T. Eugene Reagan, Julien Beuzelin, Blake Wilson, Allan Showler and M.O. Way

From an entomology integrated pest management perspective, invasive species have several characteristics that make them difficult to control. As the insect is brought in or migrates into new areas, only rarely does the new pest bring along its natural enemies, which can include parasites, predators and diseases. The Mexican rice borer’s move into the Lower Rio Grande Valley of Texas sugarcane area in 1980 is a prime example. Within a year of its discovery in the Valley, some farmers found damage so severe that they were unable to successfully harvest their sugarcane fields. As the insect moved out of the Valley and through the Texas Rice Belt toward Louisiana, rice farmers began using three insecticide applications directed mostly to stem borer control. Economic loss projections for this "alien" pest are expected to reach as high as $220 million for sugarcane and $45 million for rice industries in Louisiana in the next few years.

Scientists in Louisiana and Texas are studying two fundamentally different approaches. The first approach uses certain aspects of Mexican rice borer biology to help farmers better protect their sugarcane crop on an individual field basis. The second approach to pest management involves a "landscape ecology" perspective understanding that several grass weeds serve as important hosts to the perpetuation of this insect.

Many years of entomology research with Texas A&M scientists on insecticidal control of the Mexican rice borer have often shown inconsistent results on sugarcane. Much of this earlier work was modeled after the successful sugarcane borer scouting program in Louisiana. More recently, LSU AgCenter scientists in collaboration with those in Texas have been studying pheromone trap monitoring of adult Mexican rice borer moths to help predict treatable larval infestations.

Experiments in the summer of 2010 involved greenhouse studies at the U.S. Department of Agriculture research facility in Weslaco, Texas, and large field aerial application studies in the Lower Rio Grande Valley near Santa Rosa. Scientists found that in comparing Mexican rice borer larval feeding immediately after egg hatch (neonate larvae), the data show that more than three quarters of the new larvae on susceptible Louisiana variety HoCP 00-950 bore into the plant within one day, where they are mostly protected from insecticides. The remaining 24 percent of the larvae bore into the plant within six days on average. On the Mexican rice borer-resistant HoCP 85-845 variety, more than 41 percent of the neonates escaped potential insecticidal exposure one day after hatching out of the egg masses. See Table 1.

Using weekly pheromone trap collections of the Mexican rice borer to help improve the timing of insecticidal control of larvae, researchers treated five fields, ranging in size from 35 to 78 acres with insecticides. The treatments were made on Aug. 21, 2009 (Figure 1). Each field was divided into three separate plots for aerial spraying of labeled rates of Baythroid (beta-cyfluthrin) and Diamond (novaluron) at a time when plant infestations averaged 14 percent. The data on insect damage show substantial control – 6.8 percent bored internodes (Diamond) versus 20.4 percent on the nontreated sites. See Table 2. The study was repeated in 2010.

Early studies conducted in Mexico during the 1920s found that virtually any large grass could host the Mexican rice borer. However, the role of noncrop hosts in Mexican rice borer population changes has only recently received consideration for pest management. LSU AgCenter scientists have conducted studies for two years to determine Mexican rice borer infestations in noncrop hosts under natural conditions. Three farms were selected in the upper, middle and lower Texas rice production area. On each farm, noncultivated habitats adjacent to rice fields were sampled year-round on a six-to-eight-week basis. Average densities of the Mexican rice borer ranged from 0.3 to 5.7 borers per square meter throughout the year. Early annual grasses such as ryegrass, brome and canarygrass were infested during the spring, whereas the perennial johnsongrass and vaseygrass were infested throughout the year. See Figure 2 and Table 3. Johnsongrass was the most prevalent host (41 percent to 78 percent relative abundance), but vaseygrass (13 percent to 40 percent relative abundance) harbored up to 70 percent of all borer infestations during the 2007-2008 winter.

These studies confirm that noncrop hosts play a role in Mexican rice borer population overwintering and build-up during the spring. In addition, the relative importance of these noncrop hosts changes at different times of the year, with vaseygrass representing a major overwintering host. The manipulation of noncrop hosts may decrease a significant proportion of Mexican rice borer populations, thus decreasing infestations in crop fields. Ongoing research involves simulations of different landscape weed management strategies (mowing, insecticide applications, other cultural practices) to predict their effect on the Mexican rice borer populations.

Even though this alien invasive species has been in the extreme southwestern portion of Louisiana since December 2008, it has not yet caused detectable losses to the state sugarcane and rice industries. Successful management of Mexican rice borer infestations will have to rely on a comprehensive strategy that includes protecting individual crop fields, but also decreasing areawide pest populations. Improved crop production practices involve using irrigation because plants are more vulnerable in droughty conditions, planting resistant varieties and better timing of insecticides using pheromone trap-assisted scouting. Decreasing areawide populations involves managing Mexican rice borer noncrop hosts and resistant crop varieties. 

T. Eugene Reagan, Austin C. Thompson Endowed Professor, Julien Beuzelin and Blake Wilson, Graduate Research Assistants, Department of Entomology, LSU AgCenter, Baton Rouge, La.; Allan Showler, Research Entomologist, USDA-ARS Kika de la Garza Subtropical Agricultural Research Center, Weslaco, Texas; and M.O. Way, Professor of Entomology, Texas A&M University AgriLife Research Center, Beaumont, Texas

(This article was published in the fall 2010 issue of Louisiana Agriculture.)