Transgenic Bt corn varietal resistance against the Mexican rice borer, Eoreuma loftini (Dyar) (Lepidoptera: Crambidae) and implications to sugarcane

Crop Protection 48 (2013) 57e62

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Transgenic Bt corn varietal resistance against the Mexican rice borer, Eoreuma loftini (Dyar) (Lepidoptera: Crambidae) and implications to sugarcane Allan T. Showler a, *, Steven C. Cook b, Veronica Abrigo a a b

USDA-ARS IFNRRU, Kika de la Garza Subtropical Agricultural Research Center, 2413 East Highway 83, Bldg. 201, Weslaco, TX 78596, USA USDA-ARS Honeybee Lab, Kika de la Garza Subtropical Agricultural Research Center, Weslaco, TX 78596, USA

a r t i c l e i n f o

a b s t r a c t

Article history: Received 19 November 2012 Received in revised form 29 January 2013 Accepted 9 February 2013

The Mexican rice borer, Eoreuma loftini (Dyar), attacks crops including corn, Zea mays L., rice, Oryza sativa L., sorghum, Sorghum bicolor (L.) Moench, and sugarcane, Saccharum spp. Strongly resistant varieties of any kind, native or otherwise, have not been identified. A field plot corn variety test using two transgenic Bt varieties, Pioneer 31G71, expressing the Cry1F insecticidal protein, and Golden Acres 28V81, expressing the Cry1A.105, Cry2Ab2, and Cry3Bb1 insecticidal proteins, and two non-Bt controls, Dekalb DKC 69-72 and BH Genetics 9050, all four commonly grown in the Lower Rio Grande Valley of Texas, showed that, although oviposition preference was not affected, 28V81 resisted larval stalk boring to the extent that Mexican rice borer injury was almost non-existent. Pioneer 31G71 was infested nearly as much as the controls, but larval development to adulthood was reduced by z70%. Rearing larvae on 5, 50, 500, and 5000 mg of corn leaf tissue per ml of artificial diet showed that, while the three lowest concentrations did not affect larval growth and development, the high concentration of 28V81 reduced survivorship to the pupal stage, decreased weight of 4-wk-old larvae, and prolonged development to pupation. Lower numbers of pheromone trap-captured adults at the edges of commercial Bt and non-Bt corn fields showed that populations were lower at the Bt cornfields, suggesting a lesser rate of adult production. Because corn is a preferred host plant over sugarcane, sorghum or rice, use of resistant transgenic Bt corn varieties will likely protect the crop from the substantial injury that can be caused by the pest. This study also suggests that Bt genes might result in similarly strong resistance when inserted in other vulnerable crops such as sugarcane. Published by Elsevier Ltd.

Keywords: Bacillus thuringiensis Cultivar Integrated pest management Saccharum Trap crop Variety Zea mays

1. Introduction The invasive Mexican rice borer, Eoreuma loftini (Dyar) (Lepidoptera: Crambidae), originally from western Mexico (Van Zwaluwenberg, 1926; Johnson, 1984), is a stem boring pest which was first detected in the United States on sugarcane, Saccharum spp., of the Lower Rio Grande Valley, Texas, in the early 1980s (Johnson, 1981; Johnson and Van Leerdam, 1981). Since then, the Mexican rice borer has become the dominant stem boring insect pest of sugarcane (Youm et al., 1988; Legaspi et al., 1997), representing >95% of the total sugarcane stem borer population (Legaspi et al., 1999). Mexican rice borers mainly oviposit within folds of dry host plant leaves (Showler and Castro, 2010b), and early instars feed on green * Corresponding author. Tel.: þ1 830 792 0319; fax: þ1 830 792 0314. E-mail address: [email protected] (A.T. Showler). 0261-2194/$ e see front matter Published by Elsevier Ltd. http://dx.doi.org/10.1016/j.cropro.2013.02.007

leaf tissue before boring into host plant stalks where the later instars tunnel vertically and horizontally (Van Zwaluwenberg, 1926; Legaspi et al., 1997), resulting in stunting, lodging, reduced juice quality, and “dead hearts” (dead whorl center) (Legaspi et al., 1997). Larvae are exposed on the exterior of the sugarcane plant, and judging from the small diameter entry holes on corn, Zea mays L., for only a day or two before boring into the host plant’s stalk, and the tunnels are packed with frass, protecting larvae from chemical and biological control tactics (Legaspi et al., 1997; Wilson et al., in press). Unlike sugarcane and sorghum, Sorghum bicolor (L.) Moench, which tend to resist lodging better than corn, infestations in susceptible varieties of corn frequently result in stalk lodging, shattering, and collapse, each of which prevent the production of harvestable ears. Larvae pupate within the stalks and adults emerge from correspondingly larger holes than the entry holes. In the Lower Rio Grande Valley, the life cycle requires 30e45 d, and there are 4e5 overlapping generations/yr (Legaspi et al., 1997; Showler and Reagan, 2012).

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By 1987, the Mexican rice borer’s range expanded from the Lower Rio Grande Valley to encompass 40 counties in South Texas; by 1989, the pest invaded rice producing areas of East Texas (Browning et al., 1989), and by 2008 it was detected in western Louisiana rice producing areas (Hummel et al., 2008, 2010). The pest has been predicted to infest all of Louisiana’s sugarcane and rice growing areas by 2035 (Reay-Jones et al., 2008). Mexican rice borers, however, have recently been found to prefer corn over sugarcane by as much as 8.2-fold (based on numbers of borer entry holes on the stalks), and this suggests that Mexican rice borers might have already spread into corn producing regions, and that hitherto noninfested areas of Texas, Louisiana, and possibly other states, might already be infested or at risk of infestation (Showler et al., 2011). Similarly, in the Republic of South Africa, the eldana borer, Eldana saccharina Walker, which also prefers corn over sugarcane (Cochereau, 1982), was detected well outside sugarcane growing regions (Assefa et al., 2008). Keeping et al. (2007) reported that Bt corn did not deter eldana borer larvae from boring into stalks, but significantly fewer adults emerged than from non Bt corn, making the Bt variety a “dead end” trap crop. The purpose of this study was to assess the resistance of varieties of Bt corn versus non-Bt corn varieties that are commonly grown in the Lower Rio Grande Valley, Texas (where the Mexican rice borer is the predominant pest of sugarcane), but are not necessarily isolines, with the aim of both protecting corn as well as for identifying a possible source of resistance if applied to sugarcane. 2. Materials and methods 2.1. Small plot field experiment A 0.46-ha field at the USDA-ARS Kika de la Garza Subtropical Agricultural Research Center in Weslaco, Hidalgo County, Texas, was divided into eight blocks (replicates) with four plots in each block; each block was separated from the others by a 2-m bare buffer “alley”. Plots were 8 rows wide (8.5 m) and 15.25 m long (0.013 ha). Four treatments were randomly assigned to each plot within each block and each block contained all four treatments. Treatments were comprised of different corn varieties, each of which was commonly grown in the Lower Rio Grande Valley of Texas where the Mexican rice borer is the key pest of sugarcane (Legaspi et al., 1997). Two of the varieties were Bt varieties and two were non-Bt varieties (controls). The Bt varieties were Pioneer 31G71 with the HX1, Herculex 1 Insect Protection gene expressing the Cry1F insecticidal protein (Pioneer Hi-Bred International, Johnston, IA) and Golden Acres 28V81 with the VT3Pro gene expressing the Cry1A.105, Cry2Ab2, and Cry3Bb1 insecticidal proteins (Golden Acres, Waco, TX), and the controls were Dekalb DKC 69-72 (Monsanto, Chesterfield, MO) and BH Genetics 9050 (BH Genetics, Ganado, TX). Although the Bt corn varieties are registered as being resistant to other pests of corn (Table 1), neither had been assessed for efficacy against the Mexican rice borer. The seed was planted on 7 March 2011 z15 cm apart by a conventional mechanical planter. Weeds were controlled by mechanical cultivation when the plants were still short, and removed manually when the corn plants were 60 cm tall. Conventional fertilizer and irrigation practices were used. On 15 April and 18 July 2011, numbers of corn stalks were counted on two separate 3-m-long sections of row in each plot. On 15 April, 50 consecutive corn stalks (beginning >5 stalks from the end of each row to avoid edge effects), were counted, and the first and fiftieth stalks were marked with plastic flagging. On 18 July, when the crop was ready for harvest, numbers of total stalks and non-yielding stalks (e.g., lodged, shattered, or collapsed) within the

Table 1 Bt corn varieties and the pests against which they are registered. Variety

Pioneer 31G71 HX1 gene

Golden Acres 28V81 VT3Pro gene DKC 69-72 (non-Bt) BH Genetics 9050 (non-Bt) a

Pests Common name

Scientific name

Black cutworm Corn earworma European corn borer Fall armyworm Lesser corn stalk borer Southern corn stalk borer Southwestern corn borer Sugarcane borer Western bean cutworm Corn earworm Corn rootworm European corn borer Fall armyworm None

Agrotis ipsilon Hufnagel Heliothis zea (Boddie) Ostrinia nubilalis Hübner Spodoptera frugiperda J.E. Smith Elasmopalpus lignosellus (Zeller) Diatraea crambidioides (Grote) Diatraea grandiosella Dyar Diatraea saccharalis (F.) Loxagrotis albicosta Smith Heliothis zea (Boddie) Diabrotica spp. Ostrinia nubilalis Hübner Spodoptera frugiperda J.E. Smith

None

“Suppression” only.

50 stalks were counted. On the same day, five randomly selected stalks from each plot were cut at the base, stripped of leaves, diameters 5 cm above the soil surface were recorded, as were numbers of internodes; Mexican rice borer entry holes, bored internodes (determined by splitting the stalks to observe tunneling), and exit holes per stalk, and numbers of injured stalks. Treatment differences were detected using ANOVA with treatment and block effects and a treatment  block interaction in the model, and means were separated using Tukey’s HSD (Analytical Software, 1998). Because normality and homogeneity of variance assumptions were not violated, data was not transformed in this experiment and in the other experiments unless stated otherwise. 2.2. Excised dry leaf oviposition preference experiment Thirty dry leaves, each z20 cm long and from the lower third of a separate corn stalk, were excised from individual, randomly selected corn plants in each of the four field plot experiment treatments. Two leaves of each variety, each pre-inspected to ensure absence of eggs, were suspended in random order from the tops of 15.40 cm  40 cm  40 cm Plexiglass containers and five newly-emerged adult male and five female Mexican rice borers were released into each container. After 7 d, the leaves were removed from the containers and egg clusters and eggs were counted. Treatment differences for egg clusters and eggs were detected using one-way ANOVA (Analytical Software, 1998). 2.3. Larval effects experiment Five meridic foods, modified after Vanderzant (1974) were used in laboratory bioassays. Each food consisted of a base diet having added wither cellulose powder (control), or a mixture of cellulose powder and lyophilized, powdered leaf material from each one of the four corn varieties (treatment foods). The base diet was prepared according to Martinez et al. (1988). One hundred ml of the base diet was placed in a preheated (70  C) blender carafe and, for each of the treatment foods the powdered corn plant tissue was added to obtain 5, 50, 500, and 5000 mg per ml of base diet. Each combination, and the control, was mixed at high speed for 1 min using a separate preheated carafe. Two ml of the final mixture of each food treatment was poured into each of 32 wells of 128-well bioassay trays. The trays were covered with paper towels and allowed to cool and harden for 24 h at room temperature.

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Using a fine camelhair paintbrush, neonate Mexican rice borer larvae were individually placed in the diet-filled wells. The wells were sealed with aerated bioassay tray covers, and the trays were kept in an incubator at 27  C, 75% RH, and on a 12:12 L:D photocycle. Larvae were checked after 24 h to ascertain mortality from handling. Neonates were allowed to feed on the diet for 2 wk, then the diet “plug” was gently removed from each well and examined for evidence of feeding (e.g., piles of debris on the surface, and tunneling into the diet). Numbers of surviving larvae were counted at the ends of wk 2, 3, and 4. Larvae were transferred by paintbrush from the diet to a scale for weighing at the end of the 4th wk, then they were placed on freshly prepared food in wells of clean bioassay trays, covered, and returned to the environmental chamber until larvae pupated. Numbers of pupae in each treatment were counted. There were seven replicates per treatment; each replicate was comprised of three larvae (each one in a separate container). Because of mortality, there were five replications in the 5000 mg/ml 28V81 diet treatments at the ends of wks 3 and 4, and after pupation. For larval weights and for determining time to pupation, there were only enough larvae at the end of 4 wk for three replicates in the high 28V81 concentration. Data across all treatments and times was analyzed using ANOVA and means were separated using Tukey’s HSD (Analytical Software, 1998). Percentages were arcsine-square root-transformed before analysis. 3. Results

59

per 3 m row on 14 April, by 18 July, only 58% and 58.5% of the stalks were producing ears in 9050 and 69-72, respectively (F ¼ 11.94; df ¼ 7, 63; P < 0.0001), but in the 31G71 and 28V81 treatments 87.7% and 99.1% of the stalks produced ears. Of the 50 consecutive stalks counted in April, 50.8% and 40.4% failed to produce ears of corn in 9050 and 69-72, respectively, but 31G71 and 28V81 only lost production from 16.4% to 2.2% of the stalks, respectively (Table 2). Numbers of stalks that were producing ears in the 9050 and 69-72 treatments were only 49.1% and 59.5%, respectively, of the 50 original stalks (F ¼ 13.3; df ¼ 7, 63; P < 0.0001), but the numbers found in the 31G71 and 28V81 plots were 83.6% and 99.75% of the original number and a difference between the Bt varieties was not detected. At harvest, treatment differences between stalk diameters and numbers of internodes per stalk were not detected (Table 3), but the numbers of stalks with one or more Mexican rice borerinjured internodes in 31G71 and 28V81 corn were 36% and 97.3% less than in the controls (F ¼ 30.78; df ¼ 3, 31; P < 0.0001) (Table 3). Numbers of bored internodes per stalk were reduced by 99.1% in 28V81 corn but differences were not detected between 31G71 and the controls (Table 3). Larval entry holes per stalk were 99% fewer on 28V81 than on the controls, but statistical differences between 31G71 and the other three varieties were not detected (Table 3). Exit holes, however, were reduced by z67.4% in 31G71, and no exit holes were found in 28V81 corn stalks (Table 3).

3.1. Small plot field experiment 3.2. Excised dry leaf oviposition preference experiment Forty one percent and 46% of the stalks in the susceptible 9050 and 69-72 varieties, respectively, were non-yielding because the stalks were lodged, shattered, or collapsed, but only 12.7% and 0.9% of the stalks failed to produce ears in the 31G71 and 28V81 plots, respectively (Table 2). Compared with the number of stalks counted

Treatment differences were not detected for numbers of Mexican rice borer egg clusters and for numbers of eggs. The pooled averages for number of egg clusters and number of eggs per leaf was 0.87  0.17 and 4.9  0.9, respectively.

Table 2 Mean numbers of corn stalks/3 m row, and numbers of corn stalks that did not produce ears of corn because of lodging, shattering, and collapse, Hidalgo County, Texas, USA, 2011. Varietya

9050 69-72 31G71 28V81 Fb P

Bt versus non-Bt

Non-Bt Non-Bt Bt Bt

No. stalks/3 m row

No. non-yielding stalks/3 m row

No. non-yielding stalks of 50 stalksc

15 April Mean (SE)

18 July Mean (SE)

Mean (SE)

Mean (SE)

21.1 (0.4) 22.5 (0.7) 21.9 (0.2) 21.6 (0.3) 1.59 0.2226

20.9 (0.4) 21.5 (1.2) 21.6 (0.2) 21.4 (0.2) 0.51 0.6943

8.6 (2.0) 9.9 (2.4) 2.7 (1.3) 0.2 (0.1) 9.58 0.0003

25.4 (5.0) a 20.2 (5.1) ab 8.2 (3.9) bc 1.1 (0.5) c 8.09 0.0009

ab a bc c

Means within the same column followed by different letter are significantly different (P < 0.05), Tukey’s HSD. a BH Genetics 9050, DKC 69-72, Pioneer 31G71, Golden Acres 28V81. b df ¼ 3, 31, n ¼ 8 replicates. c The 50 stalks were consecutive, counted on 14 April, the data shown was collected 18 July.

Table 3 Mean stalk diameters and numbers of internodes and Mexican rice borer damage per corn stalk, n ¼ 5 stalks/plot, Hidalgo County, Texas, USA, 18 July 2011. Varietya

9050 69-72 31G71 28V81 Fc P

Stalk diam (cm)b

No. internodes

No. bored internodes

No. entry holes

No. exit holes

Mean (SE)

Mean (SE)

Mean (SE)

Mean (SE)

Mean (SE)

2.7 (0.1) 2.7 (0.1) 2.7 (0.1) 2.7 (0.1) 0.41 0.7538

13.0 (0.1) 13.0 (0.1) 13.1 (0.1) 13.0 (0.1) 0.47 0.7094

2.28 (0.25) 2.15 (0.21) 1.30 (0.44) 0.02 (0.02) 12.61 0.0001

Means within each column followed by different letters are significant (P < 0.05), Tukey’s HSD. a BH Genetic 9050 (non-Bt), DKC 69-72 (non-Bt), Pioneer 31G71 (Bt), Golden Acres 28V81 (Bt). b 5 cm above soil surface. c df ¼ 3, 31.

a a a b

5.12 (0.34) 4.82 (0.78) 2.52 (0.89) 0.05 (0.05) 11.96 0.0001

a a ab b

1.40 (0.21) a 1.38 (0.17) a 0.45 (0.20) b 0 (0) b 14.41 <0.0001

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10

At the high rate (5000 mg corn leaf tissue per ml artificial diet) of 28V81 there were 68.6% fewer pupae than in the control (F ¼ 4.22; df ¼ 19, 133; P < 0.0001). The three low rates of corn tissue did not affect larval weights after 4 wk, but the high rate of 31G71 was associated with 47.4% lower weight than the control, and larval weight on 28V81 was 87.2% less than on 31G71 (F ¼ 12.92; df ¼ 4, 30; P < 0.0001) (Fig. 1). Similarly, the numbers of weeks until pupation were not influenced by the three lowest rates, but at the high rate, 31G71 was associated with a 37.5% longer development time than the control, and 28V81 was associated with a 32.6% longer development time than in the 31G71 high rate treatment (F ¼ 20.29; df ¼ 4, 27; P < 0.0001) (Fig. 2).

Our field experiment demonstrated that Mexican rice borers can cause substantial damage to susceptible varieties of corn. In regions that have large populations of Mexican rice borers, such as the Lower Rio Grande Valley, the insect’s economic impact against locally grown corn varieties should be assessed. Varieties of Bt corn grown in 2009 accounted for 63% of the United States’ 22 million ha of corn (Hutchinson et al., 2010); our results suggest that, where the Mexican rice borer is present, Bt varieties will likely constitute the first and greatest defense. The extent of tunneling injury to the susceptible corn varieties used in our study precludes growing them for economic profit unless the crop is adequately protected by one or more cultural, chemical, and biological tactics. In many parts of Africa, the eldana borer is a severe pest of sugarcane (Girling, 1972; Atkinson and Carnegie, 1989; Keeping and Meyer, 2002), and although there are a number of differences between the eldana borer and the Mexican rice borer, other aspects are similar (Showler and Reagan, 2012). Unlike the sugarcane borer, Diatraea saccharalis (F.), which was displaced in the Lower Rio Grande Valley by the arrival of the Mexican rice borer (Van Leerdam et al., 1984; Legaspi et al., 1997), the eldana and Mexican rice borers are cryptic, inserting their eggs in tight folds and curls of leaf tissue (Showler and Castro, 2010b), and blocking larval tunnels with frass, posing challenges to chemical and biological control (Legaspi et al., 1997; Showler and Reagan, 2012). The insect’s prevalence and the extent of its injury make it the key sugarcane pest of South Texas (Legaspi et al., 1997). The pest damages z20% of the sugarcane internodes in that region, causing losses of US$575/ha (Meagher

Larval weight (mg)

50

Control 9050 DKC6972 31G71 28V81

a ab

ab

40 30

b

20 10

8

Control 9050 DKC6972 31G71 28V81

bc

6

a b bc

c 4 2 0

4. Discussion

60

Weeks to pupation

3.3. Larval effects experiment

c

0 Fig. 1. Mean (SE) weights of 4-wk-old Mexican rice borer larvae reared on artificial diet incorporated with 5000 mg of corn leaf tissue per ml diet, one-way ANOVA, Tukey’s HSD (5e7 replicates, 3 larvae per replicate).

Fig. 2. Mean (SE) numbers of week until pupation by larval Mexican rice borers reared on artificial diet incorporated with 5000 mg of corn leaf tissue per ml diet, oneway ANOVA, Tukey’s HSD (5e7 replicates, 3 larvae per replicate).

et al., 1994) and US$10e20 million annually throughout the region (Legaspi et al., 1997, 1999). Larval tunneling also provides portals for red rot, Colletotrichum falcatum Went, infection, which results in the breakdown of sugar (Van Zwaluwenberg, 1926; Osborn and Phillips, 1946). Both the Mexican rice borer and the eldana borer cause more damage to sugarcane that is drought stressed or provided with excessive nitrogen fertilizer, indicating their orientation toward host plant nutritional quality, particularly in terms of nitrogenous and carbohydrate biochemicals (Showler and Reagan, 2012). Our study demonstrated that the expression of resistance by transgenic Bt corn against the Mexican rice borer closely resembled that found against the eldana borer in corn producing Bt Cry1Ab toxin (Keeping et al., 2007). That variety did not affect oviposition preference and was as infested with eldana borers as the non-Bt control (Keeping et al., 2007). Similarly, our study showed that, statistically, 31G71 was not different from the controls either, but the mean numbers of bored internodes and total numbers of larval entry holes were substantially lower (by z40% and z48%, respectively) than on the controls; levels of injury were highly variable between the 31G71 plots. Because of the observed variability and lower numbers of larval entry holes, and because 31G71 curtailed 70% of adult emergence, 31G71 is not as effective for trapping Mexican rice borers as the South African variety was for eldana borers (Showler and Reagan, 2012). Ideal dead end trap cultivars would be as attractive, or more so, compared with susceptible varieties, and stalks would be tolerant to shattering, lodging, and stalk rot diseases to extend the effectiveness of each plant (Showler and Reagan, 2012) while reducing adult emergence as much as 28V81. Because a Bt Cry1Ab variety reduced the survival of the sugarcane borer to nearly zero, it was suggested as a trap crop for that pest (McAllister et al., 2004). In the excised dry leaf experiment, we observed no oviposition preference between the varieties, but 28V81 was almost completely resistant to larval boring. Larval entry holes in the other three varieties were found almost exclusively on the six lowermost internodes, but on 28V81 what appeared to be whitish larval feeding “tracks,” 1 cm in length each, scarred the outermost tissue without penetrating the stalk. Because adult Mexican rice borers deposited eggs on 28V81, the basis of resistance appears to be antibiotic rather than antixenotic. Hence, 28V81 is also a suitable trap plant for Mexican rice borer eggs with negligible risk to the stalk. Our observation that only artificial diet incorporated with the highest concentration of Bt-corn tissue interfered with larval development and survival indicates that the Bt toxin must be

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available in relatively substantial concentrations to be effective. Because the Bt-corn plants in the field were not diluted by artificial diet, effects against Mexican rice borer larvae were apparent. The larval diet assay also demonstrated that percentage larval survival to pupation is reduced by Bt-corn tissue to a greater extent in 28V81 than in 31G71. The greater reduction in 4-wk-old larval weight by 28V81 than by 31G71, and the low survivorship and longer time needed for pupation on the 28V81 diet show that the Bt toxin in 28V81 was more deleterious to larval development. This helps to explain the strong reduction in adult emergence holes in our field-grown 28V81 corn, decimating the next generation of Mexican rice borer at the larval stage before the crop is injured. Adoption of corn that is as resistant to Mexican rice borers as 28V81 on an area-wide basis might have area-wide implications in terms of reducing overall populations of the pest. All four varieties used in our study could each serve as trap crops to divert Mexican rice borers away from sugarcane, but 31G71 and the controls would likely need to be destroyed before they contributed toward the production of adults developing within the stalks. Growing 28V81, and varieties with similarly high levels of resistance to Mexican rice borers, commercially within the regional agricultural landscape, intermingled with sugarcane fields might be the best way of “deploying” corn as a dead end trap crop to protect the sugarcane. Perhaps the most obvious possibility to emerge from our results, however, is development of sugarcane varieties with Bt resistance to the Mexican rice borer similar to that of 28V81 corn. Insertion of a Bt gene, for example, that is as protective of sugarcane against the Mexican rice borer as the VT3Pro gene found to be in 28V81 corn might provide a cornerstone tactic for sugarcane integrated pest management in areas where the Mexican rice borer is problematic. Currently there is no integrated strategy in place for Mexican rice borers in corn, but integrative tactics are being developed for protecting sugarcane, some of which are already being used for eldana borer control in parts of Africa (Showler and Reagan, 2012). Because both species are more attracted to drought stressed than to well-watered sugarcane, irrigation sufficient to avoid water deficitassociated nutritional increases, and heightened infestations of the two pests, has been recommended (Carnegie, 1981; Keeping and Meyer, 2002; Reay-Jones et al., 2005; Showler and Castro, 2010a). In the Republic of South Africa, reduction of nitrogen fertilizer to 30 kg/ha is recommended to minimize eldana borer problems in sugarcane (Carnegie, 1981), especially during periods of water deficit stress (Keeping and Meyer, 2002). Relatively high concentrations of organic matter amended to soil in the Lower Rio Grande Valley (and fertilized with nitrogen as per convention) resulted in 18% more stalk production per sugarcane stool, but this effect was offset by substantially increased Mexican rice borer infestation, causing reductions in stalk weight, length, and percentage brix relative to sugarcane with conventional nitrogen fertilizer or chicken litter (ATS, unpublished data). Although sugarcane varieties that are partially resistant to Mexican rice borers were better protected than susceptible varieties under drought conditions, water deficit nevertheless increased injury by z2.5-fold to susceptible and resistant cultivars (Reay-Jones et al., 2005); hence, the influence of water deficit stress, and excessive nitrogen, on the efficacy of resistant Bt corn varieties (e.g., 28V81) should be assessed. Although some stalkborers might become resistant to Bt corn varieties (Huang et al., 2009), the level of resistance we observed in 28V81 against the Mexican rice borer offers a valuable tool for integrated pest management. 28V81 minimized loss of stalks and accompanying ears, making it a strong tactic capable of providing nearly complete protection against the Mexican rice borer.

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