By Leslie Mertz
A moth caterpillar called the Mexican rice borer (Eoreuma loftini) that has already taken a heavy toll on sugar cane and rice crops in Texas has now moved into Louisiana and Florida, and continues to spread through the Gulf Coast region. A new article in the Journal of Integrated Pest Management provides a more complete picture of the pest and offers suggestions about how to manage them.
The history of Mexican rice borers in the United States is about a century old. It was first described in Arizona in 1917, but it drew little attention until it arrived in southern Texas in 1980. Within just a couple of years of its appearance there, it became the primary pest of sugar cane, according to Julien Beuzelin, lead author of the JIPM paper. He is an assistant professor at the Louisiana State University Agricultural Center with a joint appointment in the Department of Entomology and at the Dean Lee Research & Extension Center.
Since then, the insect has moved north and east along the Gulf Coast at a rate of about 15 miles per year. It made its way into southwestern Louisiana in 2008, and has since continued its march into and now beyond central Louisiana.
“Out of the blue in 2012, it was detected for the first time in central Florida and is now established there too,” Beuzelin said.
The Mexican rice borer causes damage to a variety of grasses, extending beyond sugar cane and rice to sorghum, corn, and non-crop grasses. In fact, Beuzelin said, it will attack any grasses that have stalks large enough for them to burrow into. He explained that the larvae hatch from eggs laid on leaves and stalks (or culms). The young caterpillars crawl onto the green parts of the plant and start feeding, and after the second or third molt, burrow into the culm.
In rice, Mexican rice borers can kill the growing part of the grain-bearing branch (or tiller), which is known as “deadhearting.” They can also kill the plant’s seed head (or panicle) and the developing grain, which is known as whiteheading because it turns the panicle white. Such damage could result in many millions of dollars of crop loss. One study suggested that in a worst-case scenario, the insect could cause more than $40 million a year in rice losses, and more than $200 million losses in sugar cane in Louisiana alone.
“We don’t think it will be as bad as that, but those are high numbers,” Beuzelin remarked.
One of the uncertainties is how far the Mexican rice borer will spread. Preliminary studies conducted in Texas compared it to a similar insect called the sugarcane borer (Diatraea saccharalis), which has moved into northern Louisiana and perhaps as far as southern Arkansas.
“Those studies found that the Mexican rice borer could probably tolerate temperatures colder than the sugarcane borer, so we think it will move at least into northern Louisiana,” Beuzelin said. “How far it will ultimately go, however, is a question mark. We hope to work on that.”For now, however, the focus is on how to control the Mexican rice borer where it already occurs. Currently, growers are mainly relying on a diamide pesticide known as chlorantraniliprole, which works well against both Mexican rice borers and another rice pest called the rice water weevil (Lissorhoptrus oryzophilus), but a concern has arisen. Chlorantraniliprole works in much the same way as another diamide that might have its registration cancelled by the U.S. Environmental Protection Agency (EPA) due to potential negative impacts on some aquatic organisms.
“Because chlorantraniliprole has the same mode of action, the entomological community is afraid this might happen with chlorantraniliprole as well,” Beuzelin said. “We don’t expect it to be taken off the market, but we just don’t know.”
Other control methods beyond pesticides are available, although many need additional study. An example is to grow resistant varieties of crop grasses, which often work well to deter pests.
“Unfortunately, this has not been much of a focus of research in the past few years, so we need to get back on that,” Beuzelin said.
Research results have highlighted some steps that growers can take. One is adjusting cutting height from the usual 16 inches to 8 inches, essentially cutting away stems that are infested with larvae.
“This can decrease the number of Mexican rice borers in the stubble,” Beuzelin said.
Another control method is to plant early. According to field experiments, later plantings (in mid-May vs. mid-March), as well as ratoon cropping, have increased infestations.
Soil amendments, particularly silicon, may also be helpful.
“This is ongoing work that we are doing, but we think the addition of silicon may be a cheap way to make rice more resistant to rice borers,” Beuzelin said.
Besides these measures, traps baited with synthetic hormones can be useful in identifying a coming infestation.
“These traps are good early-detection tools. Typically, if you find an adult in one of the traps, you will find the larvae in the field one or two years later,” Beuzelin said, noting that the advance warning gives growers a chance to weigh their options and mount a defense.
While he encourages research on control measures beyond pesticides, he is also interested in the Mexican rice borer as a model for landscape-wide management.
“Because the Mexican rice borer is an invasive species that affects multiple crops, it gives you a chance to study landscape dynamics, and try to see if managing landscape can help,” he said.
For instance, simulation studies have shown that lowering the population of the insects in adjacent non-crop grasses is correlated with a reduction in the population in the rice field, which might mean that eliminating non-crop host plants could decrease cash-crop infestations.
“In other words, instead of just taking a management approach on a field basis, it might be beneficial to manage this insect over a wider area,” Beuzelin said. “I think the Mexican rice borer would be a good model for such landscape-wide management studies.”
And that, he said, could provide needed insight and possibly control approaches for other crop pests too.
“As an entomologist, this makes the Mexican rice borer very interesting,” he said.
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Leslie Mertz, PhD, teaches summer field-biology courses, writes about science, and runs an educational insect-identification website, www.knowyourinsects.org. She resides in northern Michigan.