Evaluation of soil‐applied insecticides with Bt maize for managing corn rootworm larval injury

The western corn rootworm, Diabrotica virgifera virgifera LeConte, is considered one of the most significant insect pests of maize in North America. Larvae of other secondary subterranean pests such as grape colaspis, Colaspis brunnea (F.), and Japanese beetle, Popillia japonica Newman, can also injure maize seedlings and cause yield loss. In the past decade, maize hybrids containing Bt proteins have been used to manage the western corn rootworm; additionally, seeds are commonly treated with a neonicotinoid and fungicide combination to control secondary pests. Recently, soil‐applied insecticides have been used in conjunction with rootworm Bt hybrids (and seed‐applied insecticides) in areas with perceived risk for increased rootworm larval or secondary pest damage. We conducted a series of trials from 2009 to 2011 that examined multiple rootworm Bt hybrids and their near‐isolines, along with two soil‐applied insecticides, to determine whether the Bt plus insecticide combination resulted in an increased level of efficacy or yield. We also sampled for Japanese beetle and grape colaspis larvae to determine their potential for reducing yield. Densities of secondary pests in our trials were low and likely had no effect on maize yield. The addition of a soil‐applied insecticide to rootworm Bt hybrids improved efficacy only once across 17 location‐years, when overall corn rootworm injury was highest; an improvement in yield was never observed. Our results suggest that the use of a soil‐applied insecticide with a rootworm Bt hybrid should only be considered in scenarios with potentially significant rootworm larval populations. However, potential negative consequences related to trait durability when soil insecticides are used with rootworm Bt maize should be considered.     

Interaction among planting dates,transgenic maize,seed treatment,corn rootworm damage and grain yield

A study was conducted from 2006 to 2008 at South Charleston and Wooster, Ohio, USA, to evaluate the potential use of planting dates in combination with Bacillus thuringiensis transgenic maize and insecticidal seed treatments to manage the root feeding of western corn rootworm (Diabrotica virgifera virgifera, Coleoptera: Chrysomelidae). Three planting dates (early, middle and late), targeting late April/early, mid‐May and early June, respectively, were used. We planted six hybrid treatments consisting of two seed‐treated hybrids with seed treatment, two transgenic hybrids and two untreated hybrids, each set represented by one short and one full season maturity hybrid. When root injury was high, significant lodging and stunted growth were observed on untreated maize and declined when planting was delayed. Root injury by rootworm larval feeding was significantly reduced by later planting maize, that is, early June. The use of transgenic maize and seed treatment also significantly reduced root injury by rootworm larvae. The influence of planting date on grain yield was inconsistent from year to year. Grain yields from short season hybrids were comparable to full season hybrids especially on later plantings. These results showed that the use of a seed treatment and transgenic maize might be beneficial only when rootworm population is high and planting is early.     

Estimating western corn rootworm (Coleoptera: Chrysomelidae) larval susceptibility to event DAS‐59122‐7 maize

Susceptibility of Diabrotica virgifera virgifera (LeConte) larvae to DAS‐59122‐7 maize was evaluated using a laboratory technique that measures rootworm survival to adulthood on maize seedlings. This method produces direct measures of larval susceptibility using realistic exposure to the same range of insecticidal protein concentrations found in field‐grown DAS‐59122‐7 maize roots. First, second and third instars were reared to adulthood on DAS‐59122‐7 maize seedlings or a non‐transgenic, near‐isoline maize. Data on survival, adult gender ratio, adult weight and median emergence were collected. Overall, larval susceptibility to DAS‐59122‐7 maize was lower than earlier predictions ( Storer et al. 2006 ). Neonate survival on DAS‐59122‐7 maize was approximately 33% of isoline survival after 17 days, and the same 33% recovered and developed to adulthood when the isoline maize was substituted. Survival rate on DAS‐59122‐7 maize increased with instar. The mean survivorship was 0.5%, 26% and 65% when exposure to DAS‐59122‐7 maize began at the first, second and third instars, respectively. Exposure to DAS‐59122‐7 maize led to sub‐lethal effects on adult gender ratio, weight and median emergence. These effects decreased when exposure to DAS‐59122‐7 maize began at later instars. The killing effect of DAS‐59122‐7 maize on rootworm larvae appeared to result from the combined chronic effects and absence of a suitable host as perceived by the larvae. The relevance of these data and the methodology of estimating rootworm susceptibility to plant‐incorporated protectants are discussed in the context of the US Environmental Protection Agency’s functional definition of ‘high dose’ and use of refuge for resistance management ( EPA 1998a ). Based on these results it is evident that DAS‐59122‐7 maize does not meet the functional definitions of high dose as described by EPA (1998a,b) and ILSI (1999) , and the utility of refuge, refuge size and refuge placement for delaying rootworm resistance should be further investigated.     

Transgenic maize hybrids as a tool to control Sesamia cretica Led. compared by conventional method of control on normal hybrids

This study was conducted on Bt and normal maize hybrids during two successive sowing dates, early and late summer plantation. The Sesamia cretica larvae were observed for the first time on first week of April on different commercial maize hybrids. While there was no larva/plant on transgenic maize hybrids, YieldGard and Ageeb-YG, while larvae were found on last week of July by a very little number on maize hybrids for late one. In Early summer planting date showed highly significant differences in reduction percent, between tested compounds and control one, in the pink corn borer population. The bio-residual activity of the NeemAzal-T/S 0.5% on TWC310 was significantly less pronounced, i.e. 75.5%, than other tested ones; the highest percentage of reduction, 86.6%, was achieved by Chlorophan 0.005% on SC2031, followed by NeemAzal-T/S 0.5% on SC2031 83.5% and Chlorophan 0.005% on SC2030 83.4%, respectively. Also the average reduction percentage of S. cretica larvae differed significantly from 69.6%, on NeemAzal-T/S SC2031, to 83.0%, on Chlorophan SC2031, in the late summer planting. The different treatments had positive effects on maize yield in the two planting dates. In the first plantation date, the increased rate of control ranged from 1.04 to 1.24, when NeemAzal-T/S acted on TWC310 and transgenic IYG, respectively. And in the second plantation date, rate of control obtained was 1.06–1.30, when NeemAzal-T/S acted on SC 2031 and transgenic IYG, respectively. The tested compounds played an important role in controlling the pink corn borer insect, S. cretica, and this study suggests that Cry 1Ab-expressing corn hybrids would provide a great value as a component of corn IPM in Egypt.     

Mortality impact of MON863 transgenic maize roots on western corn rootworm larvae in the field

Mortality of western corn rootworm (Diabrotica virgifera virgifera LeConte) due to feeding on MON863 transgenic maize (Zea mays L.) expressing the Cry3Bb1 protein was evaluated at three Missouri sites in both 2003 and 2004 and at one site each in South Dakota, Nebraska and Iowa in 2004. To do this, survivorship relative to survivorship on isoline maize (i.e. the same genetic background, but without Cry3Bb1) was evaluated. Comparisons were made using low (1650–2500 eggs/m) and high (3300–3500 eggs/m) western corn rootworm egg densities. Significantly fewer beetles were recovered from MON863 than from isoline maize. Emergence from MON863 as a percentage of viable eggs ranged from 0.02% to 0.10%, whereas percentage emergence from isoline maize ranged from 1.09% to 7.14%. Survivorship on MON863 relative to survivorship on isoline averaged 1.51% when averaged across all environments and both years, so mortality because of the Cry3Bb1 protein averaged 98.49%. The average time delay to 50% cumulative beetle emergence from MON863 was 18.3 days later than from isoline maize. Females comprised 56% and 71% of total beetles recovered from MON863 in 2003 and 2004, respectively. Results are discussed in relation to insect resistance management (IRM) of western corn rootworm.     

Field-evolved resistance to Bt maize by western corn rootworm

Background

Crops engineered to produce insecticidal toxins derived from the bacterium Bacillus thuringiensis (Bt) are planted on millions of hectares annually, reducing the use of conventional insecticides and suppressing pests. However, the evolution of resistance could cut short these benefits. A primary pest targeted by Bt maize in the United States is the western corn rootworm Diabrotica virgifera virgifera (Coleoptera: Chrysomelidae).

Methodology/Principal Findings

We report that fields identified by farmers as having severe rootworm feeding injury to Bt maize contained populations of western corn rootworm that displayed significantly higher survival on Cry3Bb1 maize in laboratory bioassays than did western corn rootworm from fields not associated with such feeding injury. In all cases, fields experiencing severe rootworm feeding contained Cry3Bb1 maize. Interviews with farmers indicated that Cry3Bb1 maize had been grown in those fields for at least three consecutive years. There was a significant positive correlation between the number of years Cry3Bb1 maize had been grown in a field and the survival of rootworm populations on Cry3Bb1 maize in bioassays. However, there was no significant correlation among populations for survival on Cry34/35Ab1 maize and Cry3Bb1 maize, suggesting a lack of cross resistance between these Bt toxins.

Conclusions/Significance

This is the first report of field-evolved resistance to a Bt toxin by the western corn rootworm and by any species of Coleoptera. Insufficient planting of refuges and non-recessive inheritance of resistance may have contributed to resistance. These results suggest that improvements in resistance management and a more integrated approach to the use of Bt crops may be necessary.     

转Bt基因玉米的生态安全性研究进展

随着转基因作物的应用和推广 ,转 Bt基因作物释放后对生态环境及其它方面产生的潜在影响越来越受到重视。分别从生物活性杀虫晶体蛋白在土壤中的残留特性、杀虫晶体蛋白对土壤中非目标生物的影响、转 Bt基因玉米植株体成分的变化、转Bt基因玉米花粉中杀虫晶体蛋白的表达特性及其在田间和马力筋叶片上的散积状况、花粉中表达的杀虫晶体蛋白对君主斑蝶的毒性、君主斑蝶幼虫暴露在 Bt花粉中的概率及综合风险评价估算等方面对转 Bt基因玉米产生的杀虫晶体蛋白与土壤生态环境的相互作用、花粉对非目标生物影响的研究现状进行了综述。通过对转 Bt基因作物生态安全性的科学评价和广泛宣传 ,以确保生物技术的健康发展。     

Estimation of efficacy functions for products used to manage corn rootworm larval injury

Maize, Zea mays L., is an economically important crop grown throughout the world. Corn rootworm, Diabrotica spp. (Coleoptera: Chrysomelidae), larvae constitute a significant economic threat to maize production in the United States, where yield losses and management costs associated with corn rootworm species exceed $1 billion annually. Furthermore, the introduction of the western corn rootworm, D. virgifera virgifera LeConte, into maize‐producing regions of Europe has made managing corn rootworm larval injury an international concern. Larvae injure maize plants by feeding on root tissue and are the primary target of management activities. Products commonly used to protect root systems from injury include chemical insecticides (seed or soil applied) and genetically modified maize hybrids expressing toxins derived from Bacillus thuringiensis Berliner (Bt). The confirmation of field‐evolved resistance to various Bt toxins in populations of the western corn rootworm presents a significant management challenge. We performed a meta‐analysis to provide a broad understanding of the relative efficacy of the primary products currently being used to manage corn rootworm larval injury, including insecticidal seed treatments, soil insecticides and Bt hybrids (with and without the addition of soil insecticide). Our analysis is unique in the breadth of locations and years included – we analysed 135 individual trials conducted from 2003 through 2014 at multiple sites in both Illinois and Nebraska. Panel data were produced by pairing the mean node‐injury rating for each treatment of a given trial with the mean node‐injury rating for untreated maize. Linear regression models were developed to estimate the relationship between the potential for corn rootworm larval injury and product performance. For a given level of injury potential, the parameters estimated reveal differences in the degree of root protection offered by the various product categories analysed. Implications for developing long‐term, integrated, and sustainable practices for managing this important pest of maize are discussed.     

Susceptibility of field populations of sugarcane borer from non‐Bt and Bt maize plants to five individual Cry toxins

Abstract Sugarcane borer, Diatraea saccharalis (F.), is a major target of transgenic maize expressing Bacillus thuringiensis (Bt) proteins in South America and the US mid‐south region. Resistance development in target pest populations is a major threat to the sustainable use of Bt crops. In our field trials in 2009, a significant number of live borers and plant injury from D. saccharalis were observed in an experimental SmartStax? maize line. The objective of this study was to assess the relative susceptibility of two field populations of D. saccharalis collected from non‐Bt and Bt maize plants containing SmartStax? traits to five individual Cry proteins. The five Bt proteins included two proteins (Cry1A.105 and Cry2Ab2) that were expressed in SmartStax? maize plants and three other common Bt proteins (Cry1Aa, Cry1Ab and Cry1Ac) that were not produced in SmartStax?. Larval mortality and growth inhibition on Bt diet of the fourth generation after field collections were evaluated 7 days after release of neonates on the diet surface. The laboratory bioassays showed that 50% lethal concentration (LC₅₀) values for Cry1A.105 and Cry2Ab2 for the population originated from Bt plants were 3.55‐ and 1.34‐fold greater, respectively, than those of the population collected from non‐Bt plants. In contrast, relative to the population from non‐Bt plants, the LC₅₀ of the population sampled from Bt plants were 3.85‐, 2.5‐ and 1.64‐fold more sensitive to Cry1Aa, Cry1Ab and Cry1Ac, respectively. The results did not provide clear evidence to conclude that the observed field survival of D. saccharalis on Bt plants was associated with increased levels of resistance.     

Influence of western corn rootworm (Coleoptera: Chrysomelidae) larval injury on yield of different types of maize

Two field experiments were conducted in 1995-1996 to determine if there are common yield responses among maize hybrids to larval western corn rootworm, Diabrotica virgifera virgifera LeConte injury. Three yellow dent hybrids, five white food grade dent hybrids, and a popcorn hybrid were included in the study. The minimum level of rootworm injury as measured by root damage ratings (3.2-4.2) that significantly reduced yield was similar across the hybrids included in the study. However, the pattern of yield response to different rootworm injury levels varied among hybrids. This suggests that maize hybrids may inherently differ in their ability to tolerate rootworm injury and partition biomass in response to injury and other stresses. The complex interaction among hybrid, level of injury, and other stresses suggests that a common western corn rootworm injury-yield relationship may not exist within maize.     

Belowground resistance to western corn rootworm in lepidopteran-resistant maize genotypes

Several maize, Zea mays L., inbred lines developed from an Antiguan maize population have been shown to exhibit resistance to numerous aboveground lepidopteran pests. This study shows that these genotypes are able to significantly reduce the survival of two root feeding pests, western corn rootworm, Diabrotica virgifera virgifera LeConte, and southern corn rootworm, Diabrotica undecimpunctata howardi Barber. The results also demonstrated that feeding by the aboveground herbivore fall armyworm, Spodoptera frugiperda (J. E. Smith), before infestation by western corn rootworm reduced survivorship of western corn rootworm in the root tissues of some, but not all, genotypes. Likewise, the presence of western corn rootworm in the soil seemed to increase resistance to fall armyworm in the whorl in several genotypes. However, genotypes derived from the Antiguan germplasm with genetic resistance to lepidopterans were still more resistant to the fall armyworm and both rootworm species than the susceptible genotypes even after defense induction. These results suggest that there may be intraplant communication that alters plant responses to aboveground and belowground herbivores.     

Dispersal capacity in the Mediterranean corn borer, Sesamia nonagrioides

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

Larval mortality and development for rotation‐resistant and rotation‐susceptible populations of western corn rootworm on Bt corn

The western corn rootworm, Diabrotica virgifera virgifera LeConte, is one of the most economically important insect pests threatening the production of corn, Zea mays (L.), in the United States. Throughout its history, this insect has displayed considerable adaptability by overcoming a variety of pest management tactics, including the cultural practice of annual crop rotation. Since first reported in Illinois in the late 1980s, populations of the rotation‐resistant western corn rootworm have spread over a wide area of the eastern Corn Belt. Currently, little information is available concerning the interaction of rotation resistance with the use of genetically modified corn expressing insecticidal toxins from Bacillus thuringiensis Berliner (Bt), a popular tactic for preventing larval injury and its associated yield loss. The goal of this greenhouse experiment was to determine whether rotation‐resistant and rotation‐susceptible western corn rootworm larvae differ with respect to survival or development when exposed to single‐ or dual‐toxin (pyramided) Bt corn. Individual corn plants were infested with 225 near‐hatch eggs at the V5 (five leaf collar) growth stage. Larvae developed undisturbed on the root systems for 17 days, after which they were recovered using Berlese–Tullgren funnels. Surviving larvae were counted to estimate mortality, and head capsule widths were measured to assess development. Rotation‐resistant and rotation‐susceptible larvae had statistically similar mean levels of mortality and head capsule widths when exposed to both single‐toxin (Cry3Bb1 or Cry34/35Ab1) and pyramided (Cry3Bb1+ Cry34/35Ab1) Bt corn, suggesting that these two populations do not differ with respect to survival or development when exposed to Bt corn. Additionally, the statistically similar mean levels of mortality for larvae exposed to single‐toxin and pyramided Bt corn suggest that pyramided Bt hybrids containing the Cry3Bb1 and Cry34/35Ab1 toxins do not result in additive mortality for western corn rootworm larvae. Implications for management of this economically important pest are discussed.     

A spatial model of the development of pest resistance to a transgenic insecticidal crop: European corn borer on Bt maize

A mathematical model was constructed to describe the evolution of resistance to the Bacillus thuringiensis toxin (Bt) in an insect pest (European corn borer) population on a transgenic crop (Bt corn). The model comprises a set of partial differential equations of the reaction-diffusion type; local interactions of three competing pest genotypes formed by alleles of Bt resistance and susceptibility are described as in the Kostitzin model, and the spread of insects is modeled as diffusion. The model was used to evaluate the influence of pest characteristics on the efficacy of the high-dose/refuge strategy aiming to prevent or delay the spread of Bt resistance in pest populations. It was shown, by contrast, that a model based on Fisher-Haldane-Wright equations and formally incorporating a diffusion term cannot adequately describe the evolution of Bt resistance in a spatially inhomogeneous pest population. Further development of the proposed demo-genetic model is discussed.     

Selection for resistance to mCry3A-expressing transgenic corn in western corn rootworm

To investigate the development of resistance to mCry3A, a laboratory colony of the western corn rootworm, Diabrotica virgifera virgifera LeConte, was established from field survivors of mCry3A-expressing (MIR604) corn, Zea mays L. Feral adults emerging from MIR604 (selected) and isoline (control) field plots were collected and returned to the laboratory. Progeny of each colony was reared one generation on isoline corn and then crossed reciprocally with a nondiapausing colony. The resulting nondiapausing progeny were then reared on greenhouse corn in accordance with the wild type parent's origin (on MIR604 or isoline corn). After four, seven, and 10 total generations of selection, the resistance ratio of the selected colony was 0.5, 4.3, and 15.4 in terms of lethal concentration (LC)50 values in toxicity assays, with the latter two LC50 values being significant. After seven generations of selection in total, selected and control colonies were screened on MIR604 and isoline corn under field conditions. There was a significant colony x corn pedigree interaction in terms of plant damage. There was no significant difference in damage between MIR604 and isoline corn, whereas this difference was significant for the control colony. After 14 generations of selection, a seedling bioassay was performed. Again, there was a significant colony x corn pedigree interaction, this time in terms of the number of larvae recovered. There was no significant difference in the number of larvae recovered from MIR604 and isoline corn for the selected colony, whereas this difference was significant for the control colony, although larval size was greater on isoline corn for both colonies. Resistance has developed in western corn rootworm laboratory colonies to all Bt proteins currently registered for corn rootworm management, which emphasizes the importance of adhering to resistance management plans for maintaining product efficacy.     

Effect of four cropping systems on variant western corn rootworm (Coleoptera: Chrysomelidae) adult and egg densities and subsequent larval injury in rotated maize

The cultural practice of rotating corn, Zea mays L., with soybean, Glycine max (L.) Merrill, to manage larval injury by the western corn rootworm, Diabrotica virgifera virgifera LeConte, was used extensively throughout east central Illinois and northern Indiana until the mid-1990s. The effectiveness of this management tactic diminished due to a shift in the ovipositional behavior of the western corn rootworm. The variant western corn rootworm has since spread as far as northwestern Illinois, southern Wisconsin, southern Michigan, and eastern Ohio. The objective of this study was to evaluate the influence of four cropping systems on adult and egg densities of the western corn rootworm and to quantify the level of root injury in rotated corn after each system. The four cropping systems used included: 1) corn; 2) soybean; 3) double-cropped winter wheat, Triticum aestivum L., followed by soybean; and 4) winter wheat. Research trials were conducted near Monmouth (northwestern), DeKalb (northern), and Urbana (east central), IL, during 2003 and 2004. Results indicated variant western corn rootworm adults can be found in all four treatments at each location and consequently no crop was immune to oviposition or root injury by corn rootworm larvae in rotated corn the following season. Adults were found primarily in corn and soybean, whereas egg densities were greatest in corn plots in all three locations in both years of the study. Root injury by larvae was most abundant in corn following corn at all three sites. Of the four systems evaluated, the use of wheat demonstrated the most potential for preventing yield reducing levels of root injury in rotated corn.     

Effects of transgenic Bt corn litter on the earthworm Lumbricus terrestris

A 200-day study was carried out to investigate the impact of transgenic Bacillus thuringiensis (Bt) corn on immature and adult Lumbricus terrestris in the field and in the laboratory. Another objective of this study was to develop test methods that could be used for standard testing of the impact of transgenic plants on different earthworm species in the field and in the laboratory. For this purpose two different experiments were involved, a laboratory experiment with adult L. terrestris and a field experiment with immature L. terrestris. No lethal effects of transgenic Bt corn on immature and adult earthworms were observed. Immature L. terrestris in the field had a very similar growth pattern when fed either (Bt+) or (Bt-) corn litter. No significant differences in relative weights of (Bt+) and (Bt-) corn-fed adult L. terrestris were observed during the first 160 days of the laboratory trial, but after 200 days adult L. terrestris had a significant weight loss of 18% of their initial weight when fed (Bt+) corn litter compared to a weight gain of 4% of the initial weight of (Bt-) corn-fed earthworms. Further studies are necessary to see whether or not this difference in relative weight was due to the Bt toxin or other factors discussed in the study. Degradation of Cry1Ab toxin in corn residues was significantly slower in the field than at 10 degrees C in the laboratory. Enzyme-linked immunosorbent assay results indicated that earthworms in both experiments were exposed to the Bt toxin throughout the whole experimental time.     

Characterizing laboratory colonies of western corn rootworm (Coleoptera: Chrysomelidae) selected for survival on maize containing event DAS-59122-7

Event DAS‐59122‐7 is a novel transgenic trait designed to protect the roots and yield potential of maize from the insect pest corn rootworm Diabrotica spp. (Col.: Chrysomelidae). The increased pest status of corn rootworm, exceptional efficacy of this trait, and anticipated increases in farm efficiency and grower and environmental safety will drive adoption of this trait. Strong grower acceptance of this trait highlights the importance of science‐based and practical resistance management strategies. A non‐diapause trait was introgressed into two laboratory colonies of Diabrotica virgifera virgifera collected from geographically distinct locations: Rochelle, IL and York, NE. Both colonies were divided and each reared on maize containing event DAS‐59122‐7 or its near isoline. Selected and unselected colonies were evaluated for phenotypic change in larval development, injury potential and survival to adulthood during 10 and 11 generations. The F1 generation of both selected colonies displayed increased larval development, survivorship and measurable, but economically insignificant increases in injury potential on DAS‐59122‐7 maize. Survival rates of 0.4 and 1.3% in F1 generations of both selected colonies corroborate field estimates of survival on DAS‐59122‐7 maize. Over later generations, total phenotypic variation declined gradually and irregularly. Despite the absence of random mating, the tolerance trait could not be fixed in either population after 10 or 11 generations of selection. An allele conferring major resistance to DAS‐59122‐7 was not identified in either selected colony. The assessment also concluded that major resistance gene(s) are rare in populations of D. v. virgifera in the United States, and that a minor trait(s) conferring a low level of survival on DAS‐59122‐7 maize was present. The tolerance trait identified in this study was considered minor with respect to its impact on DAS‐59122‐7 maize efficacy, and the role this trait may play in total effective refuge for major resistance genes with recessive inheritance is the basis of future work.     

Predicting western corn rootworm (Coleoptera: Chrysomelidae) larval injury to rotated corn with Pherocon AM traps in soybeans

Crop rotation for portions of east central Illinois and northern Indiana no longer adequately protects corn (Zea mays L.) roots from western corn rootworm, Diabrotica virgifera virgifera LeConte. Seventeen growers in east central Illinois monitored western corn rootworm adults in soybean (Glycine max L.) fields with unbaited Pherocon AM traps during 1996 and 1997. In the following years (1997 and 1998), growers left untreated strips (no insecticide applied) when these fields were planted with corn. Damage to rotated corn by rootworms was more severe in untreated than in treated strips of rotated corn, ranging from minor root scarring to a full node of roots pruned. Densities of western corn rootworms in soybean fields from 1996 were significantly correlated with root injury to rotated corn the following season. Adult densities from 1997 were not significantly correlated with root injury in 1998, due to heavy precipitation throughout the spring of 1998 and extensive larval mortality. Twenty-eight additional growers volunteered in 1998 to monitor rootworm adults in soybean fields with Pherocon AM traps based on recommendations that resulted from our research efforts in 1996 and 1997. In 1999, these 28 fields were rotated to corn, and rootworm larval injury was measured in untreated strips. Based on 1996-1997 and 1998-1999 data, a regression analysis revealed that 27% of the variation in root injury to rotated corn could be explained by adult density in soybeans the previous season. We propose a sampling plan for soybean fields and a threshold for predicting western corn rootworm larval injury to rotated corn.     

Degradation of the Cry1Ab protein within transgenic Bacillus thuringiensis corn tissue in the field

Large quantities of Bacillus thuringiensis (Bt) corn plant residue are left in the field after harvest, which may have implications for the soil ecosystem. Potential impacts on soil organisms will also depend on the persistence of the Bt toxin in plant residues. Therefore, it is important to know how long the toxin persists in plant residues. In two field studies in the temperate corn-growing region of Switzerland we investigated degradation of the Cry1Ab toxin in transgenic Bt corn leaves during autumn, winter and spring using an enzyme-linked immunosorbent assay (ELISA). In the first field trial, representing a tillage system, no degradation of the Cry1Ab toxin was observed during the first month. During the second month, Cry1Ab toxin concentrations decreased to approximately 20% of their initial values. During winter, there was no further degradation. When temperatures again increased in spring, the toxin continued to degrade slowly, but could still be detected in June. In the second field trial, representing a no-tillage system, Cry1Ab toxin concentrations decreased without initial delay as for soil-incorporated Bt plants, to 38% of the initial concentration during the first 40 days. They then continued to decrease until the end of the trial after 200 days in June, when 0.3% of the initial amount of Cry1Ab toxin was detected. Our results suggest that extended pre- and post-commercial monitoring are necessary to assess the long-term impact of Bt toxin in transgenic plant residues on soil organisms.