Published in Crop Sci. 43:1729-1735 (2003).
© 2003 Crop Science Society of America
677 S. Segoe Rd., Madison, WI 53711 USA
CROP BREEDING, GENETICS & CYTOLOGY
Broad-Sense Heritabilities, Genetic Correlations, and Selection Indices for Sugarcane Borer Resistance and Their Relation to Yield Loss
S. B. Milligan*,a,
M. Balzarinib and
W. H. Whitec
a United States Sugar Corp., P.O. Drawer 1207, Clewiston, FL 33440
b Univ. of Cordoba, Cordoba, Argentina
c USDA-ARS-SRRC, Sugarcane Research Unit, 5883 USDA Road, Houma, LA 70360
* Corresponding author (smilligan{at}ussugar.com).
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ABSTRACT
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The sugarcane borer [Diatraea saccharalis (Frabricius)] causes significant damage to sugarcane (Saccharum spp.), rendering cultivar resistance important. Researchers assess borer-induced damage using up to five different measures: percentage bored internodes, percentage exited internodes, pupation success, moth production, and a damage rating. The inheritance of the different damage measures and the best approach to integrate the different variables into a simplified damage-resistance assessment has not been well studied. Furthermore, the relationships of the damage traits to sugar production have not been comparatively assessed. We planted a replicated, two-location test of 28 clones typical of the selection stage screened for borer resistance in the Louisiana sugarcane breeding programs. We recorded the five measures together with sucrose production and its components. Using appropriate variance components, the heritability, expected response to selection, and genetic correlations among the traits were used to construct selection indices of all combinations of the five damage traits studied. We used the regression coefficients of the damage traits on sucrose production as economic weights. The indices indicated that percentage bored internodes was the most effective single trait to reduce sugarcane borer damage. If data collection costs were considered, then the subjectively assessed damage rating was the most expeditious of the traits examined. High correlation values among several of the traits lead to the observation that inclusion of more than the bored internode, exited internodes, and the damage rating in an index were unnecessary.
Abbreviations: LAES, Louisiana Agricultural Experiment Station
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INTRODUCTION
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THE SUGARCANE BORER causes the majority of insect-related damage to Louisiana sugarcane (Long, 1969). Sugarcane borer larva damage cane by boring and pupating in the stalk internode. This directly reduces stalk weight by mechanical excavation, nutrient flow interruption, and if severe enough, stalk breakage (Long and Hensley, 1972). The mechanical damage acts as an avenue for pathogen introduction, causing further weight and quality losses. Borer damage commonly vitiates apical dominance, and stalks hence produce multiple lateral shoots. This diversion of resources to the lateral buds tends to reduce the sugar content of the cane (Metcalfe, 1969). The damaged stalks also tend to have higher fiber content than undamaged stalks (White and Hensley, 1987), which further lowers the economic value of the cane.
Of the four components in the current Louisiana integrated pest management program, judicious use of insecticides, plant resistance, biological control, and cultural methods, plant resistance potentially provides the most economic and environmentally effective means of control available to the production industry (Hensley, 1981). Hence, cultivar characterization and selection for resistance plays an important part of the evaluation process.
Cultivars express resistance via complex contributions and interactions of several components (Kyle and Hensley, 1970; Coburn and Hensley, 1972; Martin et al., 1975; White and Hensley, 1987; Sosa, 1988; Bessin et al., 1990; White, 1993). Thus, all of the recognized mechanisms of resistance (antibiosis, antizenosis, and tolerance) may be expressed in cane's resistance to the sugarcane borer (Painter, 1951).
Sugarcane is a clonally propagated crop with two breeding programs in Louisiana; one is conducted by the Louisiana Agricultural Experiment Station (LAES) near St. Gabriel, LA, and the other is conducted by the USDA-ARS-SRRC Sugarcane Research Unit in Houma, LA. Experimental clones are routinely screened by each program starting 
6 yr after initial clonal planting (White, 1993; Reagan et al., 1999). Clones are evaluated in similar fashion among the programs, but methodological differences exist. Both programs establish single-row (1.8 by 4.3 m) plots of the experimental clones and appropriate check cultivars in a four-replication, randomized complete block test at a single location. Red imported fire ants (Solenopsis invecta Buren), a borer predator, are typically suppressed by chemical means. This insect is an effective predator of the sugarcane borer and, when left uncontrolled, may prevent uniform borer populations from developing in small plots (Reagan et al., 1972).
The USDA program commonly enhances borer pressure by an adjacent planting of maize (Zea mays L.) which is artificially inoculated with borer larvae (White, 1993). In November, the LAES program records the percentage of the bored internodes and the percentage of the internodes with exit holes indicating a successful pupation (Bessin et al., 1990). In addition to the bored internode percentage taken in the fall, USDA program personnel also record a damage rating (White et al., 2001). Using stalk number per area and the number of exit holes per stalk, LAES personnel estimate the number of borers produced per area per year per variety (Bessin et al., 1990). Resistance is further described by the pupation success rate per bored internode. Thus, clones screened by the USDA program are typically characterized by percentage bored internode and damage rating data, whereas the LAES clones are typically characterized by the percentage bored internode, percentage exited internode, moth production, and pupation success data.
Unfortunately, little is known of the inheritance of these sugarcane borer resistance characterizations. Such knowledge is valuable in developing the best breeding approaches to increase resistance in clonal populations. Viator and Henderson (1971) found percentage bored internode to be quantitative, but provided no measures of its genetic variation, heritability, or potential gain from selection. White et al. (2001) reported narrow-sense heritabilities of the percentage bored internode and a subjective damage rating; however, the broad-sense genetic behavior of other commonly appraised traits was not evaluated. Knowledge of the interrelationships of these traits among each other and to yield loss are also lacking.
Breeders commonly consider several traits, trait interrelationships, and the predictive confidence (repeatability) in the information when evaluating cultivars. Selection indices are a method to quantitatively integrate these data. The optimum selection index procedure simultaneously weights the genetic confidence and interrelationships of the traits together with the relative economic value to provide phenotypic index values for evaluating the cultivars (Baker, 1986). When based on accurate parameter estimates, the indices will provide the best solution to maximize genetic improvement for the selection goal.
We performed this study to estimate the genetic parameters of the measures most frequently used to evaluate borer damage and their relationships to yield loss. The goal was to develop selection indices so the breeder can best collate the range of borer damage variables into one index and then make the most effective selection decision.
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MATERIALS AND METHODS
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Louisiana sugarcane clones are first objectively screened for borer resistance 6 yr after crossing (Milligan, 1994). This was our inference population. Twenty-eight experimental clones from the LAES 1993 assignment series (Bischoff et al., 1993) and three commercial checks were planted at the Glendale Plantation near Lucy, LA, and the LAES St. Gabriel Research Station in the fall of 1995. Clones were planted in single-row plots (1.8 by 4.3 m) with a four-replication, randomized complete block design at each location. Sugarcane typically provides several crops before production declines and it is replanted. The initial plant-cane crop is followed by one to many ratoon crops, harvesting one crop per year in Louisiana. First-ratoon data was recorded in November 1997. Chlorpyrifos [O,O-diethyl-O-(3,5,6-trichloro-2 pyridinyl)phosphorothioate] was banded (1.3 kg a.i. ha-1) at the St. Gabriel location to control the red imported fire ant.
The LAES and USDA borer resistance screening programs use three types of data: percentage bored internodes, percentage exited internodes, and a damage response rating. The first three response variables are used to further calculate a pupation success rate and moth production per area. Fifteen stalks per plot were sampled for stalk weight determination, sucrose content, and percentage bored and exited internodes. The pupation success rate was estimated as:
The moth production per area is defined as:
In September of each year we recorded damage ratings with the system described by White et al. (2001)(Table 1).
Mean squares were equated to expectations to solve for variance and covariance components for each trait (sucrose yield, cane yield, sucrose content, stalk weight, stalk number, internodes per stalk, % bored internodes, damage rating, % exited internodes, pupation success, moth production, and plant damage response rating). Estimation of the variance components did not use the three checks in the data set. Broad-sense heritability (H) estimates were calculated as intraclass correlations for three scenarios: (i) unreplicated plot: one location with one replication (l = r = 1), (ii) one location and four replications (l = 1, r = 4), and (iii) two locations with two replications each (l = r = 2). The heritability was estimated as:
where the variance component 
2G equaled the genotypic variance, 2GL was the genotype x location, 2E was the residual, and 2Plr was the phenotypic variance for the given selection scenario. Standard errors of the heritability were calculated using Dickerson's approximation (Dickerson, 1969).
The genetic variability of the examined traits was expressed as the genetic variance, 2G, and the genetic coefficient of variation (GCV):
Expected genetic advance (response to selection; GA) for the traits was expressed as the percentage of the trait mean:
where i is the standardized selection differential.
Broad-sense genetic correlations (rG) among the traits were calculated as:
where ij is the genetic covariance among traits i and j, i and j are the genetic standard deviations of traits i and j, respectively. Standard errors for the genetic correlations were calculated as described by Becker (1984).
The development of an optimum selection index to maximize the economic return value of the aggregate trait requires the incorporation of relative economic values. Borer damage measures have no economic value per se, but can be assessed via their relationships to sucrose yield (Mg sucrose ha-1). The relationships of the yield components (Yijr) sucrose yield, cane yield, sucrose content, stalk weight, and stalk number to the borer damage measures (Xijr) were estimated via regression (ßk) after adjusting for the overall mean (µ), location (Li), replication (Rr), genotype (Gj), and genotype x location interaction (GLij) fixed effects or:
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where Xkijr was the k borer damage measure of the ijr plot and 
ijr was the error term. To facilitate comparison of the relative effect of the different damage measures on the various yield components, the model was also fitted after standardizing the borer damage measures and the yield components.
Sucrose yield equivalents (Mg sucrose ha-1) of the expected genetic advance (GAyield) to selection for a borer damage trait k was calculated as:
where ßk was the regression coefficient of sucrose yield (Mg sucrose ha-1) on borer damage trait k derived in Eqn. [1], Hk and kP1, 4 were the heritability and phenotypic standard deviations for trait k in the one-location, four-replication (l = 1, r = 4) selection scenario.
Selection indices and predicted responses to selection were constructed according to Brim et al. (1959) and Baker (1986). Since the breeder may have different combinations of damage variable data, indices were constructed for combinations of two to five traits. We used the predicted aggregate trait gain for the one-location, four-replication (l = 1, r = 4) selection scenario (GA1,4) as the criteria for deciding the best index. The one location with the four-replication (l = 1, r = 4) selection scenario was used because it is the current practice used by both agencies. Simple phenotypic correlations among the damage measures and the index values of the entries were also calculated.
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RESULTS AND DISCUSSION
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Three damage measures, bored internodes, exited internodes, and the moth production index showed similar and the most potential genetic advance for improvement by selection under a one-location, four-replication selection scenario (GA1,4; Table 2). This was followed closely by exited internodes. Pupation success displayed about one third the potential improvement as bored internodes and the damage rating showed even less potential. The importance of genotype x location interaction 
seemed to affect exited internodes and moth production more than the bored internode measure. The ultimate effect of genotype x location interaction on selection efficiency, however, seemed quite small when the two-location, two-replication gain(GA2,2) was compared with the one-location, four-replication gain (GA1,4). Thus, the current practice of screening at only one location in a four-replication test seems adequate.
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Table 2. Broad-sense genetic variance components, heritabilities, and potential response to selection for a single-plot and replicated-selection scenarios for borer resistance and yield components.
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Growers generate income from the sugar they produce and are not explicitly penalized for borer damaged cane. Genetic correlations among the borer damage traits were rather high (Table 3). Since stalk number is a major positive component of sugar yield (Milligan et al., 1990), and percentage exit internodes is negatively correlated with yield, the correlation between moth production value and yield apparently canceled each other.
The relationship of the damage variables to yield components was estimated using a model that accounted for the production range of genetic material in the study before fitting the damage variable as a covariant (Table 4). These regression coefficients were used as the economic weights in developing the selection indices. Since the coefficients have different units for each trait and their interpretation depends on the variability of the damage trait, the same covariate model was employed using standardized variables to provide more easily compared regression coefficients (Table 5). The coefficients suggest that the damage rating showed the most potential to affect yield. Bored internodes and exited internodes showed the same and slightly less potential, while pupation success and moth production explained little yield loss.
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Table 4. Regression coefficients of borer damage measures effect on sucrose yield and its components after accounting for location, rep(location), genotype, and genotype x location effects.
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Table 5. Regression coefficients of standardized borer damage measures effect on standardized sucrose yield and its components after accounting for location, rep(location), genotype, and genotype x location effects.
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The mathematical properties of selection indices are such that the more variables in the index, the larger the predicted gain. The increases in predicted gains from using more complex indices may be erroneous because of cumulative errors from measurement and, particularly in this case, the highly correlated and often derived nature of the predictor traits (Baker, 1986).
The most effective single trait to indicate yield loss is bored internodes (Table 6). Given the considerable labor necessary to collect the damage measures (at least 12 man-days for each test) compared to the damage rating (0.5 man-days per test), the damage rating is clearly the most cost effective single measure. The best two-trait index included the bored internodes and the plant damage rating (Table 7). The best three-trait index used the bored internodes, exited internodes, and the damage rating; however, several other indices were close seconds (Table 8). The best four-variable index excluded the moth production variable (Table 9). As expected, the largest predicted response was from the five-variable index (Table 10). The predicted gain is given for all the different combinations of the damage measures because the breeder may not possess the best combination of measures (Tables 7–10). We also frequently observed different combinations of traits with similar aggregate gains as the index with the highest gain.
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Table 6. Expected response to selection for borer damage traits by selecting lower 10% (lower values) of population for individual borer damage traits.
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Table 7. Selection index weights and expected sucrose yield response by selecting top 10% (higher values) of population using all pairs of borer damage traits in index.
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Table 8. Selection index weights and expected sucrose yield response by selecting top 10% (higher values) of population using different combinations of three borer damage traits in index.
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Table 9. Selection index weights and expected sucrose yield response by selecting top 10% (higher values) of population using different combinations of four borer damage traits in index.
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Table 10. Selection index weights and expected sucrose yield response by selecting top 10% (higher values) of population using five borer damage traits in index.
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Correlations among cultivar index values using the best selection indices with two to five variables indicate that inclusion of more than three variables in the index will not change the cultivar interpretation of borer resistance (Table 11). The results suggest that the pupation success and the production index add little to the resistance assessment after considering the percentage bored and exited internodes and the damage rating.
The results suggest that the best borer damage measure to predict yield loss is bored internodes. The damage rating is also quite useful and likely more cost effective than the bored internode measure. Use of the damage rating is limited to standing cane and must generally be collected no earlier than September to observe the full plant response to borer infestation. Although exited internodes are a good predictor, the low levels of this trait at the tested infestations levels would likely make it unreliable at lower infestation levels. It would be expected that there may be no exited internodes in many of the genotypes and, hence, differentiation among the more resistant portion of the population would not be possible. The production index is a poor predictor of yield loss because of borer damage for individual genotypes per se. The nature of its determination, the production of exited internodes per stalk times stalks per area, could lead to selection against the most productive genotypes among clones of equal genotypic resistance if the more productive clones obtained the higher production with higher stalk numbers per area. Since it is a measure of potential insect generation from a genotype, comparison to the other measures is not very fair on an individual resistance basis and still may be a valuable indicator of the potential for area-wide pest pressure (Bessin et al., 1991).
A fundamental assumption in this study and made in the routine screening studies is that relative genotype responses to the high insect pressures used in the screening tests are the same as at the more typical lower levels maintained in commercial fields (<10% bored internodes). The same kind of assumption is made with the relative economic weights. It is assumed that the relative yield losses per unit damage variable are the same at lower infestation levels as the higher levels.
Results from this study and the White et al. (2001) study suggest that selection for sugarcane borer resistance should provide meaningful improvements in genotype resistance. It also indicates that selection may employ one to several measures with success and should be adequate employing a single location test.
Received for publication January 3, 2003.
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REFERENCES
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ABSTRACT
INTRODUCTION
