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CROP BREEDING, GENETICS & CYTOLOGY

Diallel Analysis of Spider Mite Resistant Maize Inbred Lines and F₁ Crosses

^a Jr., Agricultural Research and Extension Center, Texas A&M Univ., Route 3, Box 219, Lubbock, TX 79403
^b Agricultural Research and Extension Center, Texas A&M Univ., Route 3, Box 219, Lubbock, TX 79403, and Dep. of Plant and Soil Sci., Texas Tech Univ., Box 42122, Lubbock, TX 79409
^c Agricultural Research and Extension Center, Texas A&M Univ., Route 3, Box 219, Lubbock, TX 79403

^* Corresponding author (we-xu{at}tamu.edu).

	ABSTRACT

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

 
Banks grass mite [BGM; Oligonychus pratensis (Banks)] and the twospotted spider mite (Tetranychus urticae Koch) cause significant yield reductions in maize (Zea mays L.) grown in the Great Plains states. Understanding the inheritance of mite resistance will be useful in developing mite resistant hybrids. A Griffing's Method 2 diallel mating design was used to evaluate the range of resistance and combining ability of parents in F₁ crosses of seven mite resistant maize lines (T1, U2, A3, A4, A5, A7, and T9). Parent lines, F₁ crosses, and three checks (Mo17, B73 x Mo17, and Pioneer hybrid 34K77) were included in a randomized complete block design with three replications at Halfway, TX, in 2002 and Lubbock, TX, in 2001 and 2002. Following artificial infestations, mite resistance was evaluated for differences in mite infestations and feeding damage. Weekly mite densities and damage samples were used to calculate total mite densities, total damage ratings (TDRs), mite per damage ratio (M/D), and a seasonal damage ratio (SDR). Total damage rating was significant for environments, genotype, and environment x genotype interaction. Total mite density (TMD) and M/D were significant for genotype and environment x genotype interaction. The SDR was significant for environment and genotype, but no interaction. Broad-sense heritability estimates showed that >90% of the TDR and SDR were associated with genotypic effects. Crosses with T1, U2, and T9 consistently supported larger mite populations, but had higher M/D values than those with A3, A4, A5, and A7, indicating greater tolerance to mite feeding. Crosses between tolerant lines or between tolerant lines and antibiotic lines had overall better resistance than crosses between antibiotic lines. Diallel analyses showed that T1 and U2 had the highest general combining ability (GCA) for reducing damage. T1 x A4, T1 x A7, A3 x A5, and A4 x A7 were the best crosses for specific combining ability (SCA) for resistance to spider mites.

Abbreviations: BGM, Banks grass mite • GCA, general combining ability • M/D, mite per damage ratio • SCA, specific combining ability • SDR, seasonal damage ratio • TDR, total mite damage rating • TMD, total mite density

	INTRODUCTION

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

 
TWO SPECIES OF SPIDER MITES (BGM and the twospotted spider mite) infest maize fields in the Great Plains states of Colorado, Kansas, Nebraska, New Mexico, Oklahoma, and Texas (Ehler, 1973; Owens et al., 1976). Surveys (Ehler, 1973) have shown that the BGM is the predominate species, but both mites can reach damaging infestation levels during the growing season, often resulting in yield losses of 20% (Archer and Bynum, 1993). Mite populations rapidly increase during the grain-filling period and can cause extensive damage in just a few weeks when field conditions are hot and dry. Natural predators are generally unable to suppress mites when conditions favor population growth because low humidity and high temperatures are reported to reduce predation rates and reproduction of many predator species (Pickett and Gilstrap, 1986; Berry et al., 1991; T.L. Archer, 1998, personal communication). Augmentative releases of predatory mites have been reported to reduce spider mite densities, but the associated expense makes this biological control method cost prohibitive (Pickett and Gilstrap, 1986; T.L. Archer, 1998, personal communication).

Throughout the geographic region of the Great Plains states, management of mites depends on the use of insecticides and acaricides. Unfortunately, chemical resistance has been reported in both the BGM and twospotted spider mite (Ward et al., 1972; Owens et al., 1976; Ward and Tan, 1977; Bynum et al., 1990, 1997). Sloderbeck (1986) reports progressively higher levels of BGM resistance toward the Texas High Plains region. Also, mite outbreaks can be induced following insecticide applications for European corn borer [Ostrinia nubilalas (Hubner)], southwestern corn borer (Diatrea grandiosella Dyar), western bean cutworm [Loxagrotis albicosta (Smith)], corn earworm [Heliocoverpa zea (Boddie)], and corn rootworm adults (Diabrotica spp.) (Hirnyck, 1983; Bynum and Archer, 1992). Control failures may sometimes be attributed to poor application coverage in the maize canopy (Bynum and Archer, 1986). The unreliability of mite control and chemical resistance requires the pursuit of other management practices. Plant genetic resistance to mites could be a viable integrated pest management tool. Depending on mechanisms involved, resistance can be classified by a mite's response to plant defenses (antibiosis) or by the plant's ability to withstand mite damage (tolerance). Antibiosis includes the biochemical plant toxins that alter the mites' ability to survive, reproduce, and develop. Tolerance is the plant's ability to repair or recover from mite feeding damage, but does not affect the mites' population dynamics.

Vlosich (1980) evaluated three temperately adapted maize populations, public inbred lines, and single crosses for BGM resistance in 1976 to 1978 at New Mexico State University. The degree of susceptibility varied among the germplasm and F₁ crosses, but resistance was evident. Kamali et al. (1989) reported high levels of resistance to twospotted spider mite in the inbred line 41:2504B. During the 1950s, the USDA-ARS and Iowa State University's maize breeding project successfully developed and released resistant inbred lines (B49, B64, B65, and B68) with 41:2504B as the donor parent (Kamali et al., 1989). Mansour and Bar-Zur (1992) found higher mortality and lower fecundity in the carmine spider mite, T. cinnabarinus (Boisduval), on four-leaf stage maize plants than tassel plants for lines Oh43 and CI82B. An antibiosis study with two different growth stages resulted in significant differences among inbred lines when maize was at the three-leaf stage, but lines were equally susceptible to the check line when evaluated at the flowering stage (Mansour et al., 1993). From these results, Mansour et al. (1993) recommended resistance evaluations on flowering-stage of maize plants.

Another source for maize resistance to spider mites is tropical germplasm. Uhr and Goodman (1995) stated that tropical germplasm has not been extensively utilized in U.S. breeding programs because of poor agronomic traits and photoperiod sensitivity. Yet, Uhr and Goodman (1995) reported good agronomic performance of several inbred maize lines derived from 100% tropical germplasm and recommended plant breeders integrate tropically derived inbred lines into breeding programs. Archer et al. (1997) developed nine populations (sources: S1, S2, S3, S4, S5, S6, S7, S8, and S9) resistant to spider mites by crossing tropical germplasm with temperately adapted hybrid NB611 and then selected inbred lines by selfing and sib-mating for 6 generations. Laboratory assays indicated resistance in two sources was from tolerance (S1, S9), four sources were from antibiosis (S3, S4, S5, S7), and three sources were undetermined (S2, S6, S8) (T.L. Archer, 1999, unpublished data).

In this study, seven of these inbred lines (two tolerant, four antibiotic, and one unclassified, S2) were evaluated to (i) determine the range of resistance, (ii) estimate heritability of resistance, and (iii) measure combining ability for parents and F₁ crosses under different environmental conditions.

	MATERIALS AND METHODS

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

 
Experiments were conducted at the Texas A&M University Agricultural Research and Extension Centers in 2001 and 2002 at Lubbock (Lubbock County), TX, and in 2002 at Halfway (Hale County), TX. Standard agronomic farming practices were followed at each experimental site (Table 1). Experimental plots at both locations were furrow-irrigated on 15 March 2001 (62 mm), 14 March 2002 (122 mm) at Lubbock, and 12 March 2002 (94 mm) at Halfway before planting on 7 May 2001, 3 May 2002 at Lubbock, and 24 April 2002 at Halfway. At Halfway, postplanting furrow irrigation (average 51 mm) to the maize crop was initiated on 30 May and were applied every 7 to 10 d through mid-August as weather conditions and rainfall dictated. At Lubbock, furrow irrigation were also initiated on 24 May 2001(average 63 mm) and 14 May 2002 (average 110 mm) and applied on a similar schedule until the crop became too tall to cultivate for weeds. At which time, water was applied weekly (average 50 mm) through a surface drip irrigation system in alternate furrows during the remainder of the season.

Banks grass mites were artificially infested at each location to establish mite populations within the experimental plots. In 2001 and 2002 at Lubbock, an early maturing maize hybrid DK580 was planted 3 wk before the test plots to create a field nursery for BGMs. The BGM infested maize leaves, from laboratory colonies, were distributed through the early planted maize by weaving infested leaves end-to-end at the lowest one-third of the plants (Chandler et al., 1979). This field mite nursery was infested weekly from the first to third week of June each year. Then, at growth stages 3.5 or 4 (Hanway, 1971), experimental plots were infested by collecting BGM-infested leaves from the field nursery and weaving five to six leaves end-to-end per plot as previously described. As infested leaves dried, BGM migrated to experimental plants. Within 48 h after each artificial infestation, pyrethroid insecticide, esfenvalerate at 0.45 kg (AI) ha^–1, was applied in 94 L of water to the mite infested experimental plots and field nursery to reduce predator populations and to enhance spider mite numbers (Kattes and Teetes, 1978).

One week after infestation in 2001 and 2002, the total number of female spider mites and predators were counted weekly for four to six weeks on each of five randomly selected plants per plot. Beginning with the lowest leaf that was at least one-third green (Perring et al., 1981), all leaves from one side of the plant were sampled for female BGM numbers. Mite damage was measured weekly after BGM density counts. Mite feeding caused chlorotic spots on leaves and dead leaf areas. This damage was evaluated with a 1-to-10 scale; 1 = 1 to 10% of the leaf area on a plant damaged, 2 = 11 to 20% of the leaf area damaged, and 10 = 91 to 100% of the leaf area damaged (Archer et al., 1997).

Analysis of variance (SAS Institute, 1987) was performed with PROC GLM to evaluate differences among environments (Lubbock 2001, 2002; Halfway 2002), genotypes (resistant inbred lines, F₁ crosses, Mo17, B73 x Mo17, and Pioneer 34K77), and interactions between environment and genotypes for TDR, TMD, M/D, and a SDR. Environments were considered random effects, and genotypes were fixed effects. The TDR and TMD were the sums of the weekly damage rating and weekly mite density, respectively. A mite density per damage ratio was calculated on the plot basis by dividing the TMD by the TDR. Mite damage ratings were normalized by calculating a SDR. This ratio is determined from the number of weeks that it took the most susceptible check entry to reach the highest level of damage during the infestation period. The weekly damage ratings were totaled and then divided by the maximum accumulative damage possible. This equation is as follows:

where SDR is the proportion of seasonal damage, X_i is the sum of weekly damage ratings i = 1, 2,..., n (number of weeks sampled), 10 is the maximum damage rating per week, and W_n is the number of weeks sampled for the susceptible check to reach the highest damage level. Therefore, the calculated SDR was between 0 and 1.

Means were separated with least significance difference procedure when the F test was significant at P 0.05. Broad-sense heritability (H) was estimated on a mean basis from the ANOVA mean square values with environments being random and genotypes fixed, where H = ²_G/²_P (Nyquist, 1991). A Griffing Method 2 diallel mating design (Griffing, 1956) for resistant parent inbred lines and resistant F₁ crosses was analyzed for GCA and SCA using Zhang and Kang's (1997) diallel-SAS program.

	RESULTS

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

 
Inbred and Hybrid Mite Infestations and Damage
Mean M/D values, TDRs, TMD counts, and SDR ratings were analyzed across environments to determine significance of mite resistance among genotypes. The combined analysis showed no significant differences among environments for M/D and TMD (Table 2). The analysis indicated significant genotype and environment x genotype interaction effects for M/D and TMD. Environment, genotype, and environment x genotype interaction effects were significant for TDR. The SDRs were significantly different among environments and genotypes, but no significant differences were detected for the environment x genotype interaction. Proportion of variability (R²) of the models for M/D, TDR, TMD, and the SDR was 0.85, 0.86, 0.81, and 0.82, respectively, and indicated that each of these factors was a good estimate of mite populations and feeding damage.

Means of total mite densities at Lubbock 2001 (3483.5), Lubbock 2002 (2786.2), and Halfway 2002 (1719.2) were not different [LSD (0.05) = 2478.5]. Mean mite density averaged across environments for all genotypes was 2642.5 female mites per plant (Table 3). Range of mite densities among all mite resistant inbred lines was between 1770.2 to 3122.1 female mites per plant. Numerically, the lowest density of spider mites was among antibiosis resistant inbred maize lines A3, A5, and A7. The resistant inbred lines with the heaviest mite populations were U2 (an unknown type of resistance) and T9, a line that tolerates mite infestations. Another tolerant resistant source, T1, had intermediate levels of spider mite infestation, and an antibiotic resistant line, A4, averaged moderately high levels of mite infestations. Spider mite densities on U2 and T9 were significantly higher than densities on T1, A3, A5, and A7, but similar to each other and to A4. Resistant maize lines T1, A3, A5, and A7 had similar mite densities. The highly susceptible inbred check (Mo17), on average, had the second lowest female mite density (1929.9 per plant) among all genotypes.

Tolerant inbred line by tolerant inbred line crosses (T1 x U2, T1 x T9, and U2 x T9) had an average mite density of 2931.9, ranging from 2869.8 to 3046.4 (Table 3). Densities among the tolerant x tolerant crosses did not differ. Crosses between tolerant and antibiotic resistant lines (T1 x A3, T1 x A4, T1 x A5, T1 x A7, U2 x A3, U2 x A4, U2 x A5, U2 x A7, A3 x T9, A4 x T9, A5 x T9, and A7 x T9) averaged 2490.3 female mites per plant and ranged from 1980.4 to 3242.8. There were no significant differences in mite densities for crosses in either high level, intermediate level, or low level, but significant differences were present between crosses within the high and low density levels. The variability among T x A crosses suggests that the expression of tolerance and antibiosis resistance varies with the genetic background. Six crosses were between antibiotic x antibiotic resistant lines (A3 x A4, A3 x A5, A3 x A7, A4 x A5, A4 x A7, and A5 x A7). These crosses averaged 2300.5 female mites, and ranged from 1998.3 to 2733.6, but did not differ significantly.

Mean TDRs at Lubbock (16.64) in 2001 and at Lubbock (23.59) and at Halfway (19.94) in 2002 were different [LSD (0.05) = 2.19]. The highly susceptible inbred (Mo17) and the commercial hybrid (Pioneer P34K77) sustained the heaviest damage within each environment and across environments. All of the resistant inbred lines had significantly less damage than Mo17 at each environment and across environments. Comparisons of mean TDRs averaged across environments showed damage levels ranging from 18.67 to 23.22 (Table 3). The inbred lines U2, T1, and A3 had similar damage levels at 18.67, 18.89, and 20.56, respectively. The level of damage on T1 and U2 was less than damage on the other resistant inbred lines. Mean total damage averaged across environments for A3 was similar to A4, A7, and T9, but less than A5. The heaviest damage was to A5 at 23.22, an antibiotic resistant inbred line. All damage ratings to the resistant F₁ crosses were less (15.11–20.00) than damage ratings to the check hybrids, B73 x Mo17 and Pioneer 34K77, at 23.56 and 26.50, respectively. As a group, crosses with T1 had, on average, the lowest damage rating (17.06) with a range from 15.44 to 18.44. There was a broad range of damage ratings among crosses with A4 or A7, ranging from 15.11 to 19.44 for A4 and 20.00 for A7. The rating of 15.11 was for A4 x A7 cross and was different from damage ratings above 17.22. Damage ratings for crosses with U2 as a parent were close to ratings for crosses with A3. The mean damage rating averaged across environments for crosses with U2 was 17.89, which compares with the average rating for A3 crosses at 17.87. Both sets of crosses had narrow ranges of damage, 17.11 to 18.89 for U2 and 17.67 to 18.33 for A3. With the LSD (0.05) value for significance at 2.24, all of the crosses with either U2 or A3 as a parent were similar. Also, as a group, damage to crosses with A5 was similar to damage to crosses with T9. Crosses with A5 ranged from 17.22 to 20.00 and compares with crosses of T9 that ranged 16.67 to 19.44.

Mean damage averaged across environments among tolerant x tolerant crosses did not differ significantly (Table 3). The average TDR was 17.74, with a range from 16.67 to 18.89. Tolerant x antibiotic crosses had slightly smaller damage (17.64), but a broad range of damage levels, 15.44 to 19.44. T1 x A4 was different from other T x A crosses when damage was >17.68, and T1 x A7 was different from T x A crosses with damage > 18.68, and U2 x A4 differed significantly from T9 x A4. Also, there was a broad range of statistical differences in damage (15.11–20.00) among antibiotic x antibiotic (A x A) crosses. As a group, the A x A crosses had, on average, the highest numerical damage (18.07). However, A4 x A7 had one of the lowest damage levels (15.11).

The mean M/D at Lubbock in 2001 (201.5), Lubbock in 2002 (118.8), and Halfway in 2002 (87.9) were not different. The mean M/D averaged across environments was 135.06 (Table 3). Among the inbred lines, Mo17 had the lowest mean M/D value at 64.00, indicating that this inbred is highly susceptible to mite feeding. This M/D value was less than M/D values for tolerant inbred lines (T1 and T9), and U2 and A4. The resistant inbred lines had a broad range of M/Ds, 76.86 to 164.22. The U2, T9, and A4 resistant inbred lines had similar M/D values at 164.22, 143.68, and 132.61, respectively. The M/D values for A5 (76.86), A3 (92.18), and A7 (92.75) were similar to each other and different from U2, T9, and A4. The resistant inbred lines U2 and T9 (tolerant) and A4 (antibiosis) exhibited more tolerance to mite feeding than did the antibiotic resistant A5, A3, and A7. The T1 tolerant inbred had an intermediate M/D value (112.67) when compared with the other resistant inbred lines. Crossing the resistant inbred lines generally increased the level of M/D, indicating heterosis for tolerating spider mite feeding damage. Crosses with tolerant inbred lines (T1 and T9) and U2 (unknown resistant type) had, on average, high M/D values (means of 151.33, 148.9, and 152.38, respectively). Crosses with A4 had similar narrow range of M/D values at 109.08 to 127.95. Crosses with A3 and crosses with A5 had mean M/D values of 133.73 and 132.42, respectively. The check hybrids (B73 x Mo17 and Pioneer 34K77) had similar M/D values and were among the highest of the tested hybrids.

The M/Ds were greatest with tolerant x tolerant crosses (average 168.78), followed by tolerant x antibiotic crosses (average 141.91) and least with antibiotic x antibiotic crosses (average 123.79). The M/D among tolerant x tolerant crosses were 165.20, 168.35, 172.80, and were statistically similar. The M/Ds among the tolerant x antibiotic crosses had M/D ranges of 118.72 to 159.86, a difference of 41.14.

Calculating the seasonal damage rating to adjust for the differences in number of weeks sampled showed that the damage during 4 wk of infestation in 2001 was just as severe as damage in 6 wk of infestation in 2002. The mean SDR of 0.42 at Lubbock in 2001 and 0.39 at Lubbock in 2002 was similar, but they were significantly larger than SDR of 0.33 at Halfway in 2002. The overall mean SDR across environments was 0.38 (Table 3). Mean SDR values averaged across environments for the highly susceptible Mo17, Pioneer P34K77, and B73 x Mo17 was 0.60, 0.50, and 0.44 respectively. These values were significantly different. The SDR values for Mo17 and Pioneer P34K77 were greater than SDR values of all of the resistant inbred lines and resistant F₁ crosses. Mean SDR values across environments (0.36) for both T1 and U2 were similar to the SDR value of A3 (0.39), but less than SDR values for the other resistant inbred lines A4, A5, A7, and A9. Crossing the resistant inbred lines resulted in SDR values between 0.29 to 0.38. The SDR values for crosses A4 x A7, T1 x A4, and T1 x A7 were 0.29, 0.29, and 0.31, respectively, and significantly lower than the smaller SDR value (0.36) for the resistant inbred lines T1 and U2. The group having the next smallest set of SDR values was the crosses with A4. This group had a mean of 0.33 and a range from 0.29 to 0.37. Crosses made with U2, A3, or T9 as a parent had similar SDR values. Crosses with S7 had the broadest range of SDR values (0.29 to 0.38), while crosses with A5 had the greatest mean SDR value of 0.35 (Table 3).

The relationship of SDR to tolerant (T) x antibiotic (A), T x T, and A x A crosses was similar to the relationship for TDRs. There were no differences among T x T crosses (Table 3). The T x A crosses had the least damage (average 0.3349), but there were differences within the range for T x A crosses. The T x A cross at the high end of the range were T1 x A5, T9 x A4, and T9 x A7 (0.36, 0.36, and 0.35, respectively), and did not differ among these SDR values. The T x A crosses with the smallest SDR were T1 x A4 (0.29) and T1 x A7 (0.31). Damage was greatest (average 0.35) and the range of damage (0.29–0.38) was widest on A x A crosses. The A x A cross with the smallest SDR was A4 x A7, and it was significantly different from the other A x A crosses.

General and Specific Combining Ability
An analysis for the diallel mating design for including the resistant parents and the F₁ crosses was conducted to determine the GCA and SCA of genotypes for M/D, TDR, TMD, and SDR (Table 4). Each of these factors was significant across environments and among genotypes, but there were no significant environment and genotype interactions.

Mites per damage GCA estimates were significantly positive for U2 and T9 and significantly negative for A3, A4, and A5 (Table 5). The individual SCA estimates showed a significant interaction between T1 x A3, T1 x A5, A3 x T9, and A5 x T9 (Table 6). This data indicates that use of U2 and T9 in a cross will generally increase the number of spider mites a hybrid can support for each unit of damage. The negative effect of A3, A4, and A5 indicate that hybrids from these antibiotic resistant lines will have smaller mite densities per unit of damage. The specific interaction between T1, a tolerant inbred, with A3 and A5, antibiotic resistant lines, shows the tolerance of each cross is positively increased. The negative SCA estimate of A3 or A5 when crossed with T9 indicates the tolerance was not being expressed by T9.

The GCA and SCA estimates for the normalized damage (SDR) values across environments are presented in Tables 5 and 6. The negative GCA effect for T1 indicates the overall ability of T1 to withstand damage from spider mite feeding. The positive GCA effect for A5 indicates that crosses with A5 will have heavier damage. The SCA estimates show that T1 x A4, T1 x A7, A3 x A5 and A4 x T7 were good crosses for reducing damage. Reduced damage with T1 x A4 and T1 x A7 could be from combined tolerance and antibiosis. Damage reduction in A3 x A5 and A4 x A7 should be from increased antibiosis.

	DISCUSSION

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

 
These field studies provide important information concerning the performance of resistant maize inbred lines in a diallel mating design. Data from TMD, TDRs, M/D, and SDR provided good estimates of spider mite populations and relationships between maize resistant crosses to feeding damage. Comparing these different criteria identified the types of resistance involved and explained the levels of resistance among the crosses. Plant resistance from tolerance was associated with relatively large mite densities and M/Ds. Resistance from antibiosis was related to small mite densities from reduced rates of population increase and/or spider mite mortality, but M/Ds could be small. Both types of resistance resulted in reduced damage to the host plant.

Mite infestations in this study varied between test sites and years. Climatic conditions in 2002 were relatively hot and dry at Lubbock and mild and dry at Halfway. These environmental conditions explain the differences between environment and environment x genotype interactions. Populations increased rapidly and to very large densities at Lubbock in 2002 and killed the plants of susceptible checks, Mo17 and Pioneer P34K77, after 4 wk of infestation. Seasonal conditions at this site were hot and dry due to no measurable precipitation during the infestation period and daily temperatures exceeding 35°C on 13 d during June and 26 d during July. Spider mite populations in 2002 were smaller than in 2001, but populations at Lubbock were greater than at Halfway. Severe damage or dead plants were present in susceptible checks after six weeks of infestation. Calculations of broad-sense heritability estimates on mean-basis for TDR and SDR values showed that >90% of the damage was associated with genotypic effects. Heritability estimates for total mite densities and M/D were also large but less reliable in explaining the genetic contribution for resistance than TDRs and SDR.

Field populations and damage levels to resistant maize inbred lines supported laboratory findings (T.L. Archer, 1999, unpublished data) that resistance from T1 and T9 was tolerance and resistance from A3, A4, A5, and A7 was by antibiosis. This study showed that resistance from these inbreds was maintained under field conditions. The statistical similarity of U2 compared with T1 and T9 for spider mite densities, damage levels, and M/D indicated U2 resistance was tolerance. Laboratory studies should be conducted to confirm the actual type of resistance, but for this discussion, spider mite resistance of U2 will be assumed to be tolerance.

Mite densities and damage levels among resistant crosses were variable, but resistant crosses generally supported heavier mite densities and had less damage than their resistant inbred lines. Crosses with T1, U2, and T9 (tolerant lines) consistently supported larger mite infestations and had higher M/Ds than crosses with A3, A4, A5, and A7 (antibiotic lines). Crossing two tolerant lines produced improved tolerance. When tolerant inbred lines were crossed with antibiotic inbred lines, damage was similar to tolerant crosses, but overall there was a reduction in mite densities. Although crosses between antibiotic inbred lines resulted in plants supporting lower spider mite densities, damage to plants was slightly greater than crosses with tolerant lines. Therefore, crosses among resistant lines express different levels of resistance, but all have potential in developing spider mite resistant germplasm. The T1 line has been officially released as Tx202 (Xu et al., 2004).

The combining ability analyses in this study indicated that spider mite resistance was generally and specifically transferable among crosses. The GCA x environment interaction for TDRs and SDR suggested that resistance was not stable across environments. This instability may have been associated with extreme temperature and dry conditions that favored spider mite population outbreaks and caused severe plant stress. The tolerant resistant inbred lines, U2 and T9, were good sources for improving M/Ds although mite densities were great. The antibiotic resistant line, A5, was a good resistance source for reducing mite densities, but it sustained greater damage. The two resistant inbred lines making the best overall contribution to reduced damage were T1 and U2. Individually, T1 x A4, T1 x A7, A3 x A5, and A4 x A7 were the best crosses exhibiting resistance to spider mites.

Plant breeders and entomologists can use this information to select resistant maize inbred lines that can be integrated into their breeding programs. Specific resistant crosses from tolerant x tolerant, tolerant x antibiosis, and antibiosis x antibiosis crossings can be isolated for further development of new resistant inbred lines.

	ACKNOWLEDGMENTS

 
The authors would like to thank Dr. Manjit Kang for his assistance with the diallel analysis. Critical reviews of this manuscript were provided by Dr. Harlan Thorvilson, Dr. Dan Krieg, and Dr. Scott Armstrong. We thank Cody Crannell, Clinton Oswalt, Lance Jerden, Patrick McAlister, and Stephanie Moehle for technical assistance. This research was supported in part by the Texas Corn Producers Board.

	NOTES

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

 
Part of a dissertation submitted by E.D. Bynum in partial fulfillment of the requirements for the Ph.D. degree at Texas Tech University. Mention of a trademark or proprietary product does not constitute a guarantee, warranty, or endorsement of the product by Texas A&M University or Texas Tech University.

Received for publication September 3, 2003.

	REFERENCES

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

 

• Archer, T.L., and E.D. Bynum, Jr. 1993. Yield loss to corn from feeding by the Banks grass mite and two-spotted spider mite (Acari: Tetranychidae). Exp. Appl. Acarol. 17:893–903.
• Archer, T.L., F.B. Peairs, and J.A. Mihm. 1997. Mechanisms and bases of resistance in maize to mites. p. 101–105. In J.A. Mihm (ed.) Insect resistant maize: Recent advances and utilization. Proc. Int. Symp. Int. Maize and Wheat Improvement Center. 27 Nov.–3 Dec. 1994. CIMMYT, Mexico City.
• Berry, J.S., T.O. Holtzer, and J.M. Norman. 1991. MiteSim—A simulation model of the Banks grass mite (Acari: Tetranychidae) and the predatory mite, Neoseiulus fallacies (Acari: Phytoseiidae) on maize: Model development and validation. Ecol. Modell. 53:291–317.
• Bynum, E.D., Jr., and T.L. Archer. 1986. Chemical carrier coverage—Southwestern corn borer control in corn and crop oil spreading on various crops. Southwest. Entomol. 11:45–61.
• Bynum, E.D., Jr., and T.L. Archer. 1992. Banks grass mite (Acari: Tetranychidae) response to selected insecticides used for control of southwestern corn borer. J. Agric. Entomol. 9:189–198.
• Bynum, E.D., Jr., T.L. Archer, and F.W. Plapp. 1990. Action of insecticides to spider mites (Acari: Tetranychidae) on corn in the Texas High Plains: Resistance and synergistic combinations. J. Econ. Entomol. 83:1236–1242.
• Bynum, E.D., Jr., T.L. Archer, and F.W. Plapp, Jr. 1997. Comparison of Banks grass mite and twospotted spider mite (Acari: Tetranychidae): Responses to insecticides alone and in synergistic combinations. J. Econ. Entomol. 90:1125–1130.
• Chandler, L.D., T.L. Archer, C.R. Ward, and W.M. Lyle. 1979. Influences of irrigation practices on spider mite densities on field corn. Environ. Entomol. 8:196–201.
• Ehler, L.E. 1973. Spider mites associated with grain sorghum and corn in Texas. J. Econ. Entomol. 66:1220.
• Griffing, B. 1956. Concept of general and specific combining ability in relation to diallel crossing systems. Aust. J. Biol. Sci. 9:463–493.
• Hanway, J.J. 1971. How a corn plant develops. Cooperative Ext. Service Spec. Rep. 48. Iowa State Univ., Ames.
• Hirnyck, R.E. 1983. Management of Banks grass mite, Oligonychus pratensis (Banks), in field corn treated with insecticides for control of western bean cutworm, Loxagrotis albicosta (Smith). M.S. thesis. Univ. of Nebraska, Lincoln.
• Kamali, K., F.F. Dicke, and W.D. Guthrie. 1989. Resistance-susceptibility of maize genotypes to artifical infestations by twospotted spider mites. Crop Sci. 29:936–938.[Abstract/Free Full Text]
• Kattes, D.H., and G.L. Teetes. 1978. Selected factors influencing the abundance of Banks grass mites in sorghum. Bull. 1186. Texas Agric. Exp. Stn., College Station.
• Mansour, F., and A. Bar-Zur. 1992. Resistance of maize inbred lines to the carmine spider mite, Tetranychus cinnabarinus (Acari: Tetranychidae). Maydica 37:343–345.
• Mansour, F., A. Bar-Zur, and F. Abo-Moch. 1993. Resistance of maize inbred lines to the carmine spider mite, Tetranychus cinnabarinus (Boisduval) (Acari: Tetranychidae): Evaluation of antibiosis of selected lines at different growth stages. Maydica 38:309–311.
• Nyquist, W.E. 1991. Estimation of heritability and prediction of selection response in plant populations. Crit. Rev. Plant Sci. 10:235–322.
• Owens, J.C., C.R. Ward, and G.L. Teetes. 1976. Current status of spider mites in corn and sorghum. p. 38–64. In H.D. Loden and D.W. Wilkinson (ed.) Proc. 31st Annu. Corn and Sorghum Res. Conf., Chicago, IL. 7–9 Dec. 1976. Am. Seed Trade Assoc., Washington, DC.
• Perring, T.M., T.L. Archer, E.D. Bynum, Jr., and K.A. Hollingsworth. 1981. Chemical evaluation for control of the Banks grass mite, Oligonychus pratensis (Banks), on field corn. Southwest. Entomol. 6:130–135.
• Pickett, C.H., and F.E. Gilstrap. 1986. Inoculative releases of phytoseiids (Acari) for the biological control of spider mites (Acari: Tetranychidae) in corn. Environ. Entomol. 15:790–794.
• SAS Institute. 1987. SAS user's guide: Statistics. SAS Inst., Cary, NC.
• Sloderbeck, P.E. 1986. Nebraska has wimpy mites. Kansas St. Univ. Coop. Ext. Ser. In Southwest Kansas entomology update. 28 May, vol. 4, no. 12.
• Uhr, D.V., and M. Goodman. 1995. Temperate maize inbreds derived from tropical germplasm: II. Inbred yield trials. Crop Sci. 35:785–790.[Abstract/Free Full Text]
• Vlosich, E.J. 1980. An evaluation of spider mite resistance in three populations of corn, Zea mays L. M.S. thesis. New Mexico State Univ., Las Cruces.
• Ward, C.R., E.W. Huddleston, J.C. Owens, T.M. Hills, L.G. Richardson, and D. Ashdown. 1972. Control of the Banks grass mite attacking grain sorghum and corn in West Texas. J. Econ. Entomol. 65:523–529.
• Ward, C.R., and F.M. Tan. 1977. Organophosphate resistance in the Banks grass mite. J. Econ. Entomol. 70:250–252.
• Xu, W., T.L. Archer, E.D. Bynum, Jr., and G. Odvody. 2004. Registration of maize germplasm Tx202. Crop Sci. 44:1883–1884.[Free Full Text]
• Zhang, Y., and M.S. Kang. 1997. Diallel-SAS: A SAS program for Griffing's diallel analyses. Agron. J. 89:176–182.[Abstract/Free Full Text]

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