转基因抗虫棉花和玉米与节肢动物相关的生态安全性研究进展

转基因抗虫棉花和玉米自1996年商业化种植以来,已取得显著的经济、生态和社会效益。与其相关的生态安全性,特别是其对非靶标生物的影响及靶标害虫的抗性监测和治理已成为人们普遍关注的话题。本文在大量室内和田间评价工作的基础上,系统综述了国内外研究在该领域内取得的进展。结果表明： 由于Bt棉田和玉米田杀虫剂用量的减少,某些对Bt杀虫蛋白不敏感的非靶标植食害虫种群有上升的趋势; 现阶段生产上推广种植的Bt棉花和玉米花粉对家蚕、柞蚕和蜜蜂等经济昆虫以及帝王斑蝶是安全的。杀虫剂用量的减少,降低了对天敌的杀伤力,Bt田中捕食性天敌的种类和数量均显著高于常规施药田; 但Bt田内靶标害虫数量的减少和质量的降低,在一定程度上影响了寄生性天敌的种类和数量。Bt棉花和玉米的大面积种植对农田生态系统节肢动物群落结构无明显不利影响。靶标害虫田间抗性监测结果表明,无论在以大农场单一种植经营为主的发达国家如美国或澳大利亚,还是在以小农经营为主的多种寄主作物小规模交叉混合种植模式的发展中国家如中国或印度,田间并未出现10年前人们所关注和预测的靶标害虫种群抗性上升问题。究其原因,可能与发达国家严格执行了预防性的抗性治理对策及发展中国家独特的作物种植模式有关。尽管目前在田间尚未发现害虫对Bt作物产生抗性,但应用更多年份之后,害虫对Bt作物的抗性就很可能不是“是否”发生问题,而是“何时”发生的问题。因此,今后的研究重点应放在Bt棉花和玉米长期、大面积种植后,其对非靶标生物及靶标害虫抗性发展影响的长期生态效应上。     

Neighbouring crop diversity mediates the effect of Bt cotton on insect community and leaf damage in fields

Effects of large-scale cultivation of transgenic crops on agricultural biodiversity remain unclear, particularly in the context of complex ecological interactions between transgenic crops and other organisms. Here we conducted a comprehensive survey to investigate the number of species, population abundance, community evenness and dominance of insects and weeds as well as leaf damage to weeds in Bt and non-Bt cotton fields at 27 sites across northern China. The role of neighbouring crop diversity around cotton fields in controlling insects and weeds in the cotton fields was also assessed. In addition, we conducted a 3-year field experiment to verify the results of the survey. Weed diversity in Bt and non-Bt cotton fields was similar, but the species number and diversity indices of insects are significantly decreased in Bt fields aligning with reduced leaf damage to broadleaf plant species including cotton as well as crops in neighbouring plots. The leaf damage to Bt and non-Bt cotton negatively associates with the diversity of neighbouring crops in cotton fields. Our study demonstrates the neighbouring crop diversity mediates the effects of Bt crops on agricultural diversity in complex interactions among transgenic crops, in-field weed and insect communities, and neighbouring crops.

     

Evolution of resistance to transgenic crops: interactions between insect movement and field distribution

The refuge strategy is designed to delay evolution of pest resistance to transgenic crops producing Bacillus thuringiensis Berliner (Bt) toxins. Movement of insects between Bt crops and refuges of non-Bt crops is essential for the refuge strategy because it increases chances that resistant adults mate with susceptible adults from refuges. Conclusions about optimal levels of movement for delaying resistance are not consistent among previous modeling studies. To clarify the effects of movement on resistance evolution, we analyzed simulations of a spatially explicit model based partly on the interaction of pink bollworm, Pectinophora gossypiella (Saunders), with Bt cotton. We examined resistance evolution as a function of insect movement under 12 sets of assumptions about the relative abundance of Bt cotton (50 and 75%), temporal distribution of Bt cotton and refuge fields (fixed, partial rotation, and full rotation), and spatial distribution of fields (random and uniform). The results show that interactions among the relative abundance and distribution of refuges and Bt cotton fields can alter the effects of movement on resistance evolution. The results also suggest that differences in conclusions among previous studies can be explained by differences in assumptions about the relative abundance and distribution of refuges and Bt crop fields. With fixed field locations and all Bt cotton fields adjacent to at least one refuge, resistance evolved slowest with low movement. However, low movement and fixed field locations favored rapid resistance evolution when some Bt crop fields were isolated from refuges. When refuges and Bt cotton fields were rotated to the opposite crop type each year, resistance evolved fastest with low movement. Nonrecessive inheritance of resistance caused rapid resistanceevolution regardless of movement rate. Confirming previous reports, results described here show that resistance can be delayed effectively by fixing field locations and distributing refuges uniformly to ensure that Bt crop fields are not isolated from refuges. However, rotating fields provided better insect control and reduced the need for insecticide sprays.     

Landscape effects of transgenic cotton on non-target ants and beetles

Transgenic crops producing toxins from Bacillus thuringiensis (Bt) can be planted in the same field for many years, and many insects exploiting such crops must disperse to other habitats to persist. Accordingly, effects of transgenic crop farming could accumulate through time and affect insect populations across agricultural landscapes. We monitored the population density of seven ant genera and beetle families and of rare ants and beetles in 84 non-cultivated sites abutting agricultural fields in Central Arizona. We assessed the short-term (during planting year) and long-term (over 5–6 years) landscape effect of farming Cry1Ac cotton on ant and beetle density in non-cultivated sites, in addition to several local and regional variables. Landscape variables (e.g., sequence of crops planted in neighbouring fields, crop diversity, and abundance) were more frequently associated with insect density than local variables (e.g., plant productivity and diversity in non-cultivated sites). In the short-term, use of Bt relative to non-Bt cotton in neighbouring fields was positively associated with density of one ant and two beetle groups in non-cultivated sites. However, acreage of Bt cotton located within 1 km from non-cultivated sites had more negative effects than acreage of non-Bt cotton on density of one ant and one beetle group. In the long-term, the proportion of years that Bt cotton was planted in neighbouring fields was positively associated with ant density but not beetle density. Results suggest that the farming of Bt cotton in neighbouring fields frequently resulted in positive short- and long-term landscape effects on ants and beetles in non-cultivated sites, while Bt cotton planted farther away had less frequent negative short-term impacts.     

Early detection of field-evolved resistance to Bt cotton in China: cotton bollworm and pink bollworm

Transgenic crops producing Bacillus thuringiensis (Bt) toxins kill some major insect pests, but pests can evolve resistance and thereby reduce the effectiveness of such Bt crops. The main approach for slowing pest adaptation to Bt crops uses non-Bt host plants as "refuges" to increase survival of susceptible pests. To delay evolution of pest resistance to cotton producing Bt toxin Cry1Ac, several countries have required refuges of non-Bt cotton, while farmers in China have relied on "natural" refuges of non-Bt host plants other than cotton. This strategy is designed for cotton bollworm (Helicoverpa armigera), which attacks many crops and is the primary target of Bt cotton in China, but it does not apply to pink bollworm (Pectinophora gossypiella), which feeds almost entirely on cotton in China. Here we review evidence of field-evolved resistance to Cry1Ac by cotton bollworm in northern China and by pink bollworm in the Yangtze River Valley of China. For both pests, results of laboratory diet bioassays reveal significantly decreased susceptibility of field populations to Cry1Ac, yet field control failures of Bt cotton have not been reported. The early detection of resistance summarized here may spur countermeasures such as planting Bt cotton that produces two or more distinct toxins, increased planting of non-Bt cotton, and integration of other management tactics together with Bt cotton.     

Pollen- and seed-mediated transgene flow in commercial cotton seed production fields

Background

Characterizing the spatial patterns of gene flow from transgenic crops is challenging, making it difficult to design containment strategies for markets that regulate the adventitious presence of transgenes. Insecticidal Bacillus thuringiensis (Bt) cotton is planted on millions of hectares annually and is a potential source of transgene flow.

Methodology/Principal Findings

Here we monitored 15 non-Bt cotton (Gossypium hirsutum, L.) seed production fields (some transgenic for herbicide resistance, some not) for gene flow of the Bt cotton cry1Ac transgene. We investigated seed-mediated gene flow, which yields adventitious Bt cotton plants, and pollen-mediated gene flow, which generates outcrossed seeds. A spatially-explicit statistical analysis was used to quantify the effects of nearby Bt and non-Bt cotton fields at various spatial scales, along with the effects of pollinator abundance and adventitious Bt plants in fields, on pollen-mediated gene flow. Adventitious Bt cotton plants, resulting from seed bags and planting error, comprised over 15% of plants sampled from the edges of three seed production fields. In contrast, pollen-mediated gene flow affected less than 1% of the seed sampled from field edges. Variation in outcrossing was better explained by the area of Bt cotton fields within 750 m of the seed production fields than by the area of Bt cotton within larger or smaller spatial scales. Variation in outcrossing was also positively associated with the abundance of honey bees.

Conclusions/Significance

A comparison of statistical methods showed that our spatially-explicit analysis was more powerful for understanding the effects of surrounding fields than customary models based on distance. Given the low rates of pollen-mediated gene flow observed in this study, we conclude that careful planting and screening of seeds could be more important than field spacing for limiting gene flow.     

Fitness of Cry1A-resistant and -susceptible Helicoverpa armigera (Lepidoptera: Noctuidae) on transgenic cotton with reduced levels of Cry1Ac

The performance of Helicoverpa armigera (Hübner) on 15-wk-old cotton plants was compared for a susceptible strain, a near-isogenic laboratory-selected strain, and F1 progeny of the two strains. Glasshouse experiments were conducted to test the three insect types on conventional plants and transgenic plants that produced the Bacillus thuringiensis (Bt) toxin Cry1Ac. At the time of testing (15 wk), the Cry1Ac concentration in cotton leaves was 75% lower than at 4 wk. On these plants, < 10% of susceptible larvae reached the fifth instar, and none survived to pupation. In contrast, survival to adulthood on Cry1Ac cotton was 62% for resistant larvae and 39% for F1 larvae. These results show that inheritance of resistance to 15-wk-old Cry1Ac cotton is partially dominant, in contrast to results previously obtained on 4-wk-old Cry1Ac cotton. Growth and survival of resistant insects were similar on Cry1Ac cotton and on non-Bt cotton, but F1 insects developed more slowly on Cry1Ac cotton than on non-Bt cotton. Survival was lower and development was slower for resistant larvae than for susceptible and F1 larvae on non-Bt cotton. These results show recessive fitness costs are associated with resistance to Cry1Ac.     

Suppressing resistance to Bt cotton with sterile insect releases

Genetically engineered crops that produce insecticidal toxins from Bacillus thuringiensis (Bt) are grown widely for pest control. However, insect adaptation can reduce the toxins' efficacy. The predominant strategy for delaying pest resistance to Bt crops requires refuges of non-Bt host plants to provide susceptible insects to mate with resistant insects. Variable farmer compliance is one of the limitations of this approach. Here we report the benefits of an alternative strategy where sterile insects are released to mate with resistant insects and refuges are scarce or absent. Computer simulations show that this approach works in principle against pests with recessive or dominant inheritance of resistance. During a large-scale, four-year field deployment of this strategy in Arizona, resistance of pink bollworm (Pectinophora gossypiella) to Bt cotton did not increase. A multitactic eradication program that included the release of sterile moths reduced pink bollworm abundance by >99%, while eliminating insecticide sprays against this key invasive pest.     

The halo effect: suppression of pink bollworm on non-bt cotton by bt cotton in china

In some previously reported cases, transgenic crops producing insecticidal proteins from Bacillus thuringiensis (Bt) have suppressed insect pests not only in fields planted with such crops, but also regionally on host plants that do not produce Bt toxins. Here we used 16 years of field data to determine if Bt cotton caused this "halo effect" against pink bollworm (Pectinophora gossypiella) in six provinces of the Yangtze River Valley of China. In this region, the percentage of cotton hectares planted with Bt cotton increased from 9% in 2000 to 94% in 2009 and 2010. We found that Bt cotton significantly decreased the population density of pink bollworm on non-Bt cotton, with net decreases of 91% for eggs and 95% for larvae on non-Bt cotton after 11 years of Bt cotton use. Insecticide sprays targeting pink bollworm and cotton bollworm (Helicoverpa armigera) decreased by 69%. Previously reported evidence of the early stages of evolution of pink bollworm resistance to Bt cotton in China has raised concerns that if unchecked, such resistance could eventually diminish or eliminate the benefits of Bt cotton. The results reported here suggest that it might be possible to find a percentage of Bt cotton lower than the current level that causes sufficient regional pest suppression and reduces the risk of resistance.     

Pollination of invasive Eichhornia crassipes (Pontederiaceae) by the introduced honeybee (Apis mellifera L.) in South China

Heterostyly functions as an outcrossing mechanism facilitating accurate pollen transfer from anthers to stigmas of particular heights as a result of the behavior of specialist pollinators. However, heterostylous plants are also visited by generalist pollinators, which may affect the plant–pollinator mutualism. Eichhornia crassipes is a tristylous invasive species, with only the mid- and long-styled morphs (M and L) found in China. We recorded flower-visiting insects in Zhuhai, Zhongshan and Nanning in South China. We hand-pollinated the two morphs to determine their compatibility. In addition, by allowing controlled insect pollination in artificial isoplethically monomorphic and bimorphic populations, we undertook a detailed analysis of pollen deposition between the floral morphs, and fruit and seed set. Ranked by relative abundance, the flower-visiting insects were: Apis mellifera, A. cerana, Lasioglossum sp. and Eristalis arvorum. Hand pollination showed that both the M and L morphs were self-compatible, but the former was probably more so than the latter. Intra-morph pollen transfer by A. mellifera within a population was significantly greater than legitimate pollen transfer between populations, suggesting that the pollen exchange between populations was limited. Seed set of the L morph was significantly greater than that of the M morph in monomorphic populations, indicating intra-morph pollen deposition in the former was higher than in the latter. The results showed that A. mellifera was the major pollinator in South China and able to pollinate E. crassipes legitimately and to promote its fruit and seed set, even though high levels of intra-morph pollination occurred.     

Bt crop effects on functional guilds of non-target arthropods: a meta-analysis

Background

Uncertainty persists over the environmental effects of genetically-engineered crops that produce the insecticidal Cry proteins of Bacillus thuringiensis (Bt). We performed meta-analyses on a modified public database to synthesize current knowledge about the effects of Bt cotton, maize and potato on the abundance and interactions of arthropod non-target functional guilds.

Methodology/Principal Findings

We compared the abundance of predators, parasitoids, omnivores, detritivores and herbivores under scenarios in which neither, only the non-Bt crops, or both Bt and non-Bt crops received insecticide treatments. Predators were less abundant in Bt cotton compared to unsprayed non-Bt controls. As expected, fewer specialist parasitoids of the target pest occurred in Bt maize fields compared to unsprayed non-Bt controls, but no significant reduction was detected for other parasitoids. Numbers of predators and herbivores were higher in Bt crops compared to sprayed non-Bt controls, and type of insecticide influenced the magnitude of the difference. Omnivores and detritivores were more abundant in insecticide-treated controls and for the latter guild this was associated with reductions of their predators in sprayed non-Bt maize. No differences in abundance were found when both Bt and non-Bt crops were sprayed. Predator-to-prey ratios were unchanged by either Bt crops or the use of insecticides; ratios were higher in Bt maize relative to the sprayed non-Bt control.

Conclusions/Significance

Overall, we find no uniform effects of Bt cotton, maize and potato on the functional guilds of non-target arthropods. Use of and type of insecticides influenced the magnitude and direction of effects; insecticde effects were much larger than those of Bt crops. These meta-analyses underscore the importance of using controls not only to isolate the effects of a Bt crop per se but also to reflect the replacement of existing agricultural practices. Results will provide researchers with information to design more robust experiments and will inform the decisions of diverse stakeholders regarding the safety of transgenic insecticidal crops.     

Increased frequency of pink bollworm resistance to Bt toxin Cry1Ac in China

Transgenic crops producing insecticidal proteins from Bacillus thuringiensis (Bt) kill some key insect pests, but evolution of resistance by pests can reduce their efficacy. The main approach for delaying pest adaptation to Bt crops uses non-Bt host plants as "refuges" to increase survival of susceptible pests. To delay evolution of pest resistance to transgenic cotton producing Bt toxin Cry1Ac, the United States and some other countries have required refuges of non-Bt cotton, while farmers in China have relied on "natural" refuges of non-Bt host plants other than cotton. The "natural" refuge strategy focuses on cotton bollworm (Helicoverpa armigera), the primary target of Bt cotton in China that attacks many crops, but it does not apply to another major pest, pink bollworm (Pectinophora gossypiella), which feeds almost entirely on cotton in China. Here we report data showing field-evolved resistance to Cry1Ac by pink bollworm in the Yangtze River Valley of China. Laboratory bioassay data from 51 field-derived strains show that the susceptibility to Cry1Ac was significantly lower during 2008 to 2010 than 2005 to 2007. The percentage of field populations yielding one or more survivors at a diagnostic concentration of Cry1Ac increased from 0% in 2005-2007 to 56% in 2008-2010. However, the median survival at the diagnostic concentration was only 1.6% from 2008 to 2010 and failure of Bt cotton to control pink bollworm has not been reported in China. The early detection of resistance reported here may promote proactive countermeasures, such as a switch to transgenic cotton producing toxins distinct from Cry1A toxins, increased planting of non-Bt cotton, and integration of other management tactics together with Bt cotton.     

Changes in Cry1Ac Bt transgenic cotton in response to two environmental factors: temperature and insect damage

The efficacy of Cry1Ac Bacillus thuringiensis (Bt) cotton plants against field populations of Helicoverpa armigera (Hübner) has been inconsistent over the growing season. Any reduction in efficacy (where efficacy is the capacity of the plant to affect the survival of the insect) increases the opportunities for H. armigera to evolve resistance to Bt toxin. Changes in efficacy could be due to changes at the level of gene expression and/or in the physiological makeup of the plant and may be induced by environmental conditions. Two environmental factors, temperature and insect damage, were investigated. Temperature was found to affect efficacy, whether plants were grown at different temperatures continuously or were exposed to a change in temperature for a short period. Damage caused by chewing insects (H. armigera larvae) produced a dramatic increase in the efficacy of presquare Bt cotton. In contrast, damage by sucking insects (aphids) did not induce changes in efficacy. Changes in efficacy seemed to be mediated through modification of the physiological background of the plant rather than changes in the level of Cry1Ac expression or in the concentration of the Bt toxin. The impact of the non-Bt responses of plants on strains of H. armigera should be evaluated. It is possible that by enhancing existing defensive mechanisms of plants, the rate of evolution of resistance to Bt toxins could be retarded by increasing the plants overall toxicity through the additive effects of the toxins and plant defenses.     

Indirect reduction of ear molds and associated mycotoxins in Bacillus thuringiensis corn under controlled and open field conditions: utility and limitations

In 1995, ears of a experimental inbred (CG59-2) containing a synthetic Bacillus thuringiensis Cry IA (b) gene driven by PEPC, pith and pollen promoters and artificially infested with Ostrinia nubilalis (Hübner) larvae in small plot studies were free from insect damage, whereas 40-50% of the corresponding non-Bt inbred ears were damaged. Bt inbred ears that were inoculated with Aspergillus flavus Link and Fusarium proliferatum T. Matsushima (Nirenberg) or exposed to natural mold inoculum after infestation with O. nubilalis were free of visible signs of mold, as compared with approximately 30-40% of the non-Bt ears similarly treated. Results in 1996 using the same inbred with a single allele dose of the Bt gene showed similar trends. Mean total fumonisin levels for non-Bt versus Bt inbred ears were not significantly different (2.8 versus 0.8 ppm, respectively) in 1996. In paired hybrid studies run in 0.4-ha (1-acre) fields, an event 176 Bt hybrid had significantly lower amounts of damage and signs of Fusarium spp. mold, but not fumonisin, compared with a corresponding non-Bt hybrid from 1996 to 1998. However, two hybrid pairs that contained either MON810 or Bt11 constructs examined in similar fields at the same site had lower levels of fumonisin in both 1997 (30- to 40-fold) and 1998. High intrafield variability in insect infestation and presence of Helicoverpa zea (Boddie) in Bt hybrids was apparently responsible for fewer significant differences in fumonisin levels in 1998. Similar trends for all three hybrid pairs were noted in small plot trials at another site. Incidence of other ear pests or insect predators varied as much among non-Bt hybrids as they did for Bt/non-Bt hybrid pairs.     

Feeding, oviposition and survival of Liriomyza trifolii (Diptera: Agromyzidae) on Bt and non-Bt cottons

The effects of Bt transgenic cottons (Bt-I expressing cry1Ac and Bt-II expressing cry1Ab and cry2Ab or cry1Ab and cry1Fa) and non-Bt cottons on feeding, oviposition and longevity of adults, and development and survival of Liriomyza trifolii larvae were studied under laboratory conditions; and infestation on four Bt and two non-Bt cotton traits were investigated under field conditions. Laboratory choice and no-choice tests showed that L. trifolii adults were capable of distinguishing between Bt cottons and non-Bt cottons. In a choice test on younger plants (4-5 leaves), the adults were found more often and made more feeding punctures (FP) on non-Bt cottons than on Bt cottons. On older plants (8-9 leaves), adults made the most FP on non-Bt cotton followed by those on Bt-II cottons and the least on Bt-I cotton. The females oviposited more eggs (6.7 eggs per leaf) on non-Bt cotton than on Bt-I (1.7 eggs per leaf) and Bt-II (0.8 eggs per leaf) cottons on younger plants and oviposited similar numbers of eggs (0.7-1.3 eggs per leaf) on non-Bt and Bt cottons on older plants. In a no-choice test, the females also fed more FP on non-Bt cottons than on Bt cottons on both younger and older plants. The females oviposited more eggs (15.6 eggs per leaf) on non-Bt cotton than on Bt-I (8.2 eggs per leaf) and Bt-II (6.5 eggs per leaf) cottons on younger plants and similar numbers of eggs (2.5-3.3 eggs per leaf) on non-Bt and Bt cottons on older plants. Larval and puparial survivals were not different among Bt and non-Bt cottons. The occurrence and damage of leafminers on cottons in the field showed that L. trifolii infested more plants and leaves and had more mines on non-Bt cotton than on Bt cottons.     

Field Evaluation of Bt Cotton Crop Impact on Nontarget Pests: Cotton Aphid and Boll Weevil

Bt cotton plants expressing Cry1Ac protein have high specificity for the control of lepidopteran larvae. However, studies conducted in several countries have shown these plants have a differential impact on nontarget herbivores. The aim of this study was to compare the colonization rates and population abundance of the cotton aphid, Aphis gossypii Glover (Hemiptera: Aphididae) and the boll weevil, Anthonomus grandis Boheman (Coleoptera: Curculionidae), in plots of Bt (Nuopal) and non-Bt cotton (Delta Opal) in an experimental field in Brasilia, DF, Brazil. No difference was observed in the preference and colonization by winged aphids to plants from the two treatments. There was no significant difference in abundance of wingless aphids or in the production of winged aphids between treatments. Apparently, the parameters that control factors such as fecundity, survival, and dispersal were similar on both Bt and non-Bt plants. Monitoring of plants for coccinellids, a specialist predator of aphids, and ants that act on the dispersal of aphids among plants showed no significant difference between Bt and non-Bt plants, supporting the inference above. Regarding the effect on boll weevil, there was also no significant difference between treatments in the total number of fruiting structures attacked in each plot, the percentage of fruiting structures attacked per plant or on the number of weevils emerging from fruits with boll weevil damage from egg-laying, when damaged fruit samples were held in the laboratory. Based on these results, we conclude that there is no impact of Bt cotton crop expressing Cry1Ac on the nontarget herbivores tested under field conditions.     

Influence of plant severing on movement of Ostrinia nubilalis larvae in Zea mays hybrid seed production fields

Genetically engineered corn hybrids that contain a cry gene from the bacterium Bacillus thuringiensis Berliner (Bt) are gaining popularity for controlling the corn pest Ostrinia nubilalis (Hübner). Continuous use of Bt corn, however, could select for O. nubilalis that are resistant to this corn. Monitoring for insect resistance is important, because it could help maintain the Bt technology. A possible monitoring method is to collect larval insects in commercial drying bins after harvest from Bt seed production fields. A drawback to this method is that these collections may be contaminated by insects that moved as later instars from severed non-Bt male rows into the adjacent Bt female rows. These larvae have little to no exposure to Bt toxin, resulting in possible "false positives." The objectives of this study were to first find which combination of planting and severing dates produces the least number of larvae that move from non-Bt male plants to Bt female plants and to assess O. nubilalis larval movement from severed non-Bt male rows to Bt female rows. Field studies in 2002 and 2003 were designed to simulate a hybrid seed production field. Results suggest that movement of O. nubilalis larvae from male corn is minimized when corn is planted early and male plants are severed by 2 wk post-anthesis. This reduces the likelihood of false positives by reducing the number of susceptible larvae moving between Bt and non-Bt plants. Also, larvae moved to all four female rows that were adjacent to the severed rows, but there were significantly more larvae found in the closest row compared with the other three. These results could be used to develop a monitoring program to find O. nubilalis larvae with resistance to Bt corn in field populations of O. nubilalis.     

Garden Flowers: Insect Visits and the Floral Reward of Horticulturally-modified Variants

Garden flowers can be valuable to wildlife if they produce nectar,pollen and/or seeds. To provide information needed by gardenersto select wildlife-friendly plants, we investigated nectar productionand insect visits toTropaeolum majus, Consolidasp.,Antirrhinummajus, Violaxwittrockiana, Tagetes patulaandAlcea rosea, ineach case comparing a near-original flower type with a cultivarthat had spurless, doubled, peloric or enlarged flowers. Allspecies showed high secretion rates and standing crops of nectar.In most cases the horticultural modifications affected the numbersor species composition of the assemblage of insect visitors,and they generally reduced the value of the floral reward toinsects, often affecting accessibility. Effects on seed yieldwere not investigated directly here, but are likely to furtherreduce the wildlife value of modified variants.Copyright 1999Annals of Botany Company. Garden plants, horticultural modification, nectar, pollen, coevolution, insects, pollinators,Bombus, Apis mellifera,foraging.     

Molecular,biochemical and bioassay based evidence of lower allelopathic potential in genetically modified rice

This study examines allelopathic potential of genetically modified rice. The experiment was conducted on two isogenic lines Bacillus thuringiensis (Bt) and non-Bacillus thuringiensis (non-Bt). Both isogenic lines have same allelopathic ability before insect feeding and after limited insect feeding (Spodoptera litura) non-Bt rice genotype demonstrates more allelopathic potential. The S. litura cannot feed Bt rice genotype. The role of shoot herbivory in allelopathic induction is further supported when Bt plants also exhibited higher allelopathic potential after insect regurgitant application to the damaged leaves. Allelopathic potential was assessed through several methods after treatments of mechanical damage, insect feeding and insect regurgitant application to damaged rice leaves. Rhizosphere soil and leaf leachates of non-Bt rice cultivar exhibited higher allelopathic potential on lettuce and barnyard grass after herbivore feeding. Enzyme activities (PAL and C4H) responsible for biosynthesis of phenolic compounds and their concentration were significantly higher in non-Bt plant after herbivore feeding and attain the same level in Bt plants after insect regurgitant application to damaged leaves. Similarly, genes (OsPAL and OsCYC1) responsible for biosynthesis of allelopathic compounds showed high expression in non-Bt plants after herbivore feeding. Our results indicate that herbivore feeding enhance rice allelopathic potential and no insect feeding as incase of Bt plants may reduce allelopathic potential of genetically modified rice.     

Mating compatibility among helicoverpa armigera (Lepidoptera: Noctuidae) occurring on selected host plants and Bt cotton survivors

Helicoverpa armigera (Hiibner) (Lepidoptera: Noctuidae) is a well-known polyphagous insect pest. Mating compatibility among the insects occurring on different host plants is essential for free gene flow among populations. We tested the extent of crossability and fecundity of the insects that survived on Bacillus thuringiensis (Bt) cotton with those occurring on pigeon pea, Cajanus cajun (L.) Millsp., non-Bt cotton, Gossypium hirsutum L.; sunflower, Helianthus annuus; sorghum, Sorghum bicolor L. Moench.; okra, Abelmoschus moschatus Medikus; chickpea, Cicer arietinum L.; marigold, Tagetes spp.; and tomato, Lycopersicum esculentum L., crops. The insects from different crops were freely crossable with those collected from Bt cotton and among themselves. The average fecundity across different crosses ranged from 314.1 to 426.3 in direct and from 305.8 to 421.7 eggs per female in reciprocal crosses. In any given cross, a minimum of 85.89% egg hatch was recorded. Furthermore, the F1 individuals of different cross combinations were found to cross freely with their parents (BC1) and among themselves with similar fecundity and egg hatch. High crossability among H. armigera occurring on different host plants suggests that crop mosaics that may exist in countries such as India could play an important role as natural, nonstructured refugia and prolong the durability of the genes deployed for controlling this insect.