entomologist Dr. Gene Reagan’s lab, monitoring pheromone traps – exclusively for the Mexican rice borer – every two weeks across the state. After the first catch, they ramped up the trap numbers, but the MRB did not appear again until late fall 2010 south of Welsh in Jeff Davis Parish, and the number of moths quickly increased. In 2011, MRB larvae were found in Louisiana rice fields.

According to Dr. Reagan’s research, the pest moves at a pace of about 15 miles a year.

“We think this is mostly nonassisted movement, but that might change,” says Dr. Julien Beuzelin, LSU AgCenter post doctoral researcher in entomology. “When you have larvae and pupae in sugarcane and the cane is harvested, it is possible that by hauling the crop from the harvesting point to the milling point in the fall might alter the rate of dispersal.”

Dr. Natalie Hummel, LSU AgCenter entomologist, says another source may be straw that has been harvested from a rice field, which potentially could increase the 15-mile-a-year movement. However, all of the LSU AgCenter entomologists emphasize that this is not a time to panic, but rather a time to build awareness among farmers and educate them about practices to help suppress populations and develop a management plan should the MRB appear in any of their fields.

Identification Guidelines

The first step is to properly identify the pest as its appearance is similar to the sugarcane borer and rice stalk borer. Following is a description of the three “outlaws” as published by the LSU AgCenter, LDAF and Texas AgriLife:

“Mexican rice borer adults are light tan moths with delta-shaped wings. By comparison, sugarcane borer moths are larger, straw-colored moths about ¾-inch long with a series of black dots arranged in an inverted V-shaped pattern on the front wings. Mexican rice borer adults produce spherical, globular, cream-colored eggs hidden between the folds of dried leaves. After hatching, young larvae feed inside fresh leaf sheaths and then bore into the stem or stalk. This feeding causes an orange discoloration of the leaf sheath.

“Mexican rice borer larvae are whitish with a light-colored head capsule and two pair of dark purple stripes running the length of the body. By comparison, sugarcane borer larvae are yellowish or white with a series of brown spots on the back.

“Rice stalk borer adults are about one-inch long with pale white fore and hind wings tinged on the edges with metallic gold scales. The front wings are peppered with small black dots. Larvae of the rice stalk borer are pale yellow-white with two pairs of stripes running the entire length of the body and have a black head capsule (very similar to the Mexican rice borer larva in appearance). In general, the Mexican rice borer has light-colored hairs and head capsule, while the rice stalk borer has dark hairs and head capsule.”

If rice fields are located in an area where MRB has been positively identified, farmers may want to consider a seed treatment application of Dermacor X-100 as a preventative strategy.

“Dermacor stays present in the plant long enough to have some activity against stem borers,” says LSU AgCenter entomologist Mike Stout. “If a farmer is using Dermacor to control rice water weevils, then the seed treatment will help suppress stem borers if they are present in the field.

“Scouting for MRB is similar to scouting for sugarcane borer,” he adds. “Look for orangish, yellowish feeding lesions. At that point, spray a pyrethroid before the larvae get into the stem. Once that happens, the pest is protected from the insecticide.”

Since this is a new pest moving into the state, Hummel says the treatment threshold is just the presence of MRB to prevent their spread and establishment.

Deadheart, Whitehead Injury

The MRB typically moves into rice from panicle development to boot. Prior to that, the pest may already be present in such weeds as Johnsongrass and vaseygrass, but they wait to lay their eggs in the older stages of the plant and prefer rice to the grasses.

“Most of the eggs that we find are laid on dry-leaf materials, which may explain their preference for older plants,” Beuzelin says.

According to the LSU AgCenter, “Rice injury begins with feeding in leaf sheaths. Borers then tunnel inside the stem. Signs of early injury in rice are withering and death of the youngest leaf, resulting in a condition called deadheart. Most infestations are not obvious until after the boot stage. Stem feeding during panicle development causes partial or complete sterility and the whitehead condition. The white, empty panicles are lightweight and stand upright. Feeding inside the stem can also cause plants to lodge before harvest.”

With this in mind, common sense says to scout the fields to find larvae at a treatable time.

“Once you see a lot of whiteheads in the field, there is no point in treating because you have already suffered injury,” Hummel says. “Don’t apply insecticides at that time, which would only compound the amount of money lost on your crop.”

MRB Management Plan In Rice

Because the MRB causes direct yield reduction via deadheart and whitehead injury, LSU AgCenter is asking farmers to consider following a MRB management plan if they have fields in areas where the pest has been found.

1. Learn to correctly identify the MRB and participate in population monitoring. If you suspect a Mexican rice borer infestation, contact your local county agent or LDAF at (225) 952-8100.

2. If you typically have rice water weevils and are in an area where MRB is present, consider applying Dermacor X-100. The EPA recently approved a 24(c) registration for Dermacor X-100 use in water-seeded rice in Louisiana. If you don’t use Dermacor, aggressively scout your rice at the susceptible stages and spray a pyrethroid if larvae are found in the field.

3. Strongly consider stubble management practices. Reduce the harvest cutting height, unless this conflicts with environmental regulations regarding duck habitat management. In the fall, if it’s feasible, consider flooding or plowing to destroy the stubble because it can serve as an overwintering host.

4. Consider cultivar selection. “Although there are no resistant cultivars, we have found that some are more or less relatively susceptible,” Hummel says. “For example, some of the hybrids are moderately susceptible, while others are susceptible.”

5. Consider non-crop habitat management. Manage grasses, mainly Johnsongrass and vaseygrass, along field margins and ditch banks in the fall and early spring to keep MRB populations from multiplying.

In conclusion, Hummel notes, “We’re not trying to eradicate this pest, but we want to do as much as we can in management systems to keep the populations down on an area-wide basis.”

Contact Carroll Smith at (901) 767-4020 or csmith@onegrower.com.


Ongoing Behind-The-Scenes Efforts

In an effort to promote awareness and provide education about the Mexican rice borer, LSU AgCenter’s Anna Meszaros, research associate specialist in entomology, has helped coordinate the Extension outreach, which included creating and organizing MRB content into the new LSU AgCenter rice insects Web site: www.lsuagcenter.com/riceinsects. She assisted in planning and conducting training sessions and has also been an active member of the LSU AgCenter MRB research team with entomologist Dr. Julien Beuzelin.

Beuzelin, whose MRB research has been done under the supervision of LSU AgCenter’s Dr. Gene Reagan and Drs. Ted Wilson and Mo Way with Texas AgriLife at Beaumont, primarily investigated weeds and fallow rice fields.

“Year round, the grasses surrounding rice fields host Mexican rice borers,” he says. “On average, we saw densities from 1.5 to 6 borers per square meter, so weeds can be a substantial source of borers. That’s why we think it is important to have a comprehensive management strategy. While we know that borers are not present in newly planted rice, they still have to come from somewhere.

“Besides weeds, a fallow rice field containing stubble or volunteer rice can also be a source for MRB and needs to be managed properly,” Beuzelin adds. “Right now, we haven’t quantified how much managing these non-crop Mexican rice borer habitats will decrease the infestation, but we are still working on that and consider it a priority.”

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