On the march

As Mexican rice borer expands, a collaborative effort is studying new management regimes and control methods.

By Vicky Boyd
Editor

Mexican rice borer larva

Mexican rice borer larvae bore into rice stalks, where they feed and grow, protected from pesticides — Photo by Dr. Johnny Saichuk, LSU AgCenter Extension rice specialist emeritus

Since the Mexican rice borer was first confirmed in Texas in the 1980s, it has slowly expanded its range throughout
Southeast Texas and into Southwestern Louisiana, infesting rice, sugar cane and a host of grassy weeds.

Although researchers and growers have found an insecticidal seed treatment helps manage the pest in both the first and second, or ratoon, rice crops, they agree that relying on one management tool is never a good idea.

“There’s always a concern when you rely on one single tactic over and over again,” says Julien Beuzelin, an assistant professor of with the Louisiana State University AgCenter.

A collaborative research effort between LSU and Texas A&M seeks to answer some of the unknowns associated with this relatively new pest, including the role plant fertility plays, alternate host plant management, economic thresholds, variety or hybrid tolerance, and possible new chemical controls.

The project, which kicked off in 2014, is being funded by a three-year U.S. Department of Agriculture National Institute of Food and Agriculture grant. Among the researchers involved are Beuzelin; Dr. Mike Stout, L.D. Newsom Professor of Integrated Pest Management with LSU’s AgCenter; and Dr. Mo Way, an entomology professor with Texas AgriLife Research in Beaumont.

Beuzelin also was lead author of an article published in the April Journal of Integrated Pest Management that provided a historical perspective on Mexican rice borer biology and management of the pest to date. As the group of authors from LSU and Texas A&M University researched the article, they noted knowledge gaps and used the information as a basis for the USDA grant proposal.

Mexican rice borer on the march
Mexican rice borer was first identified in the Texas Rio Grande Valley in 1980, and it has spread throughout Southeast Texas. In 2008, it was confi rmed in Calcasieu Parish, La., and has expanded eastward in Louisiana by 10 to 15 miles annually. It is now found in nine Louisiana parishes: Calcasieu, Beauregard, Allen, Cameron, Jefferson Davis, Acadia, Vermilion, St. Landry and Evangeline.

The first Florida detection occurred in 2012, and four Florida counties now have confirmed populations. Human intervention, such as transportation of infested plant material, likely hastened the spread.

One unknown is how far north this new pest will move. Two related insects — the sugarcane borer and the rice stalk borer — are sporadic pests of Louisiana rice. Sugarcane borer has been found as far north as North Louisiana and South Arkansas.

Beuzelin says he suspects the Mexican rice borer also could survive in South Arkansas, but cold tolerance is an area where he wants to conduct additional studies.

After initial establishment, Mexican rice borer populations tend to increase over several years until they begin causing economic damage to rice. Traps baited with female pheromone continue to be used to detect pest expansion.

Mexican rice borer eggs

Mexican rice borers lay eggs typically in the folds of dry plant material, leaves and leaf sheaths — Photo courtesy LSU AgCenter

Current management

As Mexican rice borer populations have increased, Texas and Louisiana producers have turned to Dermacor X-100 seed treatment, which contains the active ingredient, chlorantraniliprole.

Not only does the product help manage stalk borers, but it also controls rice water weevils and fall armyworms.

This season, Stout says upwards of 70 percent of growers in Southwest Louisiana used Dermacor seed treatment.

The seed treatment also appears to reduce stalk borer infestations in the ratoon crop, possibly by reducing overall pest populations in the main crop, Way says.

Growers can obtain similar stalk borer control with two pyrethroid applications typically tankmixed with fungicide applications, he says. But pyrethroids applied at those timings don’t address rice water weevil control.

In addition, pyrethroids are broad-spectrum insecticides that could potentially affect non-target species, Beuzelin says.

“The big issue is we don’t have good thresholds on when to spray — that’s something we’re working on,” he says. Part of the research also will try to quantify yield reductions caused by Mexican rice borer.

Although relying on a single class of chemistry may contribute to resistance, Way says the wide host range (many common grass weeds) of the Mexican rice borer can mitigate the development of insecticide resistance. As a result, only a portion of the overall population is exposed to the insecticide.

“It’s everywhere all of the other host plants are,” he says. “Practically every grass you can find in Texas is a host of Mexican rice borer and they act as refuges for them.”

Among the grass hosts are johnsongrass, barnyardgrass, Amazon sprangletop and vaseygrass, also known as Paspalum.

What has Beuzelin more concerned is the Environmental Protection Agency’s effort to vacate registration for flubendiamide, marketed as Belt. It is in the same class of diamide chemistry as chlorantraniliprole.

“I don’t now how closely related they are as far as degredation in the environment,” Beuzelin says.

The EPA cites possible toxicity to benthic organisms — microorganisms that live in ocean sediments — as reason for its actions.

Flubendiamide’s registrant, Bayer CropScience, says the EPA’s logic is flawed and has requested a hearing before an EPA administrative judge.

borer signs of feeding

Early instar Mexican rice borer, along with other borer, cause this orangish discolaration at feeding sites — Photo courtesy LSU AgCenter

Cultural control

Several cultural practices aid Mexican rice borer management, including early planting, stubble cutting height and what Beuzelin referred to as “landscape-wide management.”

A trial conducted in Southeast Texas compared infestations in rice planted in mid-March, mid-April and mid-May. The heaviest Mexican rice borer and sugarcane borer infestations were in the main crop from the later planting dates and in the ratoon crop from the early planting date.

That’s because the rice was at the most attractive growth stage when female moths were seeking sites on which to lay eggs. Reducing cutting height of the main crop stubble to 8 inches from 16 inches may reduce infestations by 70 to 81 percent, according to a collaborative Texas A&M and LSU study.

By doing so, growers chop stems potentially infested with borers. Way says reducing the cutting height also helps ensure a more uniform and consistent ratoon crop.

But some growers are hesitant to adopt the practice because it slows harvest speed, increases wear and tear on combines, and delays second crop maturity, Beuzelin says.

In addition, tests Way conducted several years ago examined varietal and hybrid susceptibility. Data showed hybrids exhibit fewer whiteheads (borer-damaged heads) than conventional varieties. With newer varieties and hybrids on the market, Way says he wants to revisit the subject.

The researchers also have begun trials to examine the role high nitrogen rates may play in Mexican rice borer damage. Previous research found that nitrogen rates above recommended levels tended to increase rice borer infestations, Beuzelin says. High nitrogen rates also can increase the severity of several rice diseases.

In addition, current trials are looking at how silicon soil amendments may toughen culms and leaves.

Because of Mexican rice borer’s wide host range, Beuzelin also suspects that eliminating weedy hosts around rice fields and practicing landscape-wide management also could help reduce pest populations.