A QTL that enhances and broadens Bt insect resistance in soybean

Effective strategies are needed to manage insect resistance to Bacillus thuringiensis (Bt) proteins expressed in transgenic crops. To evaluate a multiple resistance gene pyramiding strategy, eight soybean (Glycine max) lines possessing factorial combinations of two quantitative trait loci (QTLs) from plant introduction (PI) 229358 and a synthetic Bt cry1Ac gene were developed using marker-assisted selection with simple sequence repeat markers. Field studies were conducted in 2000 and 2001 to evaluate resistance to corn earworm (Helicoverpa zea) and soybean looper (Pseudoplusia includens), and detached leaf bioassays were used to test antibiosis resistance to Bt-resistant and Bt-susceptible strains of tobacco budworm (TBW; Heliothis virescens). Based on defoliation in the field and larval weight gain on detached leaves, lines carrying a combination of cry1Ac and the PI 229358 allele at a QTL on linkage group M were significantly more resistant to the lepidopteran pests, including the Bt-resistant TBW strain, than were the other lines. This is the first report of a complementary additive effect between a Bt transgene and a plant insect resistance QTL with an uncharacterized mode of action that was introgressed using marker-assisted selection.     

Combining cry1Ac with QTL alleles from PI 229358 to improve soybean resistance to lepidopteran pests

A QTL conditioning corn earworm resistance in soybean PI 229358 and asynthetic Bacillus thuringiensis cry1Ac transgene from therecurrent parent Jack-Bt were pyramided intoBC₂F₃ plants by marker-assisted selection. Segregatingindividuals were genotyped at SSR markers linked to an anitbiosis/antixenosisQTL on linkage group M, and were tested for the presence ofcry1Ac. Marker-assisted selection was used during andafter the two backcrosses to develop a series of BC₂F₃plants with or without the crylAc transgene and the QTLconditioning for resistance BC₂F₃ plants that werehomozygous for parental alleles at markers on LG M, and whicheither had or lacked cry1Ac, were assigned to one of fourpossible genotype classes. These plants were used in no-choice, detached leaffeeding bioassays with corn earworm and soybean looper larvae (Lepidoptera:Noctuidae) to evaluate the relative antibiosis in the different genotypeclasses. Resistance was measured as larval weight gain and degree of foliageconsumption. Few larvae of either species survived on leaves expressing theCry1Ac protein. Though not as great as the effect of Cry1Ac, the PI229358-derived LG M QTL also had a detrimental effect on larval weights of bothpest species, and on defoliation by corn earworm, but did not reduce defoliation bysoybean looper. Weights of soybean looper larvae fed foliage from transgenicplants with the PI-derived QTL were lower than those of larvae fed transgenictissue with the corresponding Jack chromosomal segment. This work demonstratesthe usefulness of SSRs for marker-assisted selection in soybean, and shows thatcombining transgene-and QTL-mediated resistance to lepidopteran pests may be aviable strategy for insect control.     

PHOTOSYNTHESIS RESPONSE OF ‘CAROLINA’ CUCUMBER TO SIMULATED AND ACTUAL STRIPED CUCUMBER BEETLE (COLEOPTERA: CHRYSOMELI-DAE) DEFOLIATION*

Abstract Field studies were conducted in 1995 to compare the photosynthesis response between simulated striped cucumber beetle, Acalymma vittatum (F.), defoliation and actual A. vittatum defoliation in ‘Carolina’ cucumber. Six simulated defoliation levels (0%–100%)were used over 5 timings of defoliation (first true leaf to harvest). Plots were defoliated with scissors twice each week throughout each timing interval, and defined as continuous defoliation. In addition to continuous defoliation, one-time simulated defoliation treatments were imposed using the same timing treatments but only 4 defoliation levels (0, 25, 50, and 100%). Two cage studies, with A. vittatum adults, were used to provide estimates of actual insect defoliation injury. Beetles were placed in cages for 2 weeks at densities of 0, 1, 3, 5, and 10/plant and net photosynthetic rates were measured. No significant differences (P <0. ⁰⁵ were detected in pho-tosynthetic rates between simulated and actual insect defoliation treatments. Also, few significant differences in photosynthetic rates were detected between damaged and undamaged leaves in the simulated insect defoliation study. Results indicated that the simulated insect defoliation procedure appeared to accurately reflect the growth response of ‘Carolina’ cucumber subjected to actual A. vittatum defoliation, and that simulated defoliation results should be appropriate for developing economic injury levels for A. vittatum on cucumber.     

Determining the major Bt refuge crops for cotton bollworm in North China

Evaluation of the effectiveness of refuge strategies involved in cotton bollworm Bt resistance management would be aided by technologies that allow monitoring and quantification of key factors that affect the process under field conditions. We hypothesized that characterization of stable carbon and nitrogen isotopes in adult bollworm, Helicoverpa armigera (Hübner) (Lepidoptera: Noctuidae) moths may aid in determining the larval host that they developed upon. We found moths reared from larvae fed on peanut, soybean or cotton, respectively, could be differentiated using isotopic analyses that also corresponded to their respective larval host origins. These techniques were then used to classify feral second‐generation bollworm moths caught in Bt cotton (Gossypium hirsutum) fields into different populations based on their isotopic signatures. In 2006–2007 feral moths captured in Bt cotton fields predominantly correlated with the peanut (Arachis hypogea) having served as their larval host, indicating this is the most important refuge crop for Bt‐susceptible bollworm individuals (providing 58%?64% individuals) during independent moth peaks for the second generation in North China. The remaining feral moths correlated with soybean (Glycine max) (0?10%); other C₃ plant (20%?22%) and non‐C₃ plant (12%?14%) host types also provided some Bt‐sensitive moths. Field observations showed that peanut constitutes the primary refuge crop contributing to sustaining Bt‐susceptible moths dispersing into cotton in North China. These results suggest that peanut may be a more effective refuge to sustain Bt‐susceptible bollworm individuals and reduce the risk of development of a Bt‐resistant biotype.     

Frequency of alleles conferring resistance to Bacillus thuringiensis maize in Louisiana populations of the southwestern corn borer

Transgenic maize [Zea mays L. (Poaceae)] expressing Bacillus thuringiensis proteins (Bt maize) has become the most important tool for managing stalk borers in maize in the USA. The current strategy for delaying the evolution of resistance in target insects for Bt maize is referred to as high dose/refuge strategy. A key requirement of the strategy is that initial resistance allele frequencies in field insect populations are low (e.g., <0.001). More than 200 iso‐line families of the southwestern corn borer, Diatraea grandiosella Dyar (Lepidoptera: Crambidae), a major target stalk borer pest of Bt maize, were developed from Louisiana populations and evaluated for Bt resistance using a modified F₂ screening method during 2005. No major resistance alleles were detected in these populations. The results showed that the expected Bt resistance allele frequency in the Louisiana populations was <0.0035 with 95% probability and a detection power of 83.9 ± 0.6%. The F₂ screen indicates that Bt resistance allele frequencies in D. grandiosella are low among the Louisiana populations and should meet the rare resistance allele requirement of the ‘high dose/refuge’ strategy.     

The Bt gene <Emphasis Type="Italic">cry2Aa2</Emphasis> driven by a tissue specific ST-LS1 promoter from potato effectively controls <Emphasis Type="Italic">Heliothis virescens</Emphasis>

Expression of the Cry2Aa2 protein was targeted specifically to the green tissues of transgenic tobacco Nicotiana tabacum cv. Xanthi plants. This deployment was achieved by using the promoter region of the gene encoding the Solanum tuberosum leaf and stem specific (ST-LS1) protein. The accumulated levels of toxin in the leaves were found to be effective in achieving 100 mortality of Heliothis virescens larvae. The levels of Cry2Aa2 expression in the leaves of these transgenic plants were up to 0.21 of the total soluble proteins. Bioassays with R₁ transgenic plants indicated the inheritance of cry2Aa2 in the progeny plants. Tissue-specific expression of the Bt toxin in transgenic plants may help in controlling the potential occurrence of insect resistance by limiting the amount of toxin to only predated tissues. The results reported here validate the use of the ST-LS1 gene promoter for a targeted expression of Bt toxins in green tissues of plants.     

Effects of Bacillus thuringiensisδ-endotoxin-fed Helicoverpa armigera on the survival and development of the parasitoid Campoletis chlorideae

With the deployment of transgenic crops expressing δ‐endotoxins from Bacillus thuringiensis (Bt) for pest management, there is a need to generate information on the interaction of crop pests with their natural enemies that are important for regulation of pest populations. Therefore, we studied the effects of the Bt δ‐endotoxins Cry1Ab and Cry1Ac on the survival and development of the parasitoid Campoletis chlorideae Uchida (Hymenoptera: Ichneumonidae) reared on Helicoverpa armigera (Hübner) (Lepidoptera: Noctuidae) larvae fed on Bt toxin‐intoxicated artificial diet. The H. armigera larvae fed on artificial diet impregnated with Cry1Ab and Cry1Ac at LC₅₀ (effective concentration to kill 50% of the neonate H. armigera larvae) and ED₅₀ (effective concentration to cause a 50% reduction in larval weight) levels before and after parasitization resulted in a significant reduction in cocoon formation and adult emergence of C. chlorideae. Larval period of the parasitoid was prolonged by 2 days when fed on Bt‐intoxicated larvae. No adverse effects were observed on female fecundity. The observed effects appeared to be indirect in nature, because no Bt proteins were detected through enzyme‐linked immunosorbent assay in the C. chlorideae larvae, cocoons, or adults fed on Cry1Ab‐ or Cry1Ac‐treated H. armigera larvae. The effects of Bt toxin proteins on C. chlorideae were due to early mortality of H. armigera larvae, that is, before completion of parasitoid larval development.     

Antixenosis and larval and adult dispersal in the Mediterranean corn borer,Sesamia nonagrioides,in relation to Bt maize

Sesamia nonagrioides Lefèbvre (Lepidoptera: Noctuidae) is a key pest of maize [Zea mays L. (Poaceae)] and a main target of Bt maize in the Mediterranean area. To choose the most suitable non‐Bt refuge strategy for preventing or delaying resistance development in this maize borer, we examined its biology and behaviour. No antixenotic effects were found on numbers of eggs and egg batches per plant in choice (Bt vs. non‐Bt plants) and no‐choice assays. However, a greater ratio of young larvae dispersed from Bt than from non‐Bt plants. In addition, larvae that hatched on Bt plants tended to disperse more than those that hatched on non‐Bt plants, particularly during young growth stages. Many adults, especially females, could fly at least up to 400 m, as was found in a dispersal study with rubidium‐marked adults. The stimulation of larval dispersal by the Bt trait and the dispersal capacity of adults might compromise the efficacy of seed mixtures as an insecticide resistance management strategy.     

Evaluation of adult emergence and larval root injury for Cry3Bb1‐resistant populations of the western corn rootworm

The transgenic maize (Zea mays L.) event MON 88017 produces the Bacillus thuringiensis Berliner (Bt) toxin Cry3Bb1 to provide protection from western corn rootworm (Diabrotica virgifera virgifera LeConte) larval feeding. In response to reports of reduced performance of Cry3Bb1‐expressing maize at two locations in Illinois, we conducted a two‐year experiment at these sites to characterize suspected resistance, as well as to evaluate root injury and adult emergence. Single‐plant bioassays were performed on larvae from each population that was suspected to be resistant. Results indicate that these populations had reduced mortality on Cry3Bb1‐expressing maize relative to susceptible control populations. No evidence of cross‐resistance between Cry3Bb1 and Cry34/35Ab1 was documented for the Cry3Bb1‐resistant populations. Field studies were conducted that included treatments with commercially available rootworm Bt hybrids and their corresponding non‐Bt near‐isolines. When compared with their near‐isolines, larval root injury and adult emergence were typically reduced for hybrids expressing Cry34/35Ab1 either alone or in a pyramid. In many instances, larval root injury and adult emergence were not significantly different for hybrids expressing mCry3A or Cry3Bb1 alone when compared with their non‐Bt near‐isolines. These findings suggest that Cry34/35Ab1‐expressing Bt maize may represent a valuable option for maize growers where Cry3Bb1 resistance is either confirmed or suspected. Consistent trends in adult size (head capsule width and dry mass) for individuals recovered from emergence cages were not detected during either year of this experiment. Because of the global importance of transgenic crops for managing insect pests, these results suggest that improved decision‐making for insect resistance management is needed to ensure the durability of Bt maize.     

<Emphasis Type="Italic">Bacillus thuringiensis</Emphasis> Protein Cry6B (BGSC ID 4D8) is Toxic to Larvae of <Emphasis Type="Italic">Hypera postica</Emphasis>

Insecticidal proteins produced by strains of Bacillus thuringenesis are specific toward target pests. One of the Bt proteins, Cry 1Ac has been used successfully for controlling crop predation by polyphagous pests Helicoverpa armigera. Structurally, Bt proteins consist of three domains; domain I and III are fairly homologous in various Bt proteins while domain II is hypervariable. The hypervariable domain II is believed to be responsible for specificity toward target pest. Successful deployment of Bt proteins requires knowledge of its specificity toward the insect. Various Bt proteins have been characterized for activity against coleopteran pests. Some Bt proteins of class Cry6 have been found to be active against potato weevil. We have evaluated the activity of Cry6B protein (BGSC-4D8) against lucerne weevil, Hypera postica, which is a major pest of forage crop Medicago sativa. Results revealed that the purified Cry6B protein is significantly active against the coleopteran pest with LC₅₀ value 280 ng/μl. The leaves coated with the purified Cry6 toxin were three times less damaged as compared with the negative control.     

Evaluation of natural and engineered resistance mechanisms in potato against Colorado potato beetle in a no-choice field study

The Colorado potato beetle, Leptinotarsa decemlineata Say, is the major insect pest of potato, Solanum tuberosum L., in eastern North America and is renowned for resistance development, currently resistant to >40 insecticides worldwide. Host plant resistance may assist in delaying in resistance development to insecticides. We evaluated natural host plant resistance mechanisms (glandular trichomes and Solanum chacoense Bitter-derived resistance) and engineered resistance mechanisms (Bacillus thuringiensis [Bt] Berliner cry3A and cry1Ia1) in a no-choice cage study. Six different potato lines representing four host plant resistance mechanisms were evaluated over 2 yr. Egg masses were placed in each cage (one egg mass per plant). Almost no feeding was observed in the Bt-cry3A lines, and only minor feeding was observed in the Bt-cry1Ia1 lines in either year. On the S. chacoense-derived line, there was significantly less defoliation than on either the susceptible line or the glandular trichome line in 2003. In 2004, there was significantly higher defoliation on the S. chacoense-derived line than on the susceptible line or glandular trichome line. The defoliation of the Solanum chacoense-derived line was largely due to larvae clipping the petioles, rather than consumption of the leaves. Defoliation on the glandular trichome line did not differ significantly from the defoliation of the susceptible line, suggesting glandular trichomes may not be effective in controlling larvae and preventing defoliation. This study suggested that Bt can provide high levels of resistance, but the natural resistance mechanisms tested here are variable for control of Colorado potato beetle larvae in no-choice situations.     

Fitness of Bt‐resistant cabbage loopers on Bt cotton plants

Development of resistance to the insecticidal toxins from Bacillus thuringiensis (Bt) in insects is the major threat to the continued success of transgenic Bt crops in agriculture. The fitness of Bt‐resistant insects on Bt and non‐Bt plants is a key parameter that determines the development of Bt resistance in insect populations. In this study, a comprehensive analysis of the fitness of Bt‐resistant Trichoplusia ni strains on Bt cotton leaves was conducted. The Bt‐resistant T. ni strains carried two genetically independent mechanisms of resistance to Bt toxins Cry1Ac and Cry2Ab. The effects of the two resistance mechanisms, individually and in combination, on the fitness of the T. ni strains on conventional non‐Bt cotton and on transgenic Bt cotton leaves expressing a single‐toxin Cry1Ac (Bollgard I) or two Bt toxins Cry1Ac and Cry2Ab (Bollgard II) were examined. The presence of Bt toxins in plants reduced the fitness of resistant insects, indicated by decreased net reproductive rate (R₀) and intrinsic rate of increase (r). The reduction in fitness in resistant T. ni on Bollgard II leaves was greater than that on Bollgard I leaves. A 12.4‐day asynchrony of adult emergence between the susceptible T. ni grown on non‐Bt cotton leaves and the dual‐toxin‐resistant T. ni on Bollgard II leaves was observed. Therefore, multitoxin Bt plants not only reduce the probability for T. ni to develop resistance but also strongly reduce the fitness of resistant insects feeding on the plants.     

转CpTI-Bt基因棉和转Bt基因棉对棉铃虫幼虫存活、生长及营养利用的影响

2000年7月中旬和8月中旬, 分别测定了采自田间的转CpTI-Bt基因双价抗虫棉（SGK321, 以下简称CpTI-Bt棉）和转Bt基因抗虫棉（中30,以下简称Bt棉）对棉铃虫Helicoverpa armigera幼虫存活、生长的影响。结果表明：7月中旬两种转基因抗虫棉抗虫效果均较好,尤其是CpTI-Bt棉棉叶和花瓣对4龄幼虫3天内致死率为92%以上;8月中旬两种转基因棉的抗虫活性均明显降低,且Bt棉的杀虫活性显著低于CpTI-Bt棉,其幼虫死亡率与对照受体棉中16的死亡率之间无显著差异,仅显著抑制了幼虫的生长;石远321(SGK321受体品系)的花瓣具有一定的抗虫活性,可显著降低取食幼虫的体重,甚至造成部分幼虫死亡; CpTI-Bt棉中,花瓣和棉叶的抗虫性明显高于蕾和铃心。对5龄幼虫取食棉铃1日后的营养指标测定结果显示： 两种转基因抗虫棉处理的幼虫相对生长率和相对取食量均显著低于石远321,但两者之间无显著差异; CpTI-Bt棉处理的幼虫近似消化率显著低于石远321和Bt棉,但其食物利用率显著高于石远321和Bt棉。     

Assessment of fitness costs in Cry3Bb1‐resistant and susceptible western corn rootworm (Coleoptera: Chrysomelidae) laboratory colonies

Abstract Maize production in the United States is dominated by plants genetically modified with transgenes from Bacillus thuringiensis (Bt). Cry3Bb delta endotoxins expressed by Bt maize specifically target corn rootworms (genus Diabrotica) and have proven highly efficacious. However, development of resistance to Bt maize, especially among western corn rootworm (Diabrotica virgifera virgifera) populations, poses a significant threat to the future viability of this pest control biotechnology. The structured refuge insect resistance management (IRM) strategy implemented in the United States for Bt maize adopts a conservative approach to managing resistance by assuming no fitness costs of Bt resistance, even though these trade‐offs strongly influence the dynamics of Bt resistance within numerous agricultural pest species. To investigate the effects of Bt resistance on fitness components of western corn rootworm, we compared survivorship, fecundity and viability of five Bt‐resistant laboratory lines reared on MON863 (YieldGard Rootworm), a Bt maize product that expresses Cry3Bb1 delta endotoxin, and on its non‐transgenic isoline. Analysis of performance on the isoline maize demonstrated no fitness costs associated with Bt resistance. In fact, resistant lines emerged approximately 2–3 days earlier than control lines when reared on both MON863 and the isoline, indicating that selection for Bt resistance resulted in a general increase in the rate of larval development. In addition, resistant lines reared on Bt maize displayed higher fecundity than those reared on the isoline, which may have significant management implications. These data will be valuable for formulating improved IRM strategies for a principal agricultural pest of maize.     

Soybean Kunitz, C-II and PI-IV inhibitor genes confer different levels of insect resistance to tobacco and potato transgenic plants

In modern, highly intensive agriculture, the control of insect pests is basically achieved with the application of chemical pesticides. Heavy reliance on this sole strategy is associated with several drawbacks, and the development of alternative or complementary methods to chemical control is desirable. In this work, three soybean genes (KTi ₃ , C-II and PI-IV)coding for serine proteinase inhibitors were isolated by PCR and transferred to Agrobacterium tumefaciens EHA 105, which in turn was used for transforming tobacco leaf and potato tuber discs. Biochemical assays confirmed that transgenic plants synthesized serine proteinase inhibitors; rates of expression varied among plants. The level of insect resistance (tested with Spodoptera littoralis Boisduval) was particularly high in tobacco, where many plants caused the death of all larvae. In potatoes, larval mortality was much less frequently achieved, but the results were still encouraging in that larval weight gain was reduced by 50% in the presence of adequate amounts of inhibitor. When 8-day-old larvae were fed different KTi ₃ -expressing tobacco plants, a highly significant (P<0.01) correlation was observed between inhibitor content and larval live weight. Larval weight gain was found to be dependent on midgut proteolytic activity. On the basis of the evidence collected, it is suggested that further work is required to identify more specific inhibitors for the main proteinases of the target insect. Received: 30 March 1998 / Accepted: 9 December 1999     

How do caterpillars cope with xenobiotics? The case of Mythimna unipuncta,a species with low susceptibility to Bt

Mythimna unipuncta is a species with low susceptibility to the Bacillus thuringiensis (Bt) toxin, and this insect occasionally causes devastating damage to maize. In the study region, M. unipuncta‐developed larvae were observed moving from a non‐Bt crop to a nearby Bt crop. Although the first response of many caterpillars to xenobiotics, such as the Bt toxin, is to reduce food intake and prolong development, few studies have focused on the causes and consequences of this response in terms of resistance evolution. To clarify the causes of this response, this work compared changes in the feeding behaviour, cytochrome P450 expression and juvenile hormone titre during the last larval instar of M. unipuncta after Bt ingestion. Four P450 enzymes related to the xenobiotic metabolism of the CYP9 and CYP6 families were identified. Developed larvae fed the Bt diet reduced their food intake and CYP9 expression, experienced prolonged development and presented an altered juvenile hormone balance. The CYP9s were not increased in the larvae that consumed Bt, as previously expected, although their highest expression was observed when larval feeding increased. The high recovery capacity of the larvae contributed to their development when they were fed a non‐Bt diet. The efficiency of responses that act jointly as a defence mechanism against Bt might favour the development of field resistance to the toxin. Therefore, these responses should be further investigated for resistance management programmes.     

Growth and development of Bacillus thuringiensis Cry1Ab-susceptible and Cry1Ab-resistant sugarcane borer on diet and conventional maize plants

The sugarcane borer, Diatraea saccharalis (F.) (Lepidoptera: Crambidae), is a dominant maize borer pest and a major target of Bacillus thuringiensis (Bt)‐maize in Louisiana and the Gulf Coast area of Texas (USA). Growth and development of D. saccharalis on non‐toxic diet, diet treated with three low concentrations (0.01, 0.05, and 0.1 μg g^?1) of Cry1Ab toxin, and on non‐Bt maize plants were compared for five insect genotypes: a Bt‐susceptible strain (BT‐SS), a Cry1Ab‐resistant strain (BT‐RR), a back‐crossed and re‐selected resistant strain (BT‐R’R’), and two F₁ progeny of the BT‐SS and BT‐R’R’ strains. Fitness of the five genotypes was examined by infesting neonates on diet with/without Cry1Ab toxin in the laboratory and on intact non‐Bt maize plants in the greenhouse. Biological parameters measured were neonate‐to‐pupa development time and pupation rate, larval survival, larval and pupal weight, and sex ratio. Larvae of BT‐SS and BT‐R’R’ on non‐toxic diet and non‐Bt maize plants grew normally and there were no significant differences between the two strains in all measured parameters, suggesting a lack‐of‐fitness cost of the Cry1Ab resistance in D. saccharalis. Except for the development time on non‐Bt diet, all other parameters on both non‐Bt diet and non‐Bt maize plants were similar among the five genotypes. Larval development of BT‐SS was significantly affected on diet treated with Cry1Ab toxin at 0.05 and 0.1 μg g^?1, whereas the effect to BT‐RR and BT‐R’R’ was not significant. Pupal weight and sex ratio reared on Cry1Ab‐diet were similar and there were no significant differences among the five genotypes. Neonate‐to‐pupation rate decreased as Cry1Ab concentrations increased but the decrease was more significant for BT‐SS than for the other four genotypes. The lack‐of‐fitness costs of Bt resistance in D. saccharalis imply a greater challenge in managing Bt resistance for this maize borer species.     

Preference of Bt‐resistant and susceptible Busseola fusca moths and larvae for Bt and non‐Bt maize

The sustainability of genetically engineered insecticidal Bacillus thuringiensis Berliner (Bt) maize, Zea mays L. (Poaceae), is threatened by the evolution of resistance by target pest species. Several Lepidoptera species have evolved resistance to Cry proteins expressed by Bt maize over the last decade, including the African maize stem borer, Busseola fusca (Fuller) (Lepidoptera: Noctuidae). The insect resistance management (IRM) strategy (i.e., the high‐dose/refuge strategy) deployed to delay resistance evolution is grounded on certain assumptions about the biology and ecology of a pest species, for example, the interactions between the insect pest and crop plants. Should these assumptions be violated, the evolution of resistance within pest populations will be rapid. This study evaluated the assumption that B. fusca adults and larvae select and colonize maize plants at random, and do not show any preference for either Bt or non‐Bt maize. Gravid female B. fusca moths of a resistant and susceptible population were subjected to two‐choice oviposition preference tests using stems of Bt and non‐Bt maize plants. Both the number of egg batches as well as the total number of eggs laid on each stem were recorded. The feeding preference of Bt‐resistant and susceptible neonate B. fusca larvae were evaluated in choice test bioassays with whorl leaf samples of specific maize cultivars. Although no differential oviposition preference was observed for either resistant or susceptible female moths, leaf damage ratings indicated that neonate larvae were able to detect Bt toxins and that they displayed feeding avoidance behaviour on Bt maize leaf samples.     

Identification of molecular markers linked to quantitative trait loci for soybean resistance to corn earworm

 One hundred and thirty nine restriction fragment length polymorphisms (RFLPs) were used to construct a soybean (Glycine max L. Merr.) genetic linkage map and to identify quantitative trait loci (QTLs) associated with resistance to corn earworm (Helicoverpa zea Boddie) in a population of 103 F₂-derived lines from a cross of ‘Cobb’ (susceptible) and PI229358 (resistant). The genetic linkage map consisted of 128 markers which converged onto 30 linkage groups covering approximately 1325 cM. There were 11 unlinked markers. The F₂-derived lines and the two parents were grown in the field under a plastic mesh cage near Athens, Ga., in 1995. The plants were artificially infested with corn earworm and evaluated for the amount of defoliation. Using interval-mapping analysis for linked markers and single-factor analysis of variance (ANOVA), markers were tested for an association with resistance. One major and two minor QTLs for resistance were identified in this population. The PI229358 allele contributed insect resistance at all three QTLs. The major QTL is linked to the RFLP marker A584 on linkage group (LG) ‘M’ of the USDA/Iowa State University public soybean genetic map. It accounts for 37% of the total variation for resistance in this cross. The minor QTLs are linked to the RFLP markers R249 (LG ‘H’) and Bng047 (LG ‘D1’). These markers explain 16% and 10% of variation, respectively. The heritability (h²) for resistance was estimated as 64% in this population. Received: 15 October 1997 / Accepted: 4 November 1997     

A review on comparative data concerning <Emphasis Type="Italic">Fusarium</Emphasis> mycotoxins in Bt maize and non-Bt isogenic maize

The European corn borer reportedly promotes the infection of maize by Fusarium spp. Stalk and ear rots caused by Fusarium spp. are often related to mycotoxin accumulation in maize kernels. As a result, food and animal feed from maize are more severely contaminated with Fusarium mycotoxins: e.g. fumonisins (FUM), deoxynivalenol (DON) and zearalenone (ZEA). Bt maize is primarily an important potential tool for insect pest protection, both in the European Union and in other countries. Bt maize carrying the Bt genes is highly resistant to European corn borer larval feeding due to Bt toxin (δ toxin) production. Effective measures to combat pests therefore often have a positive side-effect in that they also reduce mycotoxin levels. Comparative analysis was used to the evaluation of the studies dealing with the reduction of Fusarium mycotoxins in Bt maize. Nineteen out of 23 studies on Bt maize came to the conclusion that Bt maize is less contaminated with mycotoxins (FUM, DON, ZEA) than the conventional control variety in each case.