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.     

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.     

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.     

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.     

The impact of Bt maize as a natal host on adult head capsule width in field populations of western corn rootworm

The success of the current resistance management plan for transgenic maize, Zea mays L. (Poaceae), targeting the rootworm complex hinges upon high rates of mating between resistant and susceptible beetles. However, differences in the fitness of adult beetles could result in assortative mating, which could, in turn, change the rate of resistance evolution. Adult head capsule widths of naturally occurring populations of western corn rootworm, Diabrotica virgifera virgifera LeConte (Coleoptera: Chrysomelidae), were examined in a variety of refuge configurations. Beetles were classified into treatments based on the hybrid – non‐Bt refuge or Bt maize targeting larval rootworms (hereafter Bt‐RW maize) – and location – proximity to other Bt‐RW or refuge plants – of the natal host plant. Treatments included the following: a refuge plant surrounded by other refuge plants, a refuge plant located near a Bt‐RW plant, a Bt‐RW plant surrounded by Bt‐RW plants, and a Bt‐RW plant located near a refuge plant. The mean head capsule width of males emerging from Bt‐RW plants was significantly smaller than the mean head capsule width of males emerging from refuge plants. These results indicate that males emerging from Bt‐RW maize plants may be exposed to sublethal doses of the Bt toxin as larvae. No differences were detected between females emerging from refuge plants compared with Bt‐RW plants. Overall mean head capsule width decreased as the season progressed, regardless of treatment. The diminished head capsule width of western corn rootworm males emerging from Bt‐RW maize may act to enhance resistance management, particularly in a seed mix refuge system.     

Field-evolved resistance to Bt maize by western corn rootworm: predictions from the laboratory and effects in the field

Crops engineered to produce insecticidal toxins derived from the bacterium Bacillus thuringiensis (Bt) provide an effective management tool for many key insect pests. However, pest species have repeatedly demonstrated their ability to adapt to management practices. Results from laboratory selection experiments illustrate the capacity of pest species to evolve Bt resistance. Furthermore, resistance has been documented to Bt sprays in the field and greenhouse, and more recently, by some pests to Bt crops in the field. In 2009, fields were discovered in Iowa (USA) with populations of western corn rootworm, Diabrotica virgifera virgifera LeConte, that had evolved resistance to maize that produces the Bt toxin Cry3Bb1. Fields with resistant insects in 2009 had been planted to Cry3Bb1 maize for at least three consecutive years and as many as 6years. Computer simulation models predicted that the western corn rootworm might evolve resistance to Bt maize in as few as 3years. Laboratory and field data for interactions between western corn rootworm and Bt maize indicate that currently commercialized products are not high-dose events, which increases the risk of resistance evolution because non-recessive resistance traits may enhance survival on Bt maize. Furthermore, genetic analysis of laboratory strains of western corn rootworm has found non-recessive inheritance of resistance. Field studies conducted in two fields identified as harboring Cry3Bb1-resistant western corn rootworm found that survival of western corn rootworm did not differ between Cry3Bb1 maize and non-Bt maize and that root injury to Cry3Bb1 maize was higher than injury to other types of Bt maize or to maize roots protected with a soil insecticide. These first cases of field-evolved resistance to Bt maize by western corn rootworm provide an early warning and point to the need to apply better integrated pest management practices when using Bt maize to manage western corn rootworm.     

Field screening maize germplasm for resistance and tolerance to western corn rootworms (Col.: Chrysomelidae)†

Abstract: In the hopes of lessening the current reliance on soil insecticides, developing a viable alternative for transgenic maize hybrids, and providing sustainable options for Europe, researchers recently have been developing novel maize lines that exhibit resistance and/or tolerance to corn rootworm larvae. Here we report the results of a 2‐year field experiment in a northern growing region assessing the resistance and tolerance of 10 experimental synthetic maize populations selected for varying levels of damage from western corn rootworm larvae, Diabrotica virgifera virgifera LeConte (Col.: Chrysomelidae) and four maize hybrids. Maize non‐preference, antibiosis and tolerance to rootworms was evaluated using previously established methods, including: the Iowa 1–6 root damage rating scale, root fresh weight, compensatory root growth ratings and adult rootworm emergence. Among the experimental synthetic maize populations, BS29‐11‐01 was the most susceptible, and had a mean root damage rating that was greater than the highly susceptible maize hybrid B37 × H84. This line also had the lowest mean root fresh weight and one of the lowest mean compensatory root growth ratings. In contrast, CRW8‐3 appeared to be tolerant to western corn rootworms, and had the lowest mean root damage rating, which was comparable with that of the non‐transgenic hybrid DeKalb^® 46‐26.     

Tritrophic interactions among Bt maize,an insect pest and entomopathogens: effects on development and survival of western corn rootworm

Agricultural systems often provide a model for testing ecological hypotheses, while ecological theory can enable more effective pest management. One of the best examples of this is the interaction between host‐plant resistance and natural enemies. With the advent of crops that are genetically modified to produce insecticidal toxins from the bacterium Bacillus thuringiensis (Bt), a new form of host‐plant resistance has been introduced to agroecosystems. How Bt crops interact with natural enemies, especially insect pathogens in below‐ground systems, is not well understood, but provides a unique opportunity to study below‐ground tritrophic interactions. In this study, we used two species of entomopathogenic fungi and three species of entomopathogenic nematodes to determine how this community of soil‐borne natural enemies might interact with Bt maize (event 59122, expressing the insecticidal protein Cry34/35Ab1) to affect survival and development of western corn rootworm (Diabrotica virgifera virgifera), which is an obligate root feeder and a serious pest of maize. We ran two experiments, one in a greenhouse and one in a growth chamber. Both experiments consisted of a fully crossed design with two maize treatments (Bt maize and non‐Bt maize) and two entomopathogen treatments (present or absent). The community of entomopathogens significantly increased mortality of western corn rootworm, and Bt maize increased larval developmental time and mortality. Entomopathogens and Bt maize acted in an independent and additive manner, with both factors increasing the mortality of western corn rootworm. Results from this study suggest that entomopathogens may complement host‐plant resistance from Bt crops.     

Supercooling and cold-hardiness in eggs of western and northern corn rootworms

Oviposition by northern corn rootworms, Diabrotica barberi Smith and Lawrence, and western corn rootworms, Diabrotica virgifera virgifera LeConte (Coleoptera: Chrysomelidae), key pests of corn in the Great Plains of the USA, occurs in the soil during late summer. Overwintering eggs are exposed to variable soil moisture and temperatures below ?5 °C. The winter mortality of eggs in the soil is a primary factor that determines the potential for larval injury to corn the following spring. Our studies aimed to determine the comparative supercooling capacities of northern and western corn rootworm eggs and to assess egg mortality following brief exposure to extreme low temperature, ranging from ?12.0 to ?21.5 °C, under three moisture regimes. Eggs of northern corn rootworm were supercooled to a temperature as low as ?27 °C, and survived supercooling to a greater extent than did western corn rootworm eggs. Moisture treatment prior to supercooling had little effect on northern corn rootworm eggs. Western corn rootworm eggs were more resistant than northern corn rootworm eggs to the effects of desiccation followed by supercooling. The survival of northern corn rootworm eggs was better than western corn rootworms under dry conditions, followed by exposure to temperatures of ?12.0 and ?17.5 °C, but was very low at ?21.5 °C, regardless of the moisture regime. The results suggest that moisture and temperature may interact in the soil environment to determine the overwintering survival of corn rootworms. It is evident from these studies that both rootworm species experience mortality at temperatures well above the supercooling points of the eggs, but that differences exist in the effects of substrate moisture treatments on the cold‐hardiness of eggs from the two species.     

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.     

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.     

Volatile semiochemical impact on trapping and distribution in maize of northern and western corn rootworm beetles (Coleoptera: Chrysomelidae)

Abstract 1 Field studies evaluated plant attractants and analogues as tools to move corn rootworm beetles (Diabrotica spp.) into areas to be treated with toxic baits for population suppression via mass removal/annihilation of reproductive adults. 2 When dispensed from sticky traps in maize, 2‐phenyl‐1‐ethylamine and 2‐phenyl‐1‐ethanol captured more northern corn rootworm, Diabrotica barberi, than did 4‐methoxyphenethanol. Only 2‐phenyl‐1‐ethanol attracted the western corn rootworm, Diabrotica virgifera virgifera, but not until maize matured beyond milk stage. 3 Attraction of D. barberi to the amine, alone or blended with 2‐phenyl‐1‐ethanol, occurred before and after maize flowered but not during intervening silk or blister stages. Attraction recurred during early milk stage at or before 50% emergence of adult female D. barberi or D. v. virgifera, respectively, and before populations declined for the season. 4 Synergistic interaction of 2‐phenyl‐1‐ethylamine with 2‐phenyl‐1‐ethanol in attracting D. barberi females did not occur until maize matured to late milk stage. 5 The amine‐alcohol blend (0.44 point sources m^?2) doubled the density of D. barberi but not D. v. virgifera when applied to small plots within mostly milk‐stage or younger maize. Traps without bait within attractant‐treated plots captured more female, but not male, D. barberi than did traps in untreated control plots, hinting that females accounted for most of the observed increase in beetle density. 6 The results suggest that attractants can be used despite phenological limitations to concentrate preovipositional females within field areas and thus to complement a variety of corn rootworm control strategies.     

Western corn rootworm and Bt maize: challenges of pest resistance in the field

Crops genetically engineered to produce insecticidal toxins from the bacterium Bacillus thuringiensis (Bt) manage many key insect pests while reducing the use of conventional insecticides. One of the primary pests targeted by Bt maize in the United States is the western corn rootworm, Diabrotica virgifera virgifera LeConte. Beginning in 2009, populations of western corn rootworm were identified in Iowa, USA that imposed severe root injury to Cry3Bb1 maize. Subsequent laboratory bioassays revealed that these populations were resistant to Cry3Bb1 maize, with survival on Cry3Bb1 maize that was three times higher than populations not associated with such injury. Here we report the results of research that began in 2010 when western corn rootworm were sampled from 14 fields in Iowa, half of which had root injury to Cry3Bb1 maize of greater than 1 node. Of these samples, sufficient eggs were collected to conduct bioassays on seven populations. Laboratory bioassays revealed that these 2010 populations had survival on Cry3Bb1 maize that was 11 times higher and significantly greater than that of control populations, which were brought into the laboratory prior to the commercialization of Bt maize for control of corn rootworm. Additionally, the developmental delays observed for control populations on Cry3Bb1 maize were greatly diminished for 2010 populations. All 2010 populations evaluated in bioassays came from fields with a history of continuous maize production and between 3 and 7 y of Cry3Bb1 maize cultivation. Resistance to Cry34/35Ab1 maize was not detected and there was no correlation between survival on Cry3Bb1 maize and Cry34/35Ab1 maize, suggesting a lack of cross resistance between these Bt toxins. Effectively dealing with the challenge of field-evolved resistance to Bt maize by western corn rootworm will require better adherence to the principles of integrated pest management.     

A selective insecticidal protein from Pseudomonas mosselii for corn rootworm control

The coleopteran insect western corn rootworm (WCR, Diabrotica virgifera virgifera) is an economically important pest in North America and Europe. Transgenic corn plants producing Bacillus thuringiensis (Bt) insecticidal proteins have been useful against this devastating pest, but evolution of resistance has reduced their efficacy. Here, we report the discovery of a novel insecticidal protein, PIP‐47Aa, from an isolate of Pseudomonas mosselii. PIP‐47Aa sequence shows no shared motifs, domains or signatures with other known proteins. Recombinant PIP‐47Aa kills WCR, two other corn rootworm pests (Diabrotica barberi and Diabrotica undecimpunctata howardi) and two other beetle species (Diabrotica speciosa and Phyllotreta cruciferae), but it was not toxic to the spotted lady beetle (Coleomegilla maculata) or seven species of Lepidoptera and Hemiptera. Transgenic corn plants expressing PIP‐47Aa show significant protection from root damage by WCR. PIP‐47Aa kills a WCR strain resistant to mCry3A and does not share rootworm midgut binding sites with mCry3A or AfIP‐1A/1B from Alcaligenes that acts like Cry34Ab1/Cry35Ab1. Our results indicate that PIP‐47Aa is a novel insecticidal protein for controlling the corn rootworm pests.     

Validation of a nested error component model to estimate damage caused by corn rootworm larvae

The corn rootworm complex (Coleoptera: Chrysomelidae) constitutes a significant threat to maize production in the United States, and more recently, in Europe. We conducted an analysis of readily available field trial data to validate an existing damage function for corn rootworm larvae. We used a nested error component model with unbalanced panel data to describe the relationship between yield loss and root injury caused by these insects. These data were collected by personnel with the Insect Management and Insecticide Evaluation Programme (Department of Crop Sciences, University of Illinois) and represent 19 location‐years. To our knowledge, this is the largest data set used to estimate a damage function for corn rootworm larvae. Unlike many experiments examining the relationship between root injury and yield loss caused by corn rootworm larvae, the data set used for our analysis includes many Bt maize hybrids. Our model suggests that for each node of roots injured by corn rootworm larvae, a yield loss of approximately 15% can be expected. Statistically significant variance components included an effect of location and experimental error. We speculate that variation in weather across experimental sites was the principal factor contributing to the significant effect of location. The substantial experimental error observed for our model highlights the limitations of utilizing a multi‐year, geographically diverse damage function for predicting yield loss because of root injury on a small scale. We discuss major factors contributing to the variance components estimated by our model and suggest techniques for improving future analyses of the damage function for corn rootworm larvae.     

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.     

Assessment of establishment and persistence of entomopathogenic nematodes for biological control of western corn rootworm

Abstract:  The use of entomopathogenic nematodes (EPN) is potentially one ecological approach to control the invasive alien western corn rootworm ( Diabrotica virgifera virgifera LeConte, Col., Chrysomelidae) in Europe. This study investigated the establishment and the short- and long-term persistence of Heterorhabditis bacteriophora Poinar (Rhabditida: Heterorhabditidae), Heterorhabditis megidis Poinar, Jackson and Klein (Rh., Heterorhabditidae) and Steinernema feltiae Filipjev (Rh., Steinernematidae) in three maize fields in southern Hungary, using the insect-baiting technique. All three EPN species equally established and persisted in maize fields. The timing of application (April or June) did not influence the establishment of EPN species. EPNs persisted for 2–5 months, i.e. they survived up to and throughout D. v. virgifera larval occurrence in the soil. Results demonstrate that D. v. virgifera larvae can potentially be controlled by EPNs during the same year of EPN application but no long-term control effect is expected under intensive maize cultivation practices.     

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.     

Minnesota field population of western corn rootworm (Coleoptera: Chrysomelidae) shows incomplete resistance to Cry34Ab1/Cry35Ab1 and Cry3Bb1

In the United States of America, the western corn rootworm, Diabrotica virgifera virgifera LeConte (Coleoptera: Chrysomelidae), is commonly managed with transgenic corn (Zea mays L.) expressing insecticidal proteins from the bacteria Bacillus thuringiensis Berliner (Bt). Colonies of this pest have been selected in the laboratory on each commercially available transformation event and several resistant field populations have also been identified; some field populations are also resistant. In this study, progeny of a western corn rootworm population collected from a Minnesota corn field planted to SmartStax^® corn were evaluated for resistance to corn hybrids expressing Cry3Bb1 (event MON88017) or Cry34/35Ab1 (event DAS‐59122‐7) and to the individual constituent proteins in diet‐overlay bioassays. Results from these assays suggest that this population is resistant to Cry3Bb1 and is incompletely resistant to Cry34/35Ab1. In diet toxicity assays, larvae of the Minnesota (MN) population had resistance ratios of 4.71 and >13.22 for Cry34/35Ab1 and Cry3Bb1 proteins, respectively, compared with the control colonies. In all on‐plant assays, the relative survival of the MN population on the DAS‐59122‐7 and MON88017 hybrids was significantly greater than the control colonies. Larvae of the MN population had inhibited development when reared on DAS‐59122‐7 compared with larvae reared on the non‐Bt hybrid, indicating resistance was incomplete. Overall, these results document resistance to Cry3Bb1 and an incomplete resistance to Cry34/35Ab1 in a population of WCR from a SmartStax^® performance problem field.     

Western corn rootworm (Diabrotica virgifera virgifera LeConte) population dynamics

1 The western corn rootworm Diabrotica virgifera virgifera LeConte is a major insect pest of field maize, Zea mays L. Larvae can cause substantial injury by feeding on maize roots. Larval feeding may destroy individual roots or root nodes, and reduce plant growth, stability, and yield. Costs associated with managing corn rootworms in continuous maize are annually one of the largest expenditures for insect management in the United States Corn Belt.
2 Even though D. virgifera virgifera has been studied intensively for over 50 years, there is renewed interest in the biology, ecology, and genetics of this species because of its ability to rapidly adapt to management tactics, and its aggressive invasive nature.
3 This article provides a comprehensive review of D. virgifera virgifera population dynamics, specifically: diapause, larval and adult development, seasonality, spatial and temporal dynamics at local and landscape scales, invasiveness in North America and Europe, and non-trophic interactions with other arthropods.
4 Gaps in current knowledge are identified and discussed especially within the context of challenges that scientists in North America and Europe are currently facing regarding pest dynamics and the need to develop appropriate management strategies for each geographic area.