Management of feeding damage and survival of southwestern corn borer and sugarcane borer (Lepidoptera: Crambidae) with Bacillus thuringiensis transgenic field corn

The efficacy of transgenic corn hybrids expressing an insecticidal Bacillus thuringiensis (Bt) delta-endotoxin from different transformation events was evaluated in field corn, Zea mays L., against the southwestern corn borer, Diatraea grandiosella Dyar, and sugarcane borer, Diatraea saccharalis (F.). Susceptibilities of neonates and third instars were determined on Bt and non-Bt corn plants (V6 and R1 stages) in field plots and corn leaf tissue feeding exposure in laboratory bioassays. Bt corn hybrids associated with MON810 and CBH351 transformation events sustained significantly less injury by southwestern corn borer and sugarcane borer during mid-whorl stage infestations compared with their respective non-Bt hybrid equivalents. Southwestern corn borer and sugarcane borer feeding injury to ear leaf-sheath and husk tissues during the silking stage of corn was significantly reduced in MON810 and CBH351 Bt corn compared with their respective non-Bt hybrids. However, resistance levels to feeding injury in Bt hybrids associated with the MON810 event were significantly higher than that in the hybrid associated with the CBH351 event. Southwestern corn borer and sugarcane borer caused more feeding injury to husk tissue than to ear leaf-sheath tissue in both Bt and non-Bt hybrids infested during the silking stage. Laboratory performance of the MON810 event against southwestern corn borer and sugarcane borer varied among hybrids associated with the same event. Third instars of southwestern corn borer were highly susceptible to MON810 Bt corn hybrids in leaf tissue experiments. However, sugarcane borer larvae were susceptible to the MON810 event only in one of the Bt hybrids evaluated. Sugarcane borer mortality was significantly lower after 96 h of feeding exposure on CBH351 Bt corn leaf tissue than on MON810 Bt corn leaf tissue. Plant resistance to southwestern corn borer and sugarcane borer increased as plants matured, independent of the presence of a Bt construct. These results are essential to estimate the importance of Bt transgenic corn in areas of southern United States and other areas where mixed populations of southwestern corn borer and sugarcane borer are predominant and cause severe damage to corn production.     

Late-instar European corn borer (Lepidoptera: Crambidae) tunneling and survival in transgenic corn hybrids

Field studies were conducted in 1996 and 1997 to determine injury by and survival of late-instar European corn borer, Ostrinia nubilalis (Hübner), on genetically altered Bacillus thuringiensis Berliner corn, Zea mays L. Cry1Ab events 176, Bt11, MON810, and MON802; Cry1Ac event DBT418; and Cry9C event CBH351 were evaluated. Plants of each corn hybrid were manually infested with two third-, fourth-, or fifth-instar O. nubilalis. Larvae were held in proximity to the internode of the plant above the ear with a mesh sleeve. Larvae were put on the plants during corn developmental stages V8, V16, R1, R3, R4, R5, and R6. This study shows that not all B. thuringiensis hybrids provide the same protection against O. nubilalis injury. Hybrids with B. thuringiensis events Bt11, MON810, MON802, and CHB351 effectively protected the corn against tunneling by late-instar O. nubilalis. Event 176 was effective in controlling late-instar O. nubilalis during V12 and V16 corn developmental stages; however, significant tunneling occurred by fourth instars during R3 and R5. Event DBT418 was not effective in controlling late-instar O. nubilalis during corn vegetative or reproductive stages of development. Whether the B. thuringiensis hybrids satisfied high- and ultra-high-dose requirements is discussed.     

DiPel-selected Ostrinia nubilalis larvae are not resistant to transgenic corn expressing Bacillus thuringiensis Cry1Ab

The survival of KS-SC DiPel-resistant and -susceptible European corn borer, Ostrinia nubilalis (Hübner), was evaluated on different tissues from corn, Zea mays L., hybrids, including a nontransgenic and two transgenic corn plants (events MON810 and Bt11) expressing high doses of Bacillus thuringiensis (Bt) Cry1Ab. The survival of Bt-resistant and -susceptible third instars was similar after a 5-d exposure to transgenic plant tissues. Survivors eventually died when returned to Bt corn tissues, but many were able to continue development when transferred to non-Bt corn tissues. Survival of resistant and susceptible larvae also was evaluated in bioassays with dilutions of leaf extracts from the three corn hybrids incorporated in an artificial diet. In these assays, survival was significantly higher for resistant O. nubilalis neonates at three of the five dilutions compared with the susceptible strain, but the resistance ratio was only 2.2- and 2.4-fold for MON810 and Bt11, respectively. The data demonstrate that Bt-resistant and unselected control O. nubilalis larvae were similar in susceptibility to MON810 and Bt11 event corn hybrids. Although we were unable to evaluate the Cry1Ab protein that larvae were exposed to in the transgenic tissue because of company restrictions, Cry1Ab protoxin produced in Escherichia coli was incubated with extracts from non-Bt corn leaves to simulate the in planta effect on the transgenic protein. Cry1Ab protoxin was hydrolyzed rapidly by enzymes in the corn extract into peptide fragments with molecular masses ranging from 132 to 74 kDa, and eventually 58 kDa. Overall, these data suggest that plant enzymes hydrolyze transgenic toxin to one that is functionally activated. Therefore, resistant insect populations with reduced proteinase activity do not seem to pose a threat to the efficacy of commercial MON810 and Bt11 corn hybrids.     

Plant-incorporated Bacillus thuringiensis resistance for control of fall armyworm and corn earworm (Lepidoptera: Noctuidae) in corn

Fall armyworm, Spodoptera frugiperda (J.E. Smith), and corn earworm, Helicoverpa zea (Boddie), perennially cause leaf and ear damage to corn, Zea mays L., in the southeastern United States. Transgenic Bacillus thuringiensis (Bt) hybrids with the Bt11, MON810, or 176 events expressing the Cry1Ab insecticidal endotoxin from were evaluated for control fall armyworm and corn earworm at seven locations in Georgia during 1999 and 2000. Corn was planted at the recommended time for each location and 1 and 2 mo later in the southern locations. All Bt events consistently reduced whorl infestation and damage, although event 176 did not prevent whorl damage in the later plantings in the southern locations in both years. All events also reduced seedling damage by the lesser cornstalk borer, Elasmopalpus lignosellus (Zeller), in one trial and stalk infestations and tunnel length by southwestern corn borers, Diatraea grandiosella Dyar, in another trial. Hybrids containing Bt11 and MON810 events reduced ear infestations in all trials, although reductions were small in later plantings. Nevertheless, both events reduced grain damage from earworms and armyworms by an average +/- SE of 52.5 +/- 5.1% in all trials. The hybrid containing event 176 did not reduce ear infestations and damage. Total grain aflatoxin concentrations were not significantly affected by Bt resistance in any trial (N = 17). Yield responses were variable with the prevention of yield loss being proportional to the severity of insect damage. Although plantings made after the recommended time did not consistently benefit from Bt resistance, Bt11 and MON810 events were effective in reducing damage to field corn when large infestations occurred. The Bt11 and MON810 events mitigated the risk of severe lepidopteran damage to corn, thereby making later plantings of corn feasible in double-cropping systems.     

转Bt基因抗虫玉米对亚洲玉米螟幼虫取食行为的影响

室内研究了转cry1Ab杀虫蛋白基因的Bt抗虫玉米MON81 0和Bt1 1对亚洲玉米螟Ostriniafurnacalis初孵幼虫和3龄幼虫的取食行为、取食选择性和存活率的影响。在48h的非选择性试验中玉米螟初孵幼虫在MON81 0和Bt1 1玉米心叶组织上的幼虫取食率随时间的增加而下降,在对照玉米上的幼虫取食率随时间的增加而上升,两者间差异极显著。初孵幼虫接虫到MON81 0和Bt1 1玉米叶片48h的累计死亡率分别为67 .5 %和47 .5 % ,而在对照玉米上死亡率均为0. 3龄幼虫在Bt和非Bt玉米穗轴组织上的幼虫取食率随时间的增加呈上升趋势,第48h时在Bt和非Bt玉米上的幼虫取食率分别达到77 5 %和1 0 0 % ,差异极显著。选择性试验中,第4～48h内,初孵幼虫在Bt玉米上的幼虫取食率呈下降趋势,第48h时MON81 0和Bt1 1与各自非Bt对照的组合中初孵幼虫的累计死亡率分别为2 5 .0 %和1 7. 5 % ,二者差异不显著。3龄幼虫在Bt玉米和非Bt玉米上的幼虫取食率均随时间的延长而增加,但在非Bt玉米的幼虫取食率增加速度快,与Bt玉米差异极显著。Bt玉米对玉米螟幼虫取食有抑制和忌避作用。     

Assessment of experimental Bt events against fall armyworm and corn earworm in field corn

Performance of experimental Bacillus thuringiensis (Bt) MON events alone and pyramided with MON810 were evaluated over 3 yr in Georgia and Alabama. Ability of events to prevent whorl defoliation by the fall armyworm, Spodoptera frugiperda (J. E. Smith), and natural ear feeding damage by the corn earworm, Helicoverpa zea (Boddie) was assessed. In each year, near-isogenic hybrids with novel single transformation events and crosses pyramided with the MON810 event were compared with the standard single MON810 event and nontransformed susceptible control. Events were tested for resistance to whorl damage by manual infestations of fall armyworm and ear damage by natural infestations of corn earworm. All Bt events tested reduced fall armyworm whorl damage ratings per plant compared with the susceptible hybrid. All Bt treatments also had considerably less ear infestation and damage by corn earworm compared with the nontransgenic isoline. The MON841, MION849, and MON851 events reduced ear damage by H. zea but were not as effective as other novel events and were not advanced for further testing after the 1999 season. Pyramiding events compared with single events did not improve control of fall armyworm whorl damage, but they generally did prevent more ear damage by corn earworm. The MON84006 event singly and pyramided with MON810 had superior control of whorl-stage damage by S. frugiperda and ear damage by H. zea compared with MON810. Deployment of new events and genes could provide additional tools for managing the potential for insect resistance to Bt toxins. Furthermore, improved control of whorl and ear infestations by H. zea and S. frugiperda would increase the flexibility of planting corn, Zea mays L., and permit double cropping of corn in areas where these pests perennially reach damaging levels.     

Seed mixtures as a resistance management strategy for European corn borers (Lepidoptera: Crambidae) infesting transgenic corn expressing Cry1Ab protein

Dispersal of neonate European corn borers, Ostrinia nubilalis (Hübner), in seed mixtures of transgenic corn expressing Cry1Ab protein (Bt+) and nontransgenic corn (Bt-) was evaluated in a 2-yr field study. The main objective was to determine if larval dispersal limits the effectiveness of seed mixtures as a resistance management strategy. Mixtures evaluated included (1) all Bt+ plants, (2) every fifth plant Bt- with remaining plants Bt+, (3) every fifth plant Bt+ with remaining plants Bt-, and (4) all Bt- plants. The transformation events MON 802 (B73 BC1F2 x Mol7) and MON 810 (B73 BC1F1 x Mo17), which express the Cry1Ab endotoxin isolated from Bacillus thuringiensis subsp. kurstaki, were used as the sources of Bt+ seed in 1994 and 1995, respectively (YieldGard, Monsanto, St. Louis, MO). At corn growth stage V6-V8, subplots within each mixture (15-20 plants each) were infested so that every fifth plant in mixtures 1 and 4, every Bt- plant in mixture 2, and every Bt+ plant in mixture 3 received two egg masses. Larval sampling over a 21-d period indicated increased neonate dispersal off of Bt+ plants, reduced survival of larvae that dispersed from Bt+ plants to Bt- plants, and a low incidence of late-instar movement from Bt- plants to Bt+ plants. Computer simulations based on mortality and dispersal estimates from this study indicate that seed mixtures will delay the evolution of resistant European corn borer populations compared with uniform planting of transgenic corn. However, resistant European corn borer populations likely will develop faster in seed mixes compared with separate plantings of Bt and non-Bt corn.     

Effects of bacillus thuringiensis transgenic corn on corn earworm and fall armyworm (Lepidoptera: Noctuidae) densities

We examined 17 pairs of near-isogenic hybrids of Bacillus thuringiensis (Bt) (176, Mon810, and Bt11) and non-Bt corn, Zea mays L., to examine the effects of Bt on larval densities of Helicoverpa zea (Boddie) and Spodoptera frugiperda (J. E. Smith) (Lepidoptera: Noctuidae) during 2 yr. During ear formation, instar densities of H. zea and S. frugiperda were recorded for each hybrid. We found that H. zea first, second, and fifth instar densities were each affected by Mon810 and Bt11 Bt corn but not by 176 corn. Surprisingly, first and second instars were found in higher numbers on ears of Mon810 and Bt11 corn than on non-Bt corn. Densities of third and fourth instars were equal on Bt and non-Bt hybrids, whereas densities of fifth instars were lower on Bt plants. S. frugiperda larval densities were only affected during 1 yr when second, and fourth to sixth instars were lower on ears of Mon810 and Bt11 hybrids compared with their non-Bt counterparts. Two likely explanations for early instar H. zea densities being higher on Bt corn than non-Bt corn are that (1) Bt toxins delay development, creating a greater abundance of early instars that eventually die, and (2) reduced survival of H. zea to later instars on Bt corn decreased the normal asymmetric cannibalism or H. zea-S. frugiperda intraguild predation of late instars on early instars. Either explanation could explain why differences between Bt and non-Bt plants were greater for H. zea than S. frugiperda, because H. zea is more strongly affected by Bt toxins and more cannibalistic.     

Comparison of Bacillus thuringiensis corn hybrids and insecticide-treated isolines exposed to bivoltine European corn borer (Lepidoptera: Crambidae) in South Dakota

The performances of Bacillus thuringiensis (Bt) corn hybrids and permethrin-treated isolines were evaluated in South Dakota from 1997 to 1999. The corn hybrids were exposed to natural seasonal fluctuations of a bivoltine ecotype European corn borer population. Larval injury parameters, grain yields, and gross incomes were quantified during the 3-yr study. The use of either Bt hybrids or whorl-applied permethrin granules resulted in significant yield advantages in only 1 out of 3 yr of performance testing. Under high corn borer pressure in 1997, the performances of the Bt corn hybrids and permethrin-treated isolines were similar, and significantly better than the untreated isolines. However, injury-free corn did not necessarily translate into higher yields in some of the hybrid groups. Grain moisture at harvest, which can result in moisture penalty or dockage, was significantly higher in some Bt corn hybrids. Neither the Bt hybrids nor the permethrin-treated isolines produced significant advantages in yields in 1998 and 1999 when infestations of first-generation corn borer were almost nonexistent and infestations of second-generation European corn borers were moderate.     

Biotechnology and the European corn borer: measuring historical farmer perceptions and adoption of transgenic Bt corn as a pest management strategy

A 3-yr, multi-state survey of farmers who had planted transgenic Bacillus thuringiensis (Bt) corn was conducted to evaluate perceptions of Bt corn performance and its utility as a management option for European corn borer, Ostrinia nubilalis (Hübner). A questionnaire was sent to farmers in Illinois, Iowa, Kansas, Minnesota, Nebraska, and Pennsylvania who had grown Bt corn during the growing seasons of 1996, 1997, or 1998. There were 7,427 usable questionnaires returned with the following response percentages: 1996 (42.1%), 1997 (35.0%), and 1998 (22.6%). Adoption rates, based on percentage of acreage planted to Bt corn, increased dramatically from 1996 (10.5%) to 1998 (40.7%). The states growing the highest percentage of Bt corn were Minnesota, Iowa, and then Nebraska However, Illinois, was adopting Bt corn at the fastest rate. Historical use of insecticides did not influence the adoption of Bt corn. In addition, of those farmers who used insecticides to control European corn borer, the percentage that decreased their use of insecticides nearly doubled from 13.2% (1996) to 26.0% (1998) over this 3-yr period. The primary reason farmers planted Bt corn was to eliminate the yield loss caused by European corn borer. Scouting for European corn borers decreased from 91% (scouting 2.2 times a year) in 1996 to 75% (scouting 1.8 times a year) in 1998. The percentage of farmers not scouting for European corn borers increased from 9.6% (1996) to 25% (1998). Most farmers believed yields of Bt hybrids were either similar to or greater than the yields of non-Bt hybrids. Minnesota farmers perceived the greatest yield advantages. Farmers are becoming more aware of insect resistance management guidelines; however, they also clearly show preferences for having the flexibility to use different spatial plantings of Bt and non-Bt corn. Finally, after having planted Bt corn and obtained excellent control of European corn borer, most farmers believed that this insect had been causing more yield loss than they previously had suspected in their non-Bt corn. The data represented here provide an historical foundation for how transgenic Bt corn was used by farmers during the first 3 yr of commercial availability, their initial perceptions on the performance of this technology, and their attitudes regarding management of the European corn borer.     

转Bt基因玉米对甜菜夜蛾幼虫存活和发育的影响

在室内测定了2种转Cry1Ab基因的Bt玉米MON810和Bt11不同组织对甜菜夜蛾 Spodoptera exigua （Hübner）初孵幼虫以及心叶对4龄幼虫存活和发育的影响,在田间比较了甜菜夜蛾幼虫取食Bt 和非Bt玉米雌穗的存活和为害情况。结果表明,转Cry1Ab基因的Bt玉米的不同组织对甜菜夜蛾初孵幼虫都具有明显的杀虫活性,取食Bt玉米心叶、苞叶、籽粒时甜菜夜蛾均在幼虫期死亡; 取食MON810和Bt11雄穗的初孵幼虫化蛹率分别为5.2%和2.1%,羽化率为2.1%和1.0%;取食MON810和Bt11花丝的初孵幼虫化蛹率分别为1.0%和2.1%,但不能羽化。4龄幼虫取食MON810玉米心叶的化蛹率与对照差异不显著,而取食Bt11的化蛹率与对照差异显著; 取食两种Bt玉米心叶的４龄幼虫化蛹后的雌、雄蛹重和羽化率与对照组差异显著,但蛹期和平均单雌产卵量差异不显著,虽然对照组羽化的成虫平均产卵量高于Bt玉米组。田间接种初孵幼虫10 天后的调查结果表明,在MON810和Bt11玉米花丝上幼虫存活率分别为1.3%和0.3%, 而对照组分别为12.9%和16.2%;MON810和Bt11玉米雌穗被害率分别为18.3%和5.0%,而对照组分别为93.3%和95.0%,均显著低于对照组。     

Performance of transgenic corn hybrids in Missouri for insect control and yield

The efficacy of Bacillus thuringiensis-transformed corn (Zea mays L.) hybrids compared with comparable nontransformed corn hybrids for controlling first- and second-generation European corn borer, Ostrinia nubilalis (Hübner), and second-generation southwestern corn borer, Diatraea grandiosella Dyar, was determined. Yield comparisons were obtained from the same plots of corn hybrids. Both generations of European and the second-generation of southwestern corn borer were effectively controlled, but the Bt hybrids varied in degree of control. Hybrids from Ciba Seeds, DEKALB, and Mycogen had more European corn borer tunneling than those from Novartis or Cargill, and this was generally ascribed to different transgenic events. The Bt-transformed hybrids had virtually no leaf-feeding damage and less tunneling than the non-Bt corn hybrids. Some Bt corn hybrids had no tunneling, whereas other Bt hybrids had a small amount of tunneling. All of the non-Bt hybrids had significant leaf-feeding damage and stalk tunneling from both insects. Only three live European corn borer larvae (stunted) were found in the Bt corn hybrids while splitting stalks to assess tunnel length. When insect damage was significant, and in some evaluations where damage was not significant, differences in yields among hybrids were observed. No significant insect population differences were observed for five genera of beneficial insects for Bt versus non-Bt corn hybrids. Corn hybrids that have been transformed with the Bt gene provide an effective means of control for corn borers and efforts to reduce the likelihood of development of borer resistance are warranted.     

粘虫高龄幼虫对转Bt基因玉米的消化和利用

在室内用重量法研究了粘虫Mythimna separata (Walker)高龄幼虫对转Bt基因玉米MON810和Bt11叶片的消化和利用,以明确Bt玉米对暴食期幼虫取食的影响。结果表明,在连续测定的5天中,取食MON810和Bt11两种Bt玉米叶片时,幼虫存活率和取食量均显著低于各自的对照组幼虫,取食Bt玉米叶片的幼虫体重呈下降趋势,第3 天时分别比第2 天减少12.2 mg 和7.4 mg,而取食对照玉米叶片时的幼虫日增重显著的高于处理组的幼虫,第3 天的日增重分别为100.4 mg 和119.9 mg。取食Bt玉米叶片的幼虫对食物的转化率(ECI和ECD)均为负值,在最初4 天的ECI和ECD都显著低于对照组幼虫,但取食两种非Bt玉米叶片的幼虫的近似消化率(AD) 随取食时间的延长而逐渐下降,取食第5 天分别为20.6 %和15.1 %;而取食MON810和Bt11叶片时幼虫的AD均显著地高于对照组幼虫。     

Effects of transgenic bt corn on growth and development of the stalk borer Papaipema nebris (Lepidoptera: Noctuidae)

This study assessed the efficacy of two different genetic events, event Bt 11 (CrylAb) and event CBH351 (Cry 9C), in Bt corn against two instar classes of the stalk borer Papaipema nebris across three different plant stages (V1, V3, and V5) of corn, Zea mays. Class A includes instars 1 and 2, and class B includes instars 3 and 4. Stalk borer response and development over time were measured, and the data from 1999 and 2000 show that the Bt corn does have some effect on the feeding and development of P. nebris. Injury to the corn plant was reduced, although not eliminated. Stalk borer larvae caused significantly (P = 0.0001) more injury to the non-Bt plants than to the Bt plants over time. Growth and development of the larvae were slowed and mortality was higher for Bt corn than for non-Bt corn. These data suggest that planting Bt corn may benefit growers by reducing, but not eliminating, stalk borer infestations and subsequent plant injury.     

转Bt基因抗虫玉米对玉米蚜种群增长的影响

在人工气候箱条件下研究了玉米蚜（Rhopalosiphum maidis Fitch）取食表达cry1Ab杀虫蛋白Bt抗虫玉米的实验种群生命表.结果表明：两种不同Bt玉米杂交种DK647BTY （MON810转化事件）和NX4777（Bt11转化事件）对玉米蚜的生长、发育、繁殖和存活均无明显的不利影响,玉米蚜在DK647BTY和NX4777两种Bt玉米品种上的内禀增长率rm、周限增长率λ和种群净增殖率R0与各自对照之间没有显著差异;玉米蚜有翅蚜比率、各龄若虫的死亡率在Bt玉米和对照以及不同品种之间没有明显差异;Bt玉米对玉米蚜的寿命和繁殖历期也没有明显差异.表明表达cry1Ab杀虫蛋白的Bt玉米对玉米蚜的生长发育和繁殖没有明显影响.     

Corn pith weevil,Geraeus penicillus (Coleoptera: Curculionidae), in transgenic,insect‐resistant and conventional field corn hybrids

The corn pith weevil Geraeus penicillus (Herbst) is occasionally found boring in corn stalks throughout the eastern and Midwestern United States. Injury caused by G. penicillus is not typically economical, but may be confused with that of the European corn borer Ostrinia nubilalis (Hubner), an important economic pest throughout the United States. During efforts to assess European corn borer infestations, we discovered G. penicillus in field corn in south‐eastern Pennsylvania, including hybrids genetically modified (i.e. Bt hybrids) to control European corn borer among other herbivore species. Our analysis across sites indicated that tunnels of G. penicillus were significantly more abundant in transgenic Bt hybrids than non‐Bt hybrids, but comparisons of Bt hybrids and their near isolines revealed mostly similar numbers of G. penicillus tunnels, suggesting other hybrid features might be affecting the distribution of G. penicillus. Tunnels of G. penicillus were equally distributed among the three transgenic trait packages represented in our study. In plants where we found G. penicillus, tunnels were more abundant in stalks free of European corn borer damage. Our report appears to be the first to note G. penicillus feeding in Bt corn hybrids. These findings are notable because they document insect damage in Bt hybrids that may be mistaken for European corn borer damage and may provide evidence of an insect herbivore proliferating following a mild winter or possibly even moving into competitor‐free space.     

Evaluation of transgenic Bacillus thuringiensis corn hybrids against Cry1Ab-susceptible and -resistant sugarcane borer (Lepidoptera: Crambidae)

A Louisiana strain of the sugarcane borer, Diatraea saccharalis (F.) (Lepidoptera: Crambidae), was selected for resistance to the CrylAb protein of Bacillus thuringiensis (Bt) by using an F2 screening procedure. Survival of Bt-resistant, -susceptible, and -heterozygous genotypes of sugarcane borer was evaluated on vegetative and reproductive stages of five non-Bt and seven Bt field corn, Zea mays L., hybrids in a greenhouse study. Larval survival was recorded 21 d after infestation of neonates on potted plants. Larval survival across the three sugarcane borer genotypes and five non-Bt corn hybrids after 21 d ranged from 23.6 +/- 5.2% (mean +/- SEM) to 57.5 +/- 5.2%. Mean survival of Cry1Ab-resistant larvae on vegetative and reproductive plant stages was 12 and 21%, respectively. During the vegetative stages, all seven Bt corn hybrids were highly efficacious against Cry1Ab-susceptible and -heterozygous genotypes of sugarcane borer, with a larval survival rate of <2% for the Bt-susceptible genotype and < or =5% for the heterozygotes. However, 8-18% of the heterozygous genotype survived on reproductive stage plants for four of the seven Bt corn hybrids tested. The variation in performance of Bt corn cultivars at vegetative and reproductive growth stages against Cry1Ab resistant sugarcane borer suggests differential seasonal expression that may hasten resistance in the field. Bt corn hybrids expressing a "high dose" for European corn borer, Ostrinia nubilalis (Hübner), may not produce a sufficient high dose for the sugarcane borer.     

Use of transgenic Bacillus thuringiensis Berliner corn hybrids to determine the direct economic impact of the European corn borer (Lepidoptera: Crambidae) on field corn in eastern Canada

Transgenic corn expressing Bacillus thuringiensis Berliner (Bt corn) (Maximizer and Yieldgard hybrids, Novartis Seeds), non-Bt isolines and high-performance (check) hybrids were evaluated for European corn borer, Ostrinia nubilalis (Hübner), damage and grain yield in commercial strip plots across Ontario in 1996 and 1997. Bt corn hybrids reduced stalk tunneling damage by 88-100%. In 1996, minimal damage was found in locations where only one generation of European corn borer occurred per year. Bt corn proved its greatest potential for reducing the number and length of cavities below the primary ear in locations where two generations of European corn borer were present. A yield response to using Bt hybrids only occurred when levels of tunneling damage exceeded 6 cm in length. European corn borer infestations resulted in a 6 and 2.4% reduction in yield for 1996 and 1997, respectively, when Bt hybrids were compared with their non-Bt isolines. A linear relationship was found between tunnel length per plant in centimeters (x) and yield protection (%) obtained from using Bt corn (y) (y = 1.02 + 0.005x, r2 = 0.7217). At a premium of $34.58 Canadian (CDN) perhectare for Bt corn seed, an infestation of at least 6 cm of corn borer tunneling per plant was required to break even at a market price for corn of $2.50 per bushel CDN. During the period of study, low infestations (0-2 cm) of European corn borer occurred at 25% of the locations assessed, moderate infestations (4-6 cm) occurred at 42% of the locations, and high infestations (>6 cm) occurred at 33% of the locations. At a corn price of $3.00 per bushel CDN and seed premiums of $34.58 per hectare CDN, 5 cm of tunneling was required for a return on investment in Bt seed, comprising only 55% of the growers in the study. With infestations of more than 6 cm of tunneling occurring only 33% of the time, a return on seed investment would be realized in only one of three growing seasons. At a seed premium of $24.70 per hectare CDN per year, at least $74 per hectare CDN in the year of infestation would be required to make up for the two years of no return. In this study, a $74 per hectare CDN return at a corn price of $9.26 per hectare CDN with >16 cm of tunneling damage would have occurred only 7.3% of the time.     

Dispersal capacity in the Mediterranean corn borer, Sesamia nonagrioides

Corn (Zea mays L.) borers are the primary target of Bacillus thuringiensis Berliner (Bt) transgenic maize. Management of corn borer resistance to Bt requires information on larval and adult dispersal capacities, a feature that is particularly unknown in Sesamia nonagrioides Lefèbvre (Lepidoptera: Noctuidae), the most damaging corn borer in Spain. Larval dispersal was studied over a 3 year period by infesting plants with egg masses and dissecting the neighbouring plants 7, 14, and 32 days later to measure larval dispersal at several ages. The number and age of larvae were recorded in the dissected plants. Only mature larvae dispersed in significant numbers; they moved at least to rows adjacent to those containing the infested plant, and down the row five plants. The percentage of larvae that dispersed from the infested plant was density‐dependent. Adult dispersal was studied with directional light and pheromone uni‐traps over 5 and 3 year periods, respectively. Directional light traps were placed in the margins between Bt and non‐Bt maize fields, half oriented towards each of the two kinds of maize field. Pheromone traps were placed in the Bt and non‐Bt fields at increasing distances (0–100 m) from the border. The numbers of males and females caught in directional light traps were not different in traps oriented towards Bt or non‐Bt fields, but the number of males caught in the third flight in Bt fields was lower than in non‐Bt fields. These results suggest that males from adjacent Bt and non‐Bt fields mate indiscriminately with females emerging in any of the two kinds of maize fields. However, male movement in the third flight may not be sufficient to randomly distribute males between the two fields.     

Cross-pollination of nontransgenic corn ears with transgenic Bt corn: efficacy against lepidopteran pests and implications for resistance management

The efficacy of nontransgenic sweet corn, Zea mays L., hybrids cross-pollinated by Bacillus thuringiensis (Bt) sweet corn hybrids expressing Cry1Ab toxin was evaluated in both field and laboratory studies in Minnesota in 2000. Non-Bt and Bt hybrids (maternal plants) were cross-pollinated with pollen from both non-Bt and Bt hybrids (paternal plants) to create four crosses. Subsequent crosses were evaluated for efficacy in the field against European corn borer, Ostrinia nubilalis (Hübner), and corn earworm, Helicoverpa zea (Boddie), and in laboratory bioassays against O. nubilalis. Field studies indicated that crosses with maternal Bt plants led to low levels of survival for both O. nubilalis and H. zea compared with the non-Bt x non-Bt cross. However, the cross between non-Bt ears and Bt pollen led to survival rates of 43 and 63% for O. nubilalis and H. zea larvae, respectively. This intermediate level of survival also was reflected in the number of kernels damaged. Laboratory bioassays for O. nubilalis, further confirmed field results with larval survival on kernels from the cross between non-Bt ears and Bt pollen reaching 60% compared with non-Bt crossed with non-Bt. These results suggest that non-Bt refuge plants, when planted in proximity to Bt plants, and cross-pollinated, can result in sublethal exposure of O. nubilalis and H. zea larvae to Bt and may undermine the high-dose/refuge resistance management strategy for corn hybrids expressing Cry1Ab.