Evaluation of European corn borer Ostrinia nubilalis (Lepidoptera: Crambidae) larval movement and survival in structured and seed blend refuge plantings

Blending Bacillus thuringiensis (Bt) corn and non-Bt corn seeds in the same bag is a convenient and easier compliance solution to satisfy refuge regulations. However, there has been considerable debate and concern about larval movement of the target insects in a seed blend because of its potential effect on the development of resistance. Reported here are studies to determine the larval movement of European corn borers, Ostrinia nubilalis (Hübner), in a seed blend compared to a structured refuge and their effects on survival, feeding injury and larval fitness. Also evaluated is the relative performance of a purple-seeded corn hybrid as a surrogate host plant for tracking gene flow. Seed blend and structured refuge arrangements of Agrisure GT/CB/LL Bt corn expressing Cry1Ab and isoline plants were evaluated at two locations in 2013 and 2014 by manually infesting refuge plants with first and second generation corn borer larvae. The majority of leaf and stalk injury was recorded on the refuge plant, which amounted to 66–83% of the total tunnel length. In both refuge arrangements, plant injury and the number of larvae recovered significantly decreased on neighboring plants with increasing distance away from the refuge plant in the same row. The relative decline in injury was much more pronounced in the seed blend, with neighbor Bt plants experiencing very minor tunneling or no injury at all due to the high dose trait. Eighty-nine percent fewer live larvae were also found on neighbor Bt plants in the seed blend compared to those found in the structured refuge plots. Larvae that moved to Bt plants as well as older instars that were manually placed on Bt plants failed to complete development. Comparison of the number of tunnels and larvae recovered per refuge plant indicated that the seed blend may not produce as many susceptible individuals as those produced in a structured refuge arrangement. Although the purple-seeded hybrid showed lower levels of ear and stalk injury, it could serve as an appropriate surrogate refuge plant to track gene flow.     

Movement and survival of corn rootworm in seed mixtures of SmartStax® insect-protected corn

SmartStax^® insect-protected corn (Zea mays L.) contains genes for six Bacillus thuringiensis (Bt) proteins controlling both lepidopteran pests and the corn rootworm complex (Diabrotica spp.). The properties of SmartStax, particularly the multiple effective modes of action (i.e., each Bt protein provides a high level of control of the target pests with a low probability of cross-resistance among the proteins), have provided the opportunity to add to previously approved structured refuge options by combining the non-Bt refuge seed with SmartStax seed in a seed mix. Seed mixes ensure that a refuge is present in every Bt field, remove concerns about grower compliance with refuge requirements, and provide grower convenience. However, seed mixes could increase the likelihood that larval insects move between Bt and non-Bt plants and vice versa. Assessing the insect resistance management (IRM) value of a seed mix refuge requires an assessment of the amount of larval movement, and the consequences of that movement, for the key target pests. The studies here present such data for control of corn rootworm by SmartStax corn, which contains the rootworm-active protein Cry3Bb1 and the binary protein Cry34Ab1/Cry35Ab1. In a growth chamber experiment, SmartStax was most effective against first instars and significantly effective against second instars, but did not control third instars. In a field study of movement from a heavily infested non-Bt plant onto surrounding plants, a larger percentage of insects successfully dispersed from the infested plant when the surrounding plants were non-Bt plants than when they were SmartStax plants. A paired-plant study showed that few larvae migrated from infested SmartStax plants and survived on nearby non-Bt plants; larvae that migrated from infested non-Bt plants had low survival if the adjacent plants were SmartStax. Replicated field studies of plant-to-plant movement indicated that the non-Bt plants in a 5% or 10% seed mix consistently supported large populations of susceptible insects and represented a productive refuge, whereas the SmartStax plants had few or no survivors. The timing of emergence from seed mix plots containing 5% or 10% non-Bt plants was more similar to that of the non-Bt plots than that of the SmartStax plots. Thus, the available growth chamber and field data indicate that a seed mix of 5% or more will provide an effective refuge for corn rootworm in SmartStax corn.     

When and where a seed mix refuge makes sense for managing insect resistance to Bt plants

Planting of a separate structured refuge for Bt crops as part of an insect resistance management (IRM) strategy to delay resistance evolution is the most common method of refuge deployment but this strategy depends on growers planting a refuge. A seed mix refuge interspersed with a pyramided Bt product is an alternative strategy that addresses the risk of growers not planting a refuge. However, concerns exist regarding how larval movement between Bt and non-Bt plants might influence resistance evolution in a seed mix field. To understand when seed mixes are an appropriate IRM strategy, a deterministic model run probabilistically was used to examine the evolution of Bt resistance in seed mix and structured refuges under varying levels of Bt efficacy, pest fitness, refuge size, larval movement, movement penalty and grower compliance. Results from modeling simulations show that the addition of a second and third Bt toxin can delay resistance evolution longer than a single toxin, making a seed mix refuge strategy a viable option where refuge compliance is a concern. In seed mixes, resistance was shown to evolve faster compared to a responsibly implemented structured refuge and evolved fastest in seed mixes when larval movement rates were high. However, when mortality from larval movement was included in model simulations, the selection pressure from Bt was reduced and two or three Bt-pyramids with a 5% seed mix refuge were at least as durable as the same products with a 5% structured refuge, depending upon refuge compliance. These simulations show that, across a range of conditions, seed mix refugia provide an effective alternative IRM tactic for delaying resistance evolution. Under some conditions use of seed mix refugia may be a superior IRM tactic leading to longer delays to resistance, and greater durability, compared to structured refugia and is a risk adverse tactic in situations when no refuge is planted.     

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

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 ≈70%. Rearing larvae on 5, 50, 500, and 5000 μg 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.     

Southwestern corn borer damage and aflatoxin accumulation in conventional and transgenic corn hybrids

Southwestern corn borer (Diatraea grandiosella Dyar) is a major pest of corn (Zea mays L.) in the southern United States. In addition to the direct yield losses caused by southwestern corn borer, larval feeding on developing ears provides a site for fungi to enter the ear. Aspergillus flavus Link: Fries infection and the subsequent accumulation of aflatoxin in corn grain are major limitations to profitable corn production in the southern United States. This investigation was conducted to determine the effectiveness of transgenic corn hybrids expressing the δ-endotoxin insecticidal (CryIAb) proteins isolated from Bacillus thuringiensis (Bt) in reducing southwestern corn borer damage and aflatoxin accumulation. Ear damage and aflatoxin accumulation were compared among 10 pairs of conventional nonBt and transgenic Bt corn hybrids following infestation with southwestern corn borer and inoculation with A. flavus using kernel-wounding and nonwounding techniques. Both nonBt and Bt hybrids exhibited high levels of aflatoxin accumulation when inoculated with a kernel-wounding technique. When inoculated with a non-wounding technique and infested with southwestern corn borer, aflatoxin accumulation was significantly higher in nonBt than Bt hybrids. Aflatoxin accumulation was also significantly higher for nonBt hybrids inoculated with A. flavus and infested with southwestern corn borer than for hybrids that were only inoculated with A. flavus. Southwestern corn borer larval establishment was significantly higher on nonBt hybrids than on Bt hybrids. Larval survival was extremely low on the Bt hybrids. The results of this investigation indicate that these Bt hybrids should be effective in reducing aflatoxin contamination in areas where high southwestern corn borer infestations occur. The reduced levels of aflatoxin accumulation associated with Bt hybrids are likely a consequence of reduced insect damage rather than resistance to A. flavus infection or aflatoxin accumulation per se.     

Efficacy of SmartStax® insect-protected corn hybrids against corn rootworm: The value of pyramiding the Cry3Bb1 and Cry34/35Ab1 proteins

Monsanto Company and Dow AgroSciences have used conventional breeding techniques to develop the combined-trait corn (Zea mays L.) product MON 89034 × TC1507 × MON 88017 × DAS-59122-7 (“SmartStax^® corn”) that confers insect resistance against key lepidopteran pests and the corn rootworm complex (Diabrotica spp.), as well as herbicide tolerance. This product contains the Cry3Bb1, Cry34Ab1, and Cry35Ab1 proteins for corn rootworm (CRW) control. Replicated field trials were conducted in 2006 and 2007 to test the efficacy of Cry3Bb1, Cry34Ab1/Cry35Ab1, and SmartStax under both natural and controlled infestations. In both years, root damage and adult beetle emergence were significantly less for Bt-containing hybrids than for non-Bt controls. In trials with heavy insect pressure, adult beetle emergence (a measure of larval control) was significantly reduced for SmartStax hybrids compared with hybrids with the individual traits and with non-Bt controls. Similarly, strip-plot testing in 2012 in grower fields with high levels of root injury to non-Bt hybrids showed significantly lower feeding on SmartStax than on the non-Bt control or on either single-trait product. In grower fields where single-trait Cry3Bb1 products incurred heavy CRW damage in 2011, SmartStax provided consistent protection against CRW in 2012. The combination of these insecticidal proteins in a single plant provides better rootworm control than current single-trait Bt corn products and represents an effective approach for corn rootworm resistance management.     

Susceptibility of Cry1Ab-susceptible and -resistant sugarcane borer to transgenic corn plants containing single or pyramided Bacillus thuringiensis genes

Multiple independent trials were conducted to evaluate the performance of Cry1Ab-susceptible (Cry1Ab-SS), -heterozygous (Cry1Ab-RS), and -resistant (Cry1Ab-RR) genotypes of the sugarcane borer, Diatraea saccharalis (F.), on eight commercial hybrids and six experimental corn lines. The commercial varieties included two non-Bt and six Bt corn hybrids that expressed a single Bt protein (either Cry1Ab or Cry1F) targeting above-ground lepidopteran pests. The six experimental lines consisted of two non-Bt and four Bt corn lines, two expressing just the Cry1Ab protein and two containing the pyramided-genes Cry1A.105 and Cry2Ab2 (event MON 89034). Larval mortality on non-Bt corn leaf tissue ranged from 6 to 45% after 12 d across insect genotypes. The 12 d mortality of Cry1Ab-SS on leaf tissue of commercial Cry1Ab or Cry1F corn was 96-100%, whereas it was 80-96% for Cry1Ab-RS and 68-78% for Cry1Ab-RR. On intact plants, 39-64% of larvae survived on non-Bt corn plants after 21-25 d. Larval survivorship on intact plants of commercial Cry1Ab or Cry1F corn was 0-8.1% for Cry1Ab-SS, 1.3-34% for Cry1Ab-RS, and 19-51% for Cry1Ab-RR. Larvae of Cry1Ab-RR and -RS also caused significant plant injury to most of the commercial Bt corn hybrids, especially to the Cry1Ab corn. Cry1Ab resistance in D. saccharalis was incompletely dominant on commercial Bt corn hybrids. However, both experimental lines with pyramided genes of Cry1A.105 and Cry2Ab2 provided complete control of all three insect genotypes in both leaf tissue and intact plant tests. Results of this study suggest that MON 89034 should offer a means for Bt resistance management in D. saccharalis.     

Economics of long-term IPM for western corn rootworm

We used a published biological model and published economic algorithm to evaluate western corn rootworm (Diabrotica virgifera virgifera) IPM for growers over a 15 year period beginning after significant adoption of insecticidal corn (Zea mays). The primary focus of our analysis was the economic evaluation (grower profit) of transgenic insecticidal corn expressing Cry34/35Ab1 as event 59122 and its refuge planted continuously year after year (continuous corn). We chose the reference scenario for economic comparison to be the use of soil insecticides on continuous, conventional corn. The model simulated the evolution of rootworm resistance to transgenic insecticidal (Bt) corn; but did not simulate resistance to soil insecticides. We evaluated refuge sizes of 5–50% for single-trait Bt corn and 5–20% for pyramided Bt corn with two traits targeting western corn rootworm. We considered the role of block and blended (seed mixture) refuges for insect resistance management (IRM). Results demonstrated that, for pyramided Bt corn, block refuges planted in the same location within a field year after year gave the greatest overall profit for grower. If growers relocated their block refuge annually (which is the most common practice), then a 5% blended refuge gave the greatest return. For single-trait Bt corn, 10–20% blended refuges gave greater economic return compared to block refuges ranging from 5% to 50%. Single-trait Bt corn with 5–20% block refuge (with no insecticide) was superior to soil insecticide use alone in all cornfields.     

Resistance of Bt-maize (MON810) against the stem borers Busseola fusca (Fuller) and Chilo partellus (Swinhoe) and its yield performance in Kenya

A study was conducted to assess the performance of maize hybrids with Bt event MON810 (Bt-hybrids) against the maize stem borer Busseola fusca (Fuller) in a biosafety greenhouse (BGH) and against the spotted stem borer Chilo partellus (Swinhoe) under confined field trials (CFT) in Kenya for three seasons during 2013–2014. The study comprised 14 non-commercialized hybrids (seven pairs of near-isogenic Bt and non-Bt hybrids) and four non-Bt commercial hybrids. Each plant was artificially infested twice with 10 first instar larvae. In CFT, plants were infested with C. partellus 14 and 24 days after planting; in BGH, plants were infested with B. fusca 21 and 31 days after planting. In CFT, the seven Bt hybrids significantly differed from their non-Bt counterparts for leaf damage, number of exit holes, percent tunnel length, and grain yield. When averaged over three seasons, Bt-hybrids gave the highest grain yield (9.7 t ha⁻¹), followed by non-Bt hybrids (6.9 t ha⁻¹) and commercial checks (6 t ha⁻¹). Bt-hybrids had the least number of exit holes and percent tunnel length in all the seasons as compared to the non-Bt hybrids and commercial checks. In BGH trials, Bt-hybrids consistently suffered less leaf damage than their non-Bt near isolines. The study demonstrated that MON810 was effective in controlling B. fusca and C. partellus. Bt-maize, therefore, has great potential to reduce the risk of maize grain losses in Africa due to stem borers, and will enable the smallholder farmers to produce high-quality grain with increased yield, reduced insecticide inputs, and improved food security.     

Occurrence,distribution, and ear damage of Helicoverpa zea (Lepidoptera: Noctuidae) in mixed plantings of non-Bt and Bt corn containing Genuity® SmartStax™ traits

Protein contamination on refuge kernels due to cross-pollination from Bt corn to non-Bt corn ears is a major concern in the use of a seed mixture refuge strategy (“RIB”) for resistance management of ear-feeding pests. In this study, occurrence, distribution, and ear damage of the corn earworm, Helicoverpa zea (Boddie), were evaluated in three planting patterns of non-Bt and Bt corn plants containing Genuity^® SmartStax™ traits. The three planting patterns were 1) pure stands of 27 Bt plants; 2) pure stands of 27 non-Bt plants; and 3) one non-Bt plant in the center surrounded by 26 Bt plants. A total of six trials were conducted in open field conditions with natural infestations in 2011 and 2012. Egg populations of H. zea were distributed randomly or uniformly, and the number of eggs laid was similar between Bt and non-Bt corn ears regardless of the planting patterns, suggesting that females of H. zea have no egg-laying preference between Bt and non-Bt plants. Bt corn plants containing Genuity^® SmartStax™ traits were equally effective in the control of H. zea in pure stands of Bt corn and “RIB” plantings. Occurrence of larvae and ear damage on Bt corn were significantly lower than on non-Bt plants and there were no significant differences between pure stands of Bt and “RIB” plantings across all trials. However, the limited numbers of live larvae in the pure stands of Bt plants were distributed non-randomly, suggesting a possibility of uneven expression of Bt proteins or elevated larval movement in the pure stands of Bt plants. Larval occurrence (3rd–5th instars) and ear damage on the refuge ears in “RIB” plantings were similar to or greater than found on ears of pure stands of non-Bt plants. However, more studies are needed to understand the effect of pollen movement on the full life cycle of H. zea before a final conclusion on the refuge function of RIB planting can be made.     

Influence of transgenic hybrid rice expressing a fused gene derived from cry1Ab and cry1Ac on primary insect pests and rice yield

Although dozens of transgenic Bacillus thuringiensis (Bt) rice lines have been developed, none of them has been released to farmers. Under field conditions, we evaluated the influence of a hybrid Bt rice on the primary rice insect pests and rice yield in 2005 and 2006. Four treatments were evaluated, including Bt and non-Bt rice treated with insecticides when necessary, and unprotected Bt and non-Bt rice. Unprotected Bt rice exhibited stable and high control of the three primary lepidopteran pests, Chilo suppressalis Walker, Tryporyza incertulas Walker and Cnaphalocrocis edinalis Güenée. Under unprotected conditions, larval densities of these three pests in Bt plots decreased by 87.5–100% compared to those in non-Bt plots, and percentages of damaged stems and leaves remained less than 0.6% during the entire rice growing season. In early rice growth stages, populations of two important planthoppers, Nilaparvata lugens Stål and Sogatella furcifera Hovarth, were significantly affected only by protection level (protected vs unprotected). However, in late rice growth stages (filling and maturing), densities of planthoppers were significantly affected both by protection level and by rice type (Bt vs non-Bt), and densities of N. lugens were significantly higher in Bt plots than in non-Bt plots under unprotected conditions. Pesticide sprays were reduced by 60 and 50% in protected Bt vs protected non-Bt plots in 2005 and 2006, respectively. Yield of unprotected Bt rice increased by 60–65% compared to unprotected non-Bt rice, but decreased by 28–36% compared to protected Bt rice. These results show that Bt rice increased yield greatly, but still required pesticide sprays to avoid losses caused by non-target insect pests.     

转Bt基因玉米对根际土壤细菌遗传多样性的影响

连续两年在田间自然状态下,采用PCR-DGGE方法研究6个不同生育期种植转Bt基因玉米MON810对根际土壤细菌群落遗传多样性的影响。结果表明,2008年,同一生育期内转Bt基因玉米与非Bt基因玉米根际土壤细菌16S rDNA DGGE指纹图谱相似,细菌群落的遗传物质组成差异不显著,仅在抽丝期和乳熟期Bt玉米出现两条差异条带。2009年,苗期、拔节期、喇叭口期、抽雄期4个生育期两种玉米出现差异条带,抽丝期和乳熟期无差异条带。对DGGE图谱条带进行克隆测序,部分图谱中的条带所代表的DNA序列相同,最终得到51条DNA序列,共涉及主要细菌门类8个,典型差异条带分别属于芽单胞菌门（Gemmatimonadetes）、变形菌门（Proteobacteria）、厚壁菌门（Firmicutes）。总体来看,与不同品种相比,不同生育期、不同年份间细菌群落差异更为显著,Bt蛋白的积累对根际细菌群落变化有一定影响。     

棉铃虫对转基因抗虫玉米的行为反应

棉铃虫Helicoverpa armigera(Hübner)是Bt玉米的重要靶标害虫之一。规避行为可以减少害虫与Bt玉米的接触而降低生理抗性选择压力。通过比较棉铃虫对Bt玉米与常规玉米的行为反应评估棉铃虫对Bt玉米的行为规避能力,产卵选择结果显示,棉铃虫成虫在Bt玉米上的落卵量显著低于常规玉米;无选择条件下,仍然出现Bt玉米上的落卵量显著低于常规玉米的现象。幼虫实验结果显示,棉铃虫初孵幼虫在Bt玉米植株上的居留时间显著短于在常规玉米上的居留时间。研究结果证明,棉铃虫对Bt玉米具有一定的行为规避能力。     

Interaction between host plant resistance and biological activity of Bacillus thuringiensis in managing the pod borer Helicoverpa armigera in chickpea

The legume pod borer Helicoverpa armigera (Hubner) (Lepidoptera: Noctuidae) has developed high levels of resistance to conventional insecticides, and therefore, efforts are being made to develop transgenic chickpea expressing toxin genes from the bacterium Bacillus thuringiensis (Bt) for controlling this pest. However, there is an apprehension that acid exudates in chickpea might interfere with the biological activity of Bt. Therefore, we studied the biological activity of Bt (Biolep^R) on four chickpea genotypes with different levels of resistance to H. armigera under field conditions, and by incorporating lyophilized leaf and pod tissue into the artificial diet with and without Bt. The pH of the acid exudates varied from 2.1 to 2.9, and malic and oxalic acids were the major components of the acid exudates in different chickpea genotypes. There was no survival of H. armigera larvae in chickpea plants sprayed with 0.1, 0.2 and 0.5% Bt. There was a significant reduction in larval survival, larval and pupal weights and fecundity, and prolongation of larval and pupal periods in chickpea plots sprayed with Bt (0.05%) as compared to the unsprayed plots. Biological activity of Bt was lower on artificial diets with leaf or pod powder of chickpea genotypes, which might be because of a low intake of Bt toxins due to the antifeedant effects of acid exudates in the chickpea or reduction in biological activity of Bt due to the interaction of biochemical constituents in chickpea with the Bt toxins. Larval survival, larval and pupal weights, pupation and adult emergence were significantly lower on diets with leaf or pod powder of the H. armigera-resistant genotypes than on the susceptible check. Chickpea genotypes with resistance to H. armigera acted in concert with Bt to cause adverse effects on the survival and development of this insect. The results suggested that development of transgenic chickpeas expressing toxin genes form Bt will be quite effective for controlling of the pod borer, H. armigera.     

Prevalence of Helicoverpa zea (Lepidoptera: Noctuidae) on late season volunteer corn in Mississippi: Implications on Bt resistance management

The southern United States has a long growing period between corn, Zea mays L., harvest and first winter frost, so volunteer corn which germinates after harvest has a growing period sufficient for corn earworm, Helicoverpa zea (Boddie) and fall armyworm, Spodoptera frugiperda (J. E. Smith) to feed on these plants. However, lower air temperatures can limit larval development on late season volunteer corn and thereby successful pupation. Here we explore the suitability of late season volunteer corn for larval development and the potential contribution of H. zea larvae to the overwintering population. Our survey revealed the occurrence of volunteer corn in high densities, with monthly mean densities ranging from 56,000 to 143,000 plants ha⁻¹. H. zea larvae were found feeding on both vegetative and reproductive stage plants while S. frugiperda were only found on vegetative stage plants. An analysis of H. zea growing degree day (GDD) accumulations based on Mississippi weather data from 1980 to 2010 revealed that sufficient GDD to reach prepupation would always be accumulated before first frost if oviposition occurred by 9 September, with the probability of successful pupation decreasing rapidly thereafter. However, most of the H. zea larvae were oviposited after this, and could not reach pupation. Because S. frugiperda cannot overwinter in Mississippi, their ability to pupate was not examined. Low suitability of whorl stage corn for H. zea development coupled with low larval densities during this stage effectively diminish the number of larvae that complete development on late season volunteer transgenic corn expressing genes from the soil bacterium, Bacillus thuringiensis (Bt). This limits the Bt resistance risk posed by larvae developing on late season volunteer corn in all but the most southern locations in the US.     

Recruitment of adult Colorado potato beetles inBt-transgenic potato fields

A refuge of conventional potato plants adjacent to Colorado potato beetle (CPB),Leptinotarsa decemlineata (Say)-resistant,Bt-potato (transgenic) plants may reduce opportunity for the development of tolerance to the resistant plants. The refuge strategy was developed on the basis of data available for CPB recruitment in conventional potato fields. This study was undertaken to provide information on CPB recruitment inBt-transgenic potato fields. A marking experiment was conducted over the 2000 and 2001 crop seasons to determine the relative contributions of beetle populations from fields 10 m, 175 to 300 m, and 1200 to 1280 m distant to the pattern of CPB recruitment (immigration) in aBt-potato field. Season-long CPB recruitment in theBt-potato field decreased with source distance in a manner similar to that previously reported in conventional potato fields. Although marked beetles from the 10-m field plot contributed more to the recruitment than the more distant fields, they contributed only 3.3% and 6.6% of the total beetle sightings in theBt-potato field. Therefore, results suggest that a larger acreage of conventional potato fields at some distance from the resistant crop could replace the adjacent designated refuge. Seasonally, beetles from the overwintering sites provided the first recruits to the resistant field. As their contribution declined, the overwintered beetles from the refuge were recruited until the beginning of the summer population. These results suggest that locating the resistant fields close to active overwintering sites will improve the probability that the refuge strategy will be effective by ensuring the presence of recruits at the very beginning of the crop season. The substantially lower recruitment level obtained for summer than for overwintered CPB in resistant and conventional potato fields highlights the need to reconsider the applicability of the refuge strategy for the summer population.     

Larval survival and plant injury of Cry1F-susceptible, -resistant,and -heterozygous fall armyworm (Lepidoptera: Noctuidae) on non-Bt and Bt corn containing single or pyramided genes

The fall armyworm, Spodoptera frugiperda (J.E. Smith), is a major target of transgenic corn, Zea mays L., expressing Bacillus thuringiensis (Bt) proteins in both North and South America. A highly Cry1F-resistant strain of S. frugiperda was established from a field collection in Puerto Rico in 2011. In this study, three greenhouse trials were conducted to evaluate larval survival and leaf injury of Cry1F-susceptible, -resistant, and -heterozygous genotypes of S. frugiperda on whole plants of five non-Bt and eight Bt corn hybrids. The Bt corn products included two single-gene Bt corn hybrids containing Herculex^®I (Cry1F) and YieldGard^® (Cry1Ab) traits and six pyramided Bt corn hybrids representing four traits: Genuity^® VT Double Pro™, Genuity^®VT Triple Pro™, Genuity^® SmartStax™, and Agrisure^® Viptera™ 3111. In each trial, neonates of S. frugiperda were placed into the plant whorls at vegetative plant stages (V6–V10). Larvae of the three insect genotypes on non-Bt corn hybrids survived well and caused serious plant injury. Cry1Ab corn was ineffective against all three insect genotypes. On Cry1F corn plants, resistant larvae survived on 72.9% plants after 12–15 d and caused a leaf injury rating (Davis' 1 to 9 scales) of 5.7 after 7 d and 7.6 after 12–15 d. Both the larval survivorship and leaf injury rates of the resistant larvae on Cry1F corn plants were not significantly different from those observed on non-Bt corn hybrids. In contrast, no live larvae and little or no leaf injury were observed on the Cry1F corn plants that were infested with susceptible or heterozygous genotypes, or on the pyramided Bt plants. The results demonstrated that the Cry1F-resistant S. frugiperda was highly resistant to whole plants of Cry1F corn and the resistance was recessive. Hybrids that contained one of the four pyramided Bt traits were effective for managing the Cry1F resistance in S. frugiperda.     

No fitness costs associated with resistance of Busseola fusca (Lepidoptera: Noctuidae) to genetically modified Bt maize

A concern regarding planting of Bt crops is that their widespread cultivation could lead to evolution of insect resistance to Bt toxins. In South Africa, the noctuid maize stem borer (Busseola fusca [Fuller]), is resistant to Bt maize (Zea mays L.; MON810) which produces Cry1Ab protein. The presence of fitness costs in resistant populations could be a valuable component of resistance management since the non-Bt maize refuge may select against resistance. The aim of the study was to determine if there are fitness costs associated with Bt resistance of B. fusca. Life history parameters were compared between individuals of a Bt maize resistant B. fusca population when feeding on Bt or non-Bt maize. Similar comparisons were done using a control population. Field collected larvae as well as their F₁-generation were used in the study. The following parameters were compared: pupal mass, moth longevity, fecundity, fertility, larval mass and survival, and sex ratio. Except for LT50-values, no fitness costs were associated with the resistance trait in the highly resistant B. fusca population. The absence of fitness costs and presence of resistant populations may promote the use of a multi-gene strategy which would be expected to impact negatively on fitness.     

Frequency of Bacillus thuringiensis Cry1A.105 resistance alleles in field populations of the fall armyworm,Spodoptera frugiperda,in Louisiana and Florida

Fall armyworm, Spodoptera frugiperda (J.E. Smith), is a major pest of many crops and a cross-crop target of transgenic maize, cotton, and soybean containing Bacillus thuringiensis (Bt) genes. Some of the current Bt maize products for controlling lepidopteran species contain the Bt event MON 89034. The objective of this study was to determine the frequency of resistance alleles in field populations of S. frugiperda collected from Louisiana and Florida, U.S. to Cry1A.105, one of the two Bt genes in MON 89034. A total of 150 F₂ two-parent families of S. frugiperda were established using single-pair mating of field-collected individuals in 2011, which included 79 families from two locations in Louisiana and 71 families from one location in Florida. F₂ screen was conducted to detect resistance alleles in these families to Cry1A.105 protein in maize plants. Four out of the 79 Louisiana and 14 out of the 71 Florida families were identified to possess resistance alleles to the Cry1A.105 maize plants. Thus, the corresponding frequency of resistance alleles to Cry1A.105 maize was estimated to be 0.0158 with a 95% credibility interval (CI) of 0.0052–0.0323 for the Louisiana populations and 0.0559 with a 95% CI of 0.0319–0.0868 for the Florida populations. The resistant families survived on whole Cry1A.105 maize plants and demonstrated a significant level (>116-fold) of resistance to the Cry1A.105 protein in a diet-incorporated bioassay. These findings suggest that resistance allele frequency in S. frugiperda to single-gene Cry1A.105 maize in the U.S. southeast region apparently is not rare, most likely due to the selection of Cry1F resistance and its cross-resistance to Cry1A.105.     

转Bt基因抗虫玉米发展状况及对非靶标昆虫的影响

转Bt基因抗虫玉米是目前商品化进程最快的转基因作物之一,并被广泛应用于农业生产,其抗虫性和经济效益已得到普遍肯定,但外源基因的导入一定程度上改变了玉米自身的基因序列,因此转Bt基因抗虫玉米对生态安全的影响也备受关注。本文从转Bt基因抗虫玉米发展现状及对非靶标昆虫影响的角度对转Bt基因玉米的安全性研究加以综述。