T. Eugene Reagan, Francis P.F. Reay-Jones, Ben Legendre, M.O. Way and José Amador

LSU AgCenter scientists have joined forces with Texas A&M University scientists to address Mexican rice borer problems in sugarcane and rice. The first mission was to monitor Mexican rice borer movement through the Texas Rice Belt and cooperate with state regulatory agencies to protect the Louisiana sugarcane and rice crops from invasion by the new pest.

The Mexican rice borer is a devastating pest of sugarcane and a serious pest of rice that cannot be controlled (at least in sugarcane) with insecticide sprays. Basic life cycle biology of the Mexican rice borer has been studied mainly on sugarcane, which has suffered from severe infestations in Texas. The majority of female moths lay eggs (oviposit) on dried sugarcane leaves on the lower part of the plant, where they are relatively protected from predation and parasitism. The spherical, cream-colored eggs are laid in groups of 5 to 100. Young larvae quickly migrate from the oviposition site to bore into green and moist parts of the plant suitable for feeding. Toward the end of development, the larvae have tunneled in the stalk both vertically and horizontally.

Pupation takes place in tunnels after mature larvae have constructed an emergence window covered by one or two layers of plant tissue. This is a relatively protected environment compared to the sugarcane borer, which produces a hollow cavity and is therefore more accessible to parasites and predators. It is also another reason why insecticide applications are less effective in Mexican rice borer control.

The insect is active throughout the growing season in the Lower Rio Grande Valley of Texas, where many of the larvae continue to feed, develop and complete all phases of the life cycle (including oviposition) through the winter months not only in sugarcane, but in many of its alternate hosts as well.

Host Preference

One way of developing a crop resistant to the Mexican rice borer is to select a variety less attractive for egg laying (oviposition). To supplement field research at Ganado and Eagle Lake, greenhouse studies were conducted at the Weslaco Center involving different stages of sugarcane and rice. Two sugarcane varieties, LCP 85-384 and HoCP 85-845, under different water stress conditions, and two rice varieties, Cocodrie and the rice hybrid XL8, were selected.

Data in Table 1 are based on studies in which different developmental stages of sugarcane and rice were exposed to ovipositing moths. Sugarcane was more attractive than rice, and LCP 85-384, which represents 90 percent of Louisiana sugarcane acreage, was the most attractive variety. On sugarcane, the moths laid eggs only on brown leaves or brown tips of leaves. Drought-stressed sugarcane was more attractive because of increased numbers of dry leaves.

Rice generally was less attractive than sugarcane with no oviposition on the 3-4 leaf tillering stage. However, contrary to sugarcane, oviposition on rice did not occur exclusively on dry leaves, with a major portion of egg masses laid on green leaves or inserted behind leaf sheaths. Changes in chemical physiology associated with plant stage and variety were more important in explaining ovipositional preference differences in rice. Field studies are showing that modifying rice planting dates may help to avoid critical periods when yield loss from stem borers is most severe.

Variety Resistance

Criteria for field assessment of resistance to the Mexican rice borer in sugarcane include percentage bored internodes and adult emergence holes, the latter used to determine the relative impact each variety has on the potential areawide buildup or reduction of adult Mexican rice borer populations.

As shown in Table 2 from an experiment at Ganado, Texas, variety CP 70-321 was the most resistant. LCP 85-384 was the most susceptible, substantially more susceptible than NCo 310, traditionally the most susceptible variety grown in the Lower Rio Grande Valley of Texas. It is encouraging to note that there is biological resistance to this pest. However, the domestic sugarcane breeding programs have not yet taken full advantage of this germplasm in developing high yielding varieties resistant to the Mexican rice borer.

Stem borer host plant resistance studies conducted in rice each of the last several years have included both commercially produced and experimental varieties with insecticide treated (two applications of Karate Z) and untreated plots in a four replication experiment. Using number of whiteheads (borer killed panicles) and yield, Priscilla was consistently the most susceptible, followed by Concordia, Jefferson and Cypress. In hybrid varieties XL8, CLXL8, XP723 and CL161 were the most resistant.

Multiple Tactic Management Studies

A two-year field study also assessed the role that irrigation, when used in combination with variety selection and insecticide application, plays on the effective management of the Mexican rice borer. To achieve the degree of insecticide control shown in Figure 1, seven applications of Confirm (tebufen-ozide) were sprayed every two weeks (June to mid August). In this four replication test, the untreated (nonirrigated) LCP 85-384 had greater than 70 percent bored internodes, compared to nearly 40 percent under the heavy insecticide pressure.

Irrigation reduced injury levels in both varieties, which can be explained by the decreased attractiveness for oviposition on nonstressed sugarcane. Injury in both resistant and susceptible varieties still exceeded 20 percent and 40 percent bored internodes, respectively, in untreated, irrigated plots. As shown in Figure 1, only when all management tactics were used was injury suppressed below 10 percent (LCP 85-384) and 5 percent (HoCP 85-845).

Summary

Studies in the Mexican rice borer program collectively have emphasized the value of employing multiple tactics used in combination to manage insect pest infestations. If the crop plants can be made less attractive to the pest through resistant varieties or healthier management, the use of insecticides, when they work, can be more efficient. Another factor which enhances the permanency of an integrated pest management program (IPM) is the addition to Cheniere, the importance of achieving a balance among the use of diverse control tactics. If an overemphasis on any tactic (especially insecticides) can be reduced, the selection pressure on the pest population is also reduced, enhancing the permanency of the IPM system. This often includes biological control to conserve and enhance predators and parasites maintaining lower pest populations. 

For Louisiana, the anticipated problems with the Mexican rice borer will continue to demand a team approach with areawide perspectives both in sugarcane and rice. We want to develop and implement practices that not only protect the host crops, but also reduce pest populations.

T. Eugene Reagan, Austin C. Thompson Endowed Professor, and Francis P.F. Reay-Jones, graduate research assistant, Department of Entomology, LSU AgCenter, Baton Rouge, La.; Ben Legendre, professor and sugarcane specialist, LSU AgCenter St. Gabriel Research Station, St. Gabriel, La.; M.O. Way, associate professor and rice specialist, Texas A&M University Agricultural Research and Extension Center, Beaumont, Texas; and José Amador, professor and director, Texas A&M University Agricultural Research and Extension Center, Weslaco, Texas.

Acknowledgments This program has been supported by grants from the U.S. Department of Agriculture Cooperative State Research, Education and Extension Service through its Critical Issues, Southern Region IPM and Crops-at-Risk IPM programs and from the American Sugar Cane League and Texas rice producer groups. Chris Carlton, associate professor, and Dale Pollet, extension specialist, Department of Entomology, LSU AgCenter, Baton Rouge, La., and William H. White, USDA Agricultural Research Service, Sugarcane Research Unit, Houma, La., also participate in the program.

(This article was published in the winter 2005 Louisiana Agriculture Magazine.)