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

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

Modeling the development of resistance by stalk-boring lepidopteran insects (Crambidae) in areas with transgenic corn and frequent insecticide use

We simulated the population dynamics and population genetics of two bivoltine species of corn borers, the European corn borer, Ostrinia nubilalis (Hübner), and the southwestern corn borer, Diatraea grandiosella Dyar, in a hypothetical region of irrigated transgenic and nontransgenic corn where insecticide was applied only to the nontransgenic refuge crop. Over the 100-yr time horizon, resistance developed quickly in both species and to both transgenic corn and the insecticide when the allele for resistance to the respective toxin was dominant. When the allele for transgenic resistance was not dominant and the refuge location was constant over the time horizon, spraying the refuge to control southwestern corn borer had no effect on how quickly resistance to the transgenic corn developed. In contrast, the European corn borer developed resistance to transgenic corn much sooner when the refuge was sprayed once per year, and the time to 3% resistance allele frequency decreased as efficacy of the insecticide increased. Only when the refuge was treated less than once every 5 yr (10 generations) did the frequency of application decline enough to permit resistance management for the European corn borer to approximate the effectiveness of an unsprayed refuge. A consistently sprayed refuge <40% of the corn acreage was an inadequate resistance management strategy for the European corn borer even when a low efficacy insecticide (70% mortality) was used. When assumptions about European corn borer adult behavior were changed and the adults behaved similarly to adult southwestern corn borer, the development of resistance to the transgenic crop was slowed significantly.     

Evaluation of conventional resistance to European corn borer (Lepidoptera: Crambidae) and western corn rootworm (Coleoptera: Chrysomelidae) in experimental maize lines developed from a backcross breeding program

Plant resistance is a promising control method for the two most damaging insect pests of maize, Zea mays L.: the European corn borer, Ostrinia nubilalis (Hübner), and the western corn rootworm Diabrotica virgifera virgifera LeConte. Fifteen experimental lines of maize, derived from a backcross breeding program designed to introgress resistance to European corn borer from Peruvian maize into two U.S. Corn Belt adapted inbred lines, were evaluated for resistance to European corn borer and western corn rootwonrm. The experimental lines were in the second generation of backcrossing. All experimental lines were resistant to leaf blade feeding by European corn borer. These lines had low levels of 2,4-dihydroxy-7-methoxy-1,4-benzoxazin-3-one, a chemical commonly associated with leaf blade feeding resistance, indicating that this was not the mechanism of resistance to leaf blade feeding in these lines. Eleven experimental lines were resistant to leaf sheath and collar feeding by European corn borer. Useful sources of European corn borer ovipositional nonpreference and root feeding resistance to western corn rootworm were not identified. Some of the lines evaluated in this study may provide useful sources of resistance to both leaf blade and leaf sheath and collar feeding by European corn borer.     

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.     

Evaluating multiple approaches for managing western corn rootworm larvae with seed blends

Corn rootworm, Diabrotica spp., larvae represent a significant and widespread economic threat to corn, Zea mays (L.), production in the United States, where control costs and yield losses associated with these insect pests exceed $1 billion annually. Preventing root injury and associated yield loss caused by corn rootworm larvae may be accomplished by the independent use of planting time soil insecticides or transgenic Bt hybrids. However, recent reports of both confirmed and suspected Bt resistance in corn rootworm populations throughout the Corn Belt have led to significant interest in the use of these two management tactics simultaneously. Although this approach has been investigated to some extent previously, information is lacking on how the use of a soil insecticide in tandem with a Bt seed blend—Bt and refuge (non‐Bt) seed mixed into a single product—may affect root protection and yield. We describe an experiment including six trial sites conducted over a three‐year period where various seed blends and soil insecticide/seed blend combinations were evaluated. The predominant species contributing to root injury across all sites was the western corn rootworm (Diabrotica virgifera virgifera LeConte). A weighted technique is presented for evaluating root injury for seed blends that offers a reliable estimate of product performance. The addition of a soil insecticide to the seed blend treatments never resulted in significantly improved root protection and failed to provide a consistent yield benefit. Our results suggest that a soil insecticide/seed blend combination approach is not warranted. Additionally, a subanalysis performed on individual refuge and nearby Bt root systems for seed blend treatments provides insight into the spatial characteristics of root injury in seed blend scenarios.     

<Emphasis Type="Italic">Trichogramma Brassicae</Emphasis> and SLAM<Superscript>®</Superscript>, an Integrated Approach to Managing European Corn Borer and Corn Rootworms

European corn borer, Ostrinia nubilalis (Hübner) (Lepidoptera: Crambidae); western corn rootworm (WCRW), Diabrotica virgifera virgifera LeConte; and northern corn rootworm (NCRW), Diabrotica barberi (Smith & Lawrence) (Coleoptera: Chrysomelidae) are important pests of corn, Zea mays L., that occur simultaneously in the US Corn Belt. Areawide management strategies for northern and western corn rootworm in Iowa, Kansas, South Dakota, Illinois, and Indiana, and for the Mexican strain in Texas, use SLAM^®, which contains cucurbitacin, a feeding stimulant, and a small amount of the insecticide carbaryl. This management strategy is environmentally sound because it introduces a substantially lesser amount of chemical insecticide into the environment than prophylactic soil insecticide applications for WCRW management. To develop a management program for both corn pests, the compatibility of SLAM^® to control CRW, along with a biological agent to manage European corn borer, was investigated. Laboratory studies were conducted using the egg parasitoid Trichogramma brassicae (Bezdenko) (Hymenoptera: Trichogrammatidae). In experiments in which SLAM^® and T. brassicae were evaluated as components of a management system, SLAM^® did not affect parasitism or parasitoid emergence rates. Trichogramma brassicae displayed vigorous biotic fitness, emergence rates, sex ratios, and fecundity over the duration of the study. Data collected under these laboratory conditions indicated that SLAM^® had no adverse effect on the quality of T. brassicae, indicated by its ability to reproduce in the filial generation.     

Integrating biological and chemical controls in decision making: European corn borer (Lepidoptera: Crambidae) control in sweet corn as an example

As growers switch to transgenic crops and selective insecticides that are less toxic to natural enemies, natural enemies can become more important in agricultural pest management. Current decision-making guides are generally based on pest abundance and do not address pest and natural enemy toxicity differences among insecticides or the impact of natural enemies on pest survival. A refined approach to making pest management decisions is to include the impact of natural enemies and insecticides, thereby better integrating biological and chemical control. The result of this integration is a dynamic threshold that varies for each product and the level of biological control expected. To demonstrate the significance of conserved biological control in commercial production, a decision-making guide was developed that evaluates control options for European corn borer, Ostrinia nubilalis (Hübner) (Lepidoptera: Crambidae), in sweet corn, Zea mays L., where the primary natural enemies are generalist predators. Management options are lambda-cyhalothrin (broad-spectrum insecticide), spinosad (selective insecticide), Trichogramma ostriniae (Peng and Chen) (Hymenoptera: Trichogrammatidae) (parasitoid), and Bacillus thuringiensis (Bt) sweet corn (transgenic variety). The key factors influencing thresholds for all treatments are the intended market, predator populations, and the presence of alternative foods for the predators. Treatment cost is the primary factor separating the threshold for each treatment within a common scenario, with the lowest cost treatment having the lowest pest threshold. However, when the impact of a treatment on natural enemies is projected over the 3-wk control period, the impact of the treatment on predators becomes the key factor in determining the threshold, so the lowest thresholds are for broad-spectrum treatments, whereas selective products can have thresholds > 6 times higher by the third week. This decision guide can serve as a framework to help focus future integrated pest management research and to aid in the selection of pest management tools.     

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.     

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.     

Responses of transgenic maize hybrids to variant western corn rootworm larval injury

Abstract:  In 2005 and 2006, transgenic insecticidal maize hybrids (YieldGard Rootworm, MON 863, Cry3Bb1, Vector ZMIR 13L) were evaluated for their ability to limit root injury caused by western corn rootworm ( Diabrotica virgifera virgifera LeConte) larval feeding. Hybrids in each year of the experiment were planted in plots that had been devoted to a trap crop (late-planted maize interplanted with pumpkins) the previous growing season. All maize hybrids were provided by Monsanto Company and the genetic backgrounds remain unknown to the investigators. In 2005, the experiment was conducted in Urbana, Illinois. Urbana is located in east central Illinois, an area of the state in which a variant of the western corn rootworm has overcome the pest management benefits of crop rotation. Variation in root injury was noted across the maize hybrids in 2005 and the level of pruning increased from 20 July to 9 August for most hybrids. In 2006, the experiment was conducted in two locations, Monmouth and Urbana, Illinois. Monmouth is located in north-western Illinois and is within an area of the state in which densities of the variant of the western corn rootworm are lower than in east-central Illinois. In 2006, variation in root protection was again observed across the maize hybrids. Root injury differences among the hybrids were more prominent at the Urbana site. Similar to the previous year, root injury increased from the third week in July to the first week of August at both locations with this increase most noticeable at the Urbana location. We hypothesize that the variant western corn rootworm may be able to inflict more root injury to these transgenic insecticidal maize hybrids than the non-variant population of this species.     

Post-establishment movement of western corn rootworm larvae (Coleoptera: Chrysomelidae) in Central Missouri corn

If registered, transgenic corn, Zea mays L., with corn rootworm resistance will offer a viable alternative to insecticides for managing Diabrotica spp. corn rootworms. Resistance management to maintain susceptibility is in the interest of growers, the Environmental Protection Agency, and industry, but little is known about many aspects of corn rootworm biology required for an effective resistance management program. The extent of larval movement by the western corn rootworm, Diabrotica virgifera virgifera LeConte, that occurs from plant-to-plant or row-to-row after initial establishment was evaluated in 1998 and 1999 in a Central Missouri cornfield. Post-establishment movement by western corn rootworm larvae was clearly documented in two of four treatment combinations in 1999 where larvae moved up to three plants down the row and across a 0.46-m row. Larvae did not significantly cross a 0.91-m row after initial host establishment in 1998 or 1999, whether or not the soil had been compacted by a tractor and planter. In the current experiment, western corn rootworm larvae moved from highly damaged, infested plants to nearby plants with little to no previous root damage. Our data do not provide significant insight into how larvae might disperse after initial establishment when all plants in an area are heavily damaged or when only moderate damage occurs on an infested plant. A similar situation might also occur if a seed mixture of transgenic and isoline plants were used and if transgenic plants with rootworm resistance are not repellent to corn rootworm larvae.     

Planting transgenic insecticidal corn based on economic thresholds: consequences for integrated pest management and insect resistance management

A simulation model of the western corn rootworm, Diabrotica virgifera virgifera LeConte, was used to investigate whether sampling and economic thresholds can improve integrated pest management (IPM) and insect resistance management (IRM) when transgenic insecticidal crops are used for insect pest management. When transgenic corn killed at least 80% of susceptible larvae, the calculated economic threshold increased linearly as the proportion of susceptible beetles surviving the toxin increased. The use of economic thresholds slightly slowed the evolution of resistance to transgenic insecticidal crops. In areas with or without rotation-resistant western corn rootworm phenotypes, the use of sampling and economic thresholds generated similar returns compared with strategies of planting transgenic corn, Zea mays L., every season. Because transgenic crops are extremely effective, farmers may be inclined to plant transgenic crops every season rather than implementing costly and time-consuming sampling protocols.     

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

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

Economic analysis of planting dates to manage European corn borer (Lepidoptera: Crambidae) with Bt corn

Planting dates of transgenic Bacillus thuringiensis Berliner (Bt) corn were adjusted to determine the utility in managing European corn borer, Ostrinia nubilalis (Hübner). Transgenic Bt (events 176 and Bt11) corn and non-Bt corn were planted at three different times to use the early- and late- planted corn as a potential trap crop for ovipositing European corn borer moths. Grain moisture and yields were recorded to determine the economic benefits of Bt corn planted on the different dates, based on European corn borer populations and corn damage data collected before harvest. Data were recorded from three locations in southwestern, central, and northeastern Iowa for three summers (1996-1998). Economic benefits are discussed in relation to EILs and yield results. Adjusting the planting dates of Bt and non-Bt corn provided variable economic differences among planting dates in northern Iowa; however, greater economic benefits were realized when Bt corn was planted late during the planting sequence in central and southwestern Iowa. These results suggest that planting corn should be conducted in a timely manner and, if delayed or required to plant late, planting Bt corn would likely provide greater economic benefits. Although yield and economic variability were high, using Bt corn in combination with planting date adjustments may be a viable option for managing European corn borer.     

Multiacreage evaluation of aerially applied adherent malathion granules for selective insect control and indirect reduction of mycotoxigenic fungi in specialty corn

Aerially applied adherent corn flour granules containing 1% malathion were more often as, or more, effective than 15% chlorpyrifos (Lorsban 15G) granules in controlling caterpillars and sap beetles in high amylose corn in 1997 than 1996. Use of malathion granules corresponding closely in size to chlorpyrifos granules in the second year of the study apparently increased relative efficacy. Significantly less corn borer damage occurred on plants (1996) or ears (1997) within 2 wk of application for both types of insecticide granules compared with untreated plots. In 1997, there were sixfold fewer milk stage ears with more than 20 kernels damaged per ear in the malathion-treated plots compared with chlorpyrifos-treated plots, and severity of caterpillar damage was also less in malathion versus chlorpyrifos-treated plots at harvest. Control of beetles (corn rootworm adults and sap beetles) for both treatments was less effective compared with caterpillars. Significant corn rootworm adult control was noted for both chlorpyrifos and malathion in 1996 and significant sap beetle control was noted for the malathion granules in 1997. Significantly fewer live lady beetles, and more dead lady beetles were present in chlorpyrifos-treated plots compared with malathion-treated or untreated plots in 1996. The incidence and severity of Fusarium mold on ears at harvest was often indirectly reduced by both malathion treatments and chlorpyrifos treatments, with the malathion treatment significantly better than the chlorpyrifos treatment in one case.     

Insecticide enhancement with feeding stimulants in corn for western corn rootworm larvae (Coleoptera: Chrysomelidae)

Amounts of the insecticide thiamethoxam required for 50% mortality of western corn rootworm larvae, Diabrotica virgifera virgifera LeConte, were reduced 100-fold when extracts of germinating corn, Zea mays L., were used to entice neonate larvae to feed on it. In behavioral bioassays, neonate rootworm larvae fed vigorously on filter paper disks treated with liquid pressed from corn roots. Moreover, disks treated with an acetone extract of corn (dried and rewetted with water) also elicited strong feeding from larvae. Larvae wandered away from filter paper disks treated with distilled water without feeding. Dilutions of thiamethoxam were tested in the bioassay alone or with corn extract and the efficacy of this insecticide was improved by the addition of the corn extract. For solutions containing 10 ppm thiamethoxam, 95% larval mortality occurred after 30 min of exposure when corn extract was present, but only 38% mortality occurred when the same concentration of insecticide alone (no feeding stimulants) was tested. Larval mortality after 24 h was significantly higher for corn extract-treated disks with 0.01, 0.1, 1, or 10 ppm insecticide than for the same concentrations without corn extract. Thiamethoxam did not deter larval feeding on corn extract, even at the highest concentration of thiamethoxam tested.     

Nontarget arthropod abundance in areawide-managed corn habitats treated with semiochemical-based bait insecticide for corn rootworm (Coleoptera: Chrysomelidae) control

Impacts of semiochemical-based insecticidal bait applications on beneficial arthropod groups common to field corn, Zea mays L., habitats were assessed in areawide-managed field sites in South Dakota and Iowa during 1997 and 1998. Slam, a commercial bait formulation comprised of 87% cucurbitacin and 13% carbaryl insecticide, was used for management of adult rootworm, Diabrotica spp., and controls consisted of cornfield habitats without bait applications. Effects on beneficial organisms were variable, and negative impacts were infrequent. Coccinellidae, Staphylinidae, and Anthocoridae were usually more abundant in bait-treated plots than in controls that received at-plant soil insecticides, especially by 4 wk postapplication. Carabid beetle activity also had increased in bait-treated corn by proportionally greater rates than in control plots at 4 wk postapplication in two of the four site by year combinations in this study. Impacts of semiochemical-based adulticide applications on Formicidae were not consistently negative or positive. The relative lack of consistent negative impacts on nontarget arthropods suggests that other biotic and abiotic factors leading to natural population fluxes may have more influence on these groups of beneficial organisms than applications of semiochemical-based bait containing carbaryl. Overall, it seems that areawide applications of these baits for managing rootworm populations in corn are not likely to impose deleterious effects on the nontarget faunal groups we surveyed, especially in comparison with the at-plant applications of soil insecticides used as experimental controls in this study.     

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.     

Insecticidal proteins from Bacillus thuringiensis protect corn from corn rootworms.

Field tests of corn co-expressing two new delta-endotoxins from Bacillus thuringiensis (Bt) have demonstrated protection from root damage by western corn rootworm (Diabrotica virgifera virgifera LeConte). The level of protection exceeds that provided by chemical insecticides. In the bacterium, these proteins form crystals during the sporulation phase of the growth cycle, are encoded by a single operon, and have molecular masses of 14 kDa and 44 kDa. Corn rootworm larvae fed on corn roots expressing the proteins showed histopathological symptoms in the midgut epithelium.