Northern Corn Rootworm Research Articles

Multidimensional approach to formulating a specialized diet for northern corn rootworm larvae

The northern corn rootworm (NCR), Diabrotica barberi Smith & Lawrence, is a major pest of maize (Zea mays L.). This pest has developed resistance to insecticides and adapted to crop rotation and may already be in the early stages of adaptation to toxins produced by Bacillus thuringiensis (Bt). Toxicity bioassays using artificial diet have proven to be valuable for monitoring resistance in many species, but no artificial diet has been developed specifically for NCR larvae. Toward this end, we first evaluated known Diabrotica diets to identify a starting media. We then developed a specialized diet for NCR using an iterative approach. Screening designs including 8 diet components were performed to identify the principal nutritional components contributing to multiple developmental parameters (survival, weight, and molting). We then applied mixture designs coupled with response surface modeling to optimize a blend of those components. Finally, we validated an improved NCR diet formulation that supports approximately 97% survival and molting, and a 150% increase in larval weight after 10 days of feeding compared with the best previously published artificial diet. This formulation appears suitable for use in diet bioassays as a tool for evaluating the resistance of NCR populations to insecticides.

Field Trial Performance of Herculex XTRA (Cry34Ab1/Cry35Ab1) and SmartStax (Cry34Ab1/Cry35Ab1 + Cry3Bb1) Hybrids and Soil Insecticides Against Western and Northern Corn Rootworms (Coleoptera: Chrysomelidae).

Western corn rootworm, Diabrotica virgifera virgifera LeConte (Coleoptera: Chrysomelidae), and northern corn rootworm, Diabrotica barberi Smith and Lawrence (Coleoptera: Chrysomelidae), are important insect pests in corn, Zea mays L. For more than a decade, growers have been using transgenic plants expressing proteins from the bacterium Bacillus thuringiensis (Bt) to protect corn roots from feeding. In 2011, western corn rootworm populations were reported to have developed resistance to Bt hybrids expressing Cry3Bb1 and later found to be cross-resistant to hybrids expressing mCry3A and eCry3.1Ab. The identification of resistance to Cry3 (Cry3Bb1, mCry3A, and eCry3.1Ab) hybrids led to concerns about durability and efficacy of products with single traits and of products containing a pyramid of a Cry3 protein and the binary Bt proteins Cry34Ab1 and Cry35Ab1. From 2012 to 2014, 43 field trials were conducted across the central United States to estimate root protection provided by plants expressing Cry34Ab1/Cry35Ab1 alone (Herculex RW) or pyramided with Cry3Bb1 (SmartStax). These technologies were evaluated with and without soil-applied insecticides to determine if additional management measures provided benefit where Cry3 performance was reduced. Trials were categorized for analysis based on rootworm damage levels on Cry3-expressing hybrids and rootworm feeding pressure within each trial. Across scenarios, Cry34Ab1/Cry35Ab1 hybrids provided excellent root protection. Pyramided traits provided greater root and yield protection than non-Bt plus a soil-applied insecticide, and only in trials where larval feeding pressure exceeded two nodes of damage did Cry34Ab1/Cry35Ab1 single-trait hybrids and pyramided hybrids show greater root protection from the addition of soil-applied insecticides.

Evaluation of SmartStax and SmartStax PRO maize against western corn rootworm and northern corn rootworm: efficacy and resistance management.

Cases of western corn rootworm (WCR) field-evolved resistance to Cry3Bb1 and other corn rootworm (CRW) control traits have been reported. Pyramid products expressing multiple CRW traits can delay resistance compared to single trait products. We used field studies to assess the pyramid CRW corn products, SmartStax (expressing Cry3Bb1 and Cry34Ab1/Cry35Ab1) and SmartStax PRO (expressing Cry3Bb1, Cry34Ab1/Cry35Ab1 and DvSnf7), at locations with high WCR densities and possible Cry3Bb1 resistance, and to assess the reduction in adult emergence attributable to DvSnf7 and other traits. Insect resistance models were used to assess durability of SmartStax and SmartStax PRO to WCR resistance. SmartStax significantly reduced root injury compared to non-CRW-trait controls at all but one location with measurable WCR pressure, while SmartStax PRO significantly reduced root injury at all locations, despite evidence of Cry3Bb1 resistance at some locations. The advantage of SmartStax PRO over SmartStax in reducing root damage was positively correlated with root damage on non-CRW-trait controls. DvSnf7 was estimated to reduce WCR emergence by approximately 80-95%, which modeling indicated will improve durability of Cry3Bb1 and Cry34Ab1/Cry35Ab1 compared to SmartStax. The addition of DvSnf7 in SmartStax PRO can reduce root damage under high WCR densities and prolong Cry3Bb1 and Cry34Ab1/Cry35Ab1 durability. © 2017 Society of Chemical Industry.

Effects of Field History on Corn Root Injury and Adult Abundance of Northern and Western Corn Rootworm (Coleoptera: Chrysomelidae).

Western corn rootworm, Diabrotica virgifera virgifera LeConte (Coleoptera: Chrysomelidae), and northern corn rootworm, Diabrotica barberi Smith & Lawrence, are major pests of corn (Zea mays L.). Corn producing Bacillus thuringiensis (Bt) toxins are widely used to manage Diabrotica spp.; however, Bt resistance by D. v. virgifera has led to high levels of feeding injury in the field. We tested whether field history affected root injury and abundance of adult Diabrotica spp. In 2013 and 2014, four types of cornfields were sampled: 1) recently rotated fields, 2) continuous cornfields, 3) fields with a history of injury to Bt corn (past problem fields), and 4) fields with greater than one node of injury to Bt corn at the time of sampling (current problem fields). Data were collected on field history, root injury, and the abundance of adult Diabrotica spp. from each field. Root injury and the abundance of D. v. virgifera were significantly greater in current problem fields compared to the other field types, while D. barberi were significantly more abundant in recently rotated fields. Root injury and the abundance of D. v. virgifera did not differ among recently rotated fields, continuous cornfields, and past problem fields. Analysis of field history showed that recently rotated fields were characterized by significantly less Bt corn, soil-applied insecticides, and years planted to corn continuously. These results suggest that greater cropping practice diversity can reduce management inputs for Diabrotica spp.; however, its effects on resistance evolution remain undetermined.

Rootworm management: status of GM traits, insecticides and potential new tools.

Abstract Western corn rootworm ( Diabrotica virgifera virgifera LeConte) and northern corn rootworm ( Diabrotica barberi (Smith and Lawrence)) are major pests of maize in the USA. These pests have been managed with a variety of tactics over the last century. Both Diabrotica spp. have adapted to crop rotation in different ways in certain regions of the USA as well as to some of the insecticides targeted at them. D. v. virgifera has adapted to more of the chemical control measures and transgenic control methods. Discussed in this review are the challenges associated with managing both species, and how current management strategies might be combined and implemented to help manage damage from these species. Also, we discuss the potential for new technologies, such as RNA interference, to be used in the future.

Effectiveness of Bt Maize against Corn Rootworm (Coleoptera: Chrysomelidae) and Species Composition in South Dakota Ten Years Following the Introduction of Transgenic Maize

Corn rootworms, Diabrotica spp. (Coleoptera: Chrysomelidae), are the most economically important pest of maize in the United States. The larvae consume maize roots, contributing to poor nutrient uptake, lodging of the plants, and severe yield loss. Transgenic maize expressing toxins from the bacterium Bacillus thuringiensis (Bt) have been employed to combat the pest. However, corn rootworm resistance to Bt maize has recently been reported and spurred research on the effectiveness of Bt toxins against corn rootworm across the Corn Belt. Thus, our objectives were twofold. First, we quantified the efficacy of Bt maize hybrids against corn rootworm in South Dakota. Second, we surveyed populations of Diabrotica species in areas where Bt maize had been grown since its commercialization in 2003. Maize hybrids expressing Cry3Bb1, Cry34/35Ab1, and mCry3A were evaluated at two locations with previous corn rootworm infestations or in fields that were in continuous maize for three or more consecutive years. We reported damage to maize expressing Cry3Bb1 and mCry3A toxins. We also observed significantly higher numbers of western corn rootworm than northern corn rootworm in our Diabrotica species surveys, suggesting that western corn rootworm are the dominant species in the region. This research has implications for maize production and sustainable corn rootworm management in South Dakota and the Northern Plains, and will advance our knowledge of the incidence of resistance to Bt toxins and long-term impact of Bt hybrids on species diversity of Diabrotica.

Evaluation of Corn Hybrids for Rootworm Management With Insecticides and Without, 2014: Table 1

Northern and western corn rootworm (NCR/WCR) management efficacy trials were conducted to evaluate transgenic insect-resistant hybrids and granular insecticide combination treatments near Bryant, Cavour, and Colman in South Dakota. Granular insecticides were applied in-furrow to four row (∼50 ft long; spaced 30 inches apart) treatment plots at planting time. Experimental plots were arranged in an RCBD with four replications. Granular insecticides were applied with Noble metering units or SmartBox delivery system. All metering units …

Susceptibility of northern corn rootworm Diabrotica barberi (Coleoptera: Chrysomelidae) to mCry3A and eCry3.1Ab Bacillus thuringiensis proteins.

The susceptibility of the northern corn rootworm Diabrotica barberi (Smith & Lawrence) to mCry3A and eCry3.1Ab proteins derived from Bacillus thuringiensis (Bt) was determined using a diet bioassay. Northern corn rootworm neonates were exposed to different concentrations of mCry3A and eCry3.1Ab, incorporated into artificial diet. Larval mortality was evaluated after 7 d. The mCry3A and eCry3.1Ab proteins were found to be toxic to the northern corn rootworm larvae. The LC50 and LC99 values for mCry3A were 5.13 and 2482.31 μg/mL, respectively. For eCry3.1Ab, the LC50 and LC99 values were 0.49 and 213.01 μg/mL. Based on the estimated lethal concentrations, eCry3.1Ab protein was more efficacious to northern corn rootworm larvae than mCry3A. These lethal concentration values will be used as diagnostic doses for routine annual monitoring for change in susceptibility of field collected northern corn rootworm to mCry3A, and eCry3.1Ab toxins.

Effect of Seed Blends and Soil-Insecticide on Western and Northern Corn Rootworm Emergence from mCry3A+eCry3.1Ab Bt Maize.

Seed blends containing various ratios of transgenic Bt maize (Zea mays L.) expressing the mCry3A+eCry3.1Ab proteins and non-Bt maize (near-isoline maize) were deployed alone and in combination with a soil applied pyrethroid insecticide (Force CS) to evaluate the emergence of the western corn rootworm, Diabrotica virgifera virgifera LeConte, in a total of nine field environments across the Midwestern United States in 2010 and 2011. Northern corn rootworm, Diabrotica barberi Smith & Lawrence emergence was also evaluated in four of these environments. Both western and northern corn rootworm beetle emergence from all Bt treatments was significantly reduced when compared with beetle emergence from near-isoline treatments. Averaged across all environments, western corn rootworm beetle emergence from 95:5, 90:10, and 80:20 seed blend ratios of mCry3A+eCry3.1Ab: near-isoline were 2.6-, 4.2-, and 6.7-fold greater than that from the 100:0 ratio treatment. Northern corn rootworm emergence from the same seed blend treatments resulted in 2.8-, 3.2-, and 4.2-fold more beetles than from the 100:0 treatment. The addition of Force CS (tefluthrin) significantly reduced western corn rootworm beetle emergence for each of the three treatments to which it was applied. Force CS also significantly delayed the number of days to 50% beetle emergence in western corn rootworms. Time to 50% beetle emergence in the 100% mCry3A+eCry3.1Ab treatment with Force CS was delayed 13.7 d when compared with western corn rootworm beetle emergence on near-isoline corn. These data are discussed in terms of rootworm resistance management.

Synthesis of corn rootworm pheromones from commercial diols

AbstractA mixture of stereoisomers of the corn rootworm pheromones was synthesised via the Grignard coupling of protected bromohydrins with alkylcuprate as a key step. The synthesis of 8-methyldec-2- yl propanoate (I ), the northern corn rootworm Diabrotica longicornis Say pheromone, was achieved from pentane-1,5-diol in four steps with an overall yield of 35.1 % and 10-methyltridecan-2-one (II ), the southern corn rootworm Diabrotica undecimpunctata howardi Barber pheromone, was synthesised from octane-1,8-diol as commercially available starting material in five steps with an overall yield of 28.7 %.

Wolbachia Multilocus Sequence Typing of Singly Infected and Multiply Infected Populations of Northern Corn Rootworm (Coleoptera: Chrysomelidae)

Abstract The northern corn rootworm (Diabrotica barberi Smith & Lawrence) in eastern and central North America exhibits at least three distinct populations with respect to Wolbachia infection: uninfected; singly infected; multiply infected. The infected states are associated with different mtDNA haplotypes and reduced mtDNA variability. Multilocus sequence typing (MLST) was used to define strain types and examine the diversity of Wolbachia infecting northern corn rootworm. Four of the five MLST genes (coxA, hcpA, fbpA, and ftsZ) were represented by three alleles each. The remaining MLST gene (gatB) had four to six alleles. The uncertainty arose from whether to call two small clusters new alleles or artifacts of the amplification process. Singly infected individuals had a common set of alleles that defined one strain (wBarB). This strain was also a component of the multiple infections. A second strain (wBarA), restricted to the multiple infections, was defined by alleles that appeared with substantially greater frequency for each of the five loci. By default a third strain would comprise the less frequent third allele of the four loci, but it is not possible to determine to which gatB allele they are linked. Therefore, the strain is not fully defined. The diversity of gatB indicates that four to six Wolbachia strains are present in northern corn rootworm, a number that is similar to the five variants reported previously for the wsp gene. These results also highlight the very real difficulty of using the MLST system to define specific strains in a multiply infected host.

Male Reproductive Competition and Components of Female Fitness in Relation to Body Size in Northern Corn Rootworm (Coleoptera: Chrysomelidae)

AbstractIn insects, larger males generally have a reproductive advantage over smaller males when competing for mating partners. We examined male reproductive competition together with precopulation and copulation durations, female longevity, and fecundity in the northern corn rootworm in relation to the body size of males and females that were combined for mating. Longevity and fecundity were determined for individually caged, mated females. Of the females in 108 combinations of two males and one female, 35 chose not to mate. Of the females in the 73 combinations that resulted in copulation, 38 were small and 35 were large. The proportions of large and small males that mated did not vary significantly with female size, but large males were more than twice as likely as small males to mate. The precopulation duration did not vary with either male or female size, and no interaction occurred between male and female size for either the precopulation or copulation duration. However, both male and female size affected the duration of copulation, with small males copulating longer than large males and large females copulating longer than small females.Nofemale longevity or egg number differences occurredamongthe body size categories of the mating pairs. The implications of the results for insect resistance management are discussed, considering that the evolution of resistance to certain management strategies, such as resistance to the use of insecticides or of Bt maize, may be accompanied by changes in body size.

ROOTWORM LARVAL MANAGEMENT IN CORN AND ITS IMPACT ON YIELD IN EASTERN SOUTH DAKOTA USING REGISTERED INSECTICIDAL MATERIALS, 2013

Northern and western corn rootworm (NCR/WCR) management efficacy trials were conducted to evaluate transgenic insect-resistant hybrids, liquid, and granular insecticide combination treatments near Bryant, Cavour, and Coleman in South Dakota. Plots consisted of four rows (∼50 ft long; spaced 30 inches apart) and were arranged in an RCBD with four replications. Granular insecticides were applied by using Nobel metering units or computer-assisted Smartbox insecticide delivery system. Planting-time liquid insecticides were applied using CO2-powered delivery systems individually mounted on …

Supplementary data for: Wolbachia Multilocus Sequence Typing of Singly Infected and Multiply Infected Populations of Northern Corn Rootworm (Coleoptera: Chrysomelidae)

Supplementary data for: Wolbachia Multilocus Sequence Typing of Singly Infected and Multiply Infected Populations of Northern Corn Rootworm (Coleoptera: Chrysomelidae)

Effects of <I>Rag1</I> on the Preference and Performance of Soybean Defoliators

The Rag1 gene confers antibiotic resistance to soybean aphid, Aphis glycines Matsumura (Hemiptera: Aphididae), and in 2010, varieties expressing Rag1 were released for commercial use in the United States. We do not know how Rag1 varieties will influence the broader community of defoliating insects that inhabit soybean fields. In 2010 and 2011, the preference and performance of pest insects that defoliate soybeans [Glycines max (L.) Merr] were tested using Rag1 and aphid-susceptible varieties. Three coleopterans and four lepidopterans were used: northern corn rootworm, Diabrotica barberi Smith & Lawrence (Coleoptera: Chrysomelidae); southern corn rootworm, Diabrotica undecimpunctata howardi Barber (Coleoptera: Chrysomelidae); bean leaf beetle, Ceratoma trsifurcata Förster (Coleoptera: Chrysomelidae); fall armyworm, Spodoptera frugiperda (J. E. Smith) (Lepidoptera: Noctuidae); corn earworm, Helicoverpa zea (Boddie) (Lepidoptera: Noctuidae); soybean looper, Chrysodeix includens (Walker) (Lepidoptera: Noctuidae); and velvet-bean caterpillar, Anticarsia gemmatalis Hübner (Lepidoptera: Noctuidae). The preference of insects was evaluated in choice and no-choice tests using Rag1 and susceptible soybeans. Lepidopterans also were evaluated on Rag1 leaves using four nutritional indices: relative growth rate, approximate digestibility, and efficiency of conversion of ingested material. In the majority of preference tests, no effect of Rag1 was detected, and in cases where preferences were found, there was no consistent pattern of preference for Rag1 vs. susceptible leaf tissue. Helicoverpa zea demonstrated a preference for resistant leaf tissue, but this was dependent on the genetic background of the variety. Evaluations of nutritional indices indicated that three species of Lepidoptera, S. frugiperda, H. zea, and A. gemmatalis, displayed reduced conversion efficiency for Rag1 soybeans, suggesting effects of antibiosis.

Effect of Bt Maize and Soil Insecticides on Yield, Injury, and Rootworm Survival: Implications for Resistance Management

A 2-yr field experiment was conducted to determine the effects on Diabrotica spp. (Coleoptera: Chrysomelidae) of an insecticidal seed treatment (Poncho 1250, (AI)/clothianidin) and a granular insecticide (Aztec 2.1G, (AI)/tebupirimphos and cyfluthrin) alone and in combination with maize producing the insectidical toxin Cry3Bb1 derived from the bacterium Bacillus thuringiensis (Bt). Yields for Bt maize plots were significantly greater than for non-Bt maize; however, insecticides did not significantly affect yield. Insecticides significantly decreased root injury in non-Bt maize plots, but there were no significant differences in root injury between Bt maize with or without either insecticide. Maize producing the Bt toxin Cry3Bb1 and the soil-applied insecticide Aztec significantly decreased survival of western corn rootworm (Diabrotica virgifera virgifera LeConte), while only Bt maize significantly decreased survival of the northern corn rootworm (Diabrotica barberi Smith & Lawrence). For both species, Bt maize and each of the insecticides delayed emergence. In the absence of density-dependent mortality, Bt maize imposed 71 and 80% reduction in survival on the western corn rootworm and the northern corn rootworm, respectively. The data from this study do not support combining insecticide with Bt maize because the addition of insecticide did not increase yield or reduce root injury for Bt maize, and the level of rootworm mortality achieved with conventional insecticide was likely too low to delay the evolution of Bt resistance. In addition, delays in emergence from Bt maize combined with insecticides could promote assortative mating among Bt-selected individuals, which may hasten resistance evolution.

Frequency and Distribution of Extended Diapause in Nebraska Populations of <I>Diabrotica barberi</I> (Coleoptera: Chrysomelidae)

The frequency of extended diapause in populations of the northern corn rootworm, Diabrotica barberi Smith & Lawrence (Coleoptera: Chrysomelidae), was measured in eastern Nebraska. Adult collections were made during late summer in 2008 and 2009 from eight sites each year (seven sites were consistent over years). Eggs were obtained from 12 to 20 females per site and were held on moist soil under appropriate temperature profiles to facilitate egg survival, diapause development, and diapause termination. Percentage egg hatch was recorded after the first and second year for the 2008 and 2009 collections. Additional extended diapause expression was estimated for the 2008 and 2009 collections by counting remaining live eggs after the second-year egg hatch was completed. These data also were used to estimate each site's maximum egg viability. Results collectively indicate that the extended diapause trait was present in all eastern Nebraska populations; however, significant variation in the frequency of extended diapause was observed within and among D. barberi populations (first-year mean percentage egg hatch range: 2008 collections: 4.5-38.6%; 2009 collections: 10.7-42.5%). Geographically, the highest incidence of extended diapause was found along a north-south transect in eastern counties of Nebraska that parallels the Missouri River. In general, the frequency of extended diapause was lower in sites found west of the north-south transect. This study contributes to the knowledge base needed to develop appropriate D. barberi management strategies in Nebraska. These data can be used to pinpoint areas of Nebraska that may have the greatest risk of economic injury in first-year corn (Zea mays L.) when D. barberi densities are high.

Wolbachia wsp Gene Clones Detect the Distribution of Wolbachia Variants and wsp Hypervariable Regions Among Individuals of a Multistrain Infected Population of Diabrotica barberi (Coleoptera: Chrysomelidae)

Abstract The northern corn rootworm (Diabrotica barberi Smith & Lawrence) in eastern and central North America exhibits at least three distinct populations with respect to Wolbachia infection: uninfected, singly infected, and multiply infected. The infected states are associated with different mtDNA haplotypes and reduced mtDNA variability. The previous results demonstrated that the major mtDNA clades of D. barberi were infected with different variants of Wolbachia; however, the total amount of Wolbachia diversity was substantially underestimated. Here we report sequences of the Wolbachia wsp surface protein gene indicating that multiple infections are present. Analysis of the wsp sequences establishes the existence at least five distinct wsp variants. The recovery frequency of specific wsp hypervariable regions (HVR) obtained from several individuals was unequal. The most common was obtained 33 times, the least common only once. One of the Wolbachia variants was present in both singly infected and multiply infected individuals. A possible explanation for the frequency differences are that the frequency of each variant is not the same within each insect. An alternative possibility is that not all individuals are infected with all five variants and that different animals contain different combinations of the variants. The second scenario suggests that some variants are rare in the population. Comparisons of the wsp genes with sequences in GenBank show that two of the variants are most similar to variants infecting subspecies of the congeneric western corn rootworm (Diabrotica virgifera virgifera LeConte and Diabrotica virgifera zeae Krysan & Smith). Individual wsp HVRs were also aligned with GenBank sequences. The results provide evidence that some HVRs are much more common than others and that the HVRs can be shuffled to create new wsp genes.

Interactions Among Bt Maize, Entomopathogens, and Rootworm Species (Coleoptera: Chrysomelidae) in the Field: Effects on Survival, Yield, and Root Injury

A 2 yr field study was conducted to determine how a blend of entomopathogens interacted with Bt maize to affect mortality of Diabrotica spp. (Coleoptera: Chrysomelidae), root injury to maize (Zea maize L.) and yield. The blend of entomopathogens included two entomopathogenic nematodes, Steinernema carpocapsae Weiser and Heterorhabditis bacteriophora Poinar, and one entomopathogenic fungus, Metarhizium brunneum (Metschnikoff) Sorokin. Bt maize (event DAS59122-7, which produces Bt toxin Cry34/35Ab1) decreased root injury and survival of western corn rootworm (Diabrotica virgifera virgifera LeConte) and northern corn rootworm (Diabrotica barberi Smith & Lawrence) but did not affect yield. During year 1 of the study, when rootworm abundance was high, entomopathogens in combination with Bt maize led to a significant reduction in root injury. In year 2 of the study, when rootworm abundance was lower, entomopathogens significantly decreased injury to non-Bt maize roots, but had no effect on Bt maize roots. Yield was significantly increased by the addition of entomopathogens to the soil. Entomopathogens did not decrease survival of corn rootworm species. The results suggest that soil-borne entomopathogens can complement Bt maize by protecting roots from feeding injury from corn rootworm when pest abundance is high, and can decrease root injury to non-Bt maize when rootworm abundance is low. In addition, this study also showed that the addition of entomopathogens to soil contributed to an overall increase in yield.

Abundance and Distribution of Western and Northern Corn Rootworm (<I>Diabrotica</I> spp.) and Prevalence of Rotation Resistance in Eastern Iowa

The western corn rootworm Diabrotica virgifera virgifera LeConte (Coleoptera: Chrysomelidae) and the northern corn rootworm Diabrotica barberi Smith & Lawrence (Coleoptera: Chrysomelidae) are major pests of corn (Zea mays L.). Historically, crop rotation has been an effective management strategy, but both species have adapted to crop rotation in the Midwest. For both species in eastern Iowa, we measured abundance and prevalence of rotation resistance using sticky traps and emergence cages in fields of corn and soybean (Glycine max L.). Based on currently available data, we calculated the economic thresholds for these pests at two Diabrotica spp. per trap per day in cornfields and 1.5 D. v. virgifera per trap per day in soybean fields. The economic injury level of rotation-resistant D. barberi was determined to be 3.5 adult insects per emergence cage per year. Peak abundance of rootworm adults in cornfields was below economic thresholds in the majority of fields sampled, suggesting that management of rootworm larvae in continuous cornfields may not always be necessary. Rotation-resistant D. barberi was found throughout eastern Iowa using emergence cages in first-year cornfields, however, the abundance was below levels expected to impose economic injury in 14 of 17 fields evaluated. The presence of rotation-resistant D. v. virgifera, as measured by the occurrence of this insect in soybean fields, occurred only in northeastern Iowa and was also below the economic threshold. These data suggests that crop rotation remains a viable pest management strategy in eastern Iowa.