Linda Benedict | 7/7/2008 11:49:48 PM
The factor most important to resistance to the Mexican rice borer is plant vigor that minimizes the senescence, or yellowing and drying, of leaves. The Mexican rice borer is strongly attracted to dried leaves for egg laying. Mexican rice borer resistance also is influenced by the concentration or absence of certain plant chemicals, such as free amino acids in the foliage. The two borers – sugarcane and Mexican rice – interact differently with the sugarcane plant. The sugarcane borer is a vigorous-plant pest and more attracted to rapidly growing grass plants. The Mexican rice borer is a stressed-plant pest and, consequently, strongly attracted to senescing leaves.
Often the expression of plant resistance to stalk borers is influenced by the severity of infestations. Heavy borer pressure results in more bored internodes even in varieties considered resistant. Several factors contributing to seasonal, areawide sugarcane borer and Mexican rice borer infestation levels include weather, predator and parasite numbers, already existing borer populations, and effectiveness of insecticide controls.
LSU AgCenter field evaluations are conducted annually in plantings of all varieties under consideration for future commercialization. Thirty-one varieties, including four to represent susceptible and resistant standards, were planted in the fall of 2006. Twenty-four representing the L 06 series and HoCP 05 series were planted in the fall of 2007. Additionally, the standards HoCP 91-555 (susceptible), L 97-128 (susceptible) and L 03-371 (resistant) were part of the experiment. The fall 2007 planting will be evaluated in November 2008.
Percent bored internode data is collected in addition to adult (moth) emergence information from 15 stalks per plot. For several years, the sugarcane borer resistant variety evaluation has been conducted on a farm near Burns Point in St. Mary Parish. This location has a history of some of the most consistently heavy borer pest pressure in the in dustry. Additionally, the studies use a soil surface-applied insecticide to suppress borer predation and enhance infestation consistency for more reliable data.
To have high-yielding, Mexican rice borer-resistant varieties available for the Louisiana sugarcane industry, collaborative research evaluations with Texas A&M scientists are being conducted at the Texas A&M Beaumont center near Ganado, Texas. Figure 1 shows the results of destructive sampling of eight of the 10 planted varieties (two had already been dropped) evaluated in late August. Of particular significance is the discovery of possible Mexican rice borer resistance in L 03-371. This study also separated out sugarcane borer injury. Because the Mexican rice borer is a plant stress-related insect, collection of rainfall data (not shown) was also important for this research.
Studies conducted on plant resistance to the sugarcane aphid have revealed that aphid buildup will be almost four-fold less on HoCP 91-555 when compared to the most susceptible variety L 97-128 (Figure 2). Season-long monitoring of aphid populations on different varieties showed that HoCP 91-555 also maintains its resistance under field conditions (Table 1).
Of particular interest are both the substantial differences among the various commercial varieties, and the fact that there seems to be no relationship between plant resistance to aphids and borers.
These aphid studies provide data on aphid-sugarcane interactions by elucidating sugar and amino acid composition in the phloem sap of resistant and susceptible varieties and in the honeydew of aphids feeding on these varieties.
Collaborative studies on another invasive insect, the sugarcane tingid, have also shown substantial differences in pest populations where Louisiana sugarcane varieties have been grown in experiments in the Lower Rio Grande Valley sugarcane area. In this experiment, as in LSU AgCenter studies, HoCP 91-555 consistently had the lowest infestations.
In summary, plant resistance to insects plays an important role in sugarcane variety development. It provides the most permanent of all pest management approaches. It also is attractive to the farmers because there is no direct cost.
Using insect resistant sugarcane varieties in consulting. LSU AgCenter scientists train agricultural consultants to monitor and treat resistant sugarcane varieties separately from the way they scout and recommend insecticide applications on susceptible varieties. They may spend more time scouting insect pest susceptible varieties at certain times of the summer. Additionally, AgCenter research has shown that some varieties have different threshold levels.
An areawide perspective of plant resistance to insects. Plant resistance should be viewed not only from its role in reducing yield loss from the pest, but also from its role in reducing insect population pressure on an areawide basis. Without an areawide perspective, pests associated with a highly tolerant variety, even though it produces a good yield, can overwhelm the resistance of other varieties. In sugarcane, this contrast is particularly appropriate when LSU AgCenter researchers collect data on both larval entrance holes (which are smaller and round) as compared to adult moth exit holes (which are large and oval shaped). The moth exit holes signify completion of the life cycle in the plant and provide an added record of pest severity as long as the sugarcane stalk remains in the field.
In another study, the importance of combining variety resistance (HoCP 85-845) with irrigation reduced potential areawide production of Mexican rice borer moths more than fourfold as shown from studies at Ganado, Texas (Figure 3). With the Mexican rice borer, insecticides alone proved to be inadequate and impractical to manage severe pest pressure.
Thomas E. “Gene” Reagan, Austin C. Thompson Endowed Professor of Entomology; Waseem Akbar, Research Associate; and Julien Beuzelin, Research Assistant, Department of Entomology, LSU AgCenter, Baton Rouge, La.
(This article was published in the spring 2008 issue of Louisiana Agriculture.)