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.

Transgenic plants have been widely adopted by growers to manage the western corn rootworm, Diabrotica virgifera virgifera LeConte, in field corn. Because of reduced efficacy in some Nebraska fields after repeated use of Cry3Bb1-expressing hybrids, single plant bioassays were conducted in 2012 and 2013 to characterize the susceptibility of western corn rootworm populations to the rootworm-active proteins Cry3Bb1, mCry3A, and Cry34/35Ab1. Results demonstrate that there are heritable differences in susceptibility of Nebraska western corn rootworm populations to rootworm-active Bt traits. Proportional survival and corrected survival data coupled with field histories collectively support the conclusion that a level of field resistance to Cry3Bb1 has evolved in some Nebraska populations in response to selection pressure and that cross-resistance exists between Cry3Bb1 and mCry3A. There was no apparent cross-resistance between Cry34/35Ab1 and either Cry3Bb1 or mCry3A. The potential implications of these results on current and future corn rootworm management strategies are discussed.

Transgenic Bt maize that produces less than a high-dose has been widely adopted and presents considerable insect resistance management (IRM) challenges. Western corn rootworm, Diabrotica virgifera virgifera LeConte, has rapidly evolved resistance to Bt maize in the field, leading to local loss of efficacy for some corn rootworm Bt maize events. Documenting and responding to this resistance has been complicated by a lack of rapid diagnostic bioassays and by regulatory triggers that hinder timely and effective management responses. These failures are of great concern to the scientific and agricultural community. Specific challenges posed by western corn rootworm resistance to Bt maize, and more general concerns around Bt crops that produce less than a high-dose of Bt toxin, have caused uncertainty around current IRM protocols. More than 15 years of experience with IRM has shown that high-dose and refuge-based IRM is not applicable to Bt crops that produce less than a high-dose. Adaptive IRM approaches and pro-active, integrated IRM-pest management strategies are needed and should be in place before release of new technologies that produce less than a high-dose. We suggest changes in IRM strategies to preserve the utility of corn rootworm Bt maize by 1) targeting local resistance management earlier in the sequence of responses to resistance and 2) developing area-wide criteria to address widespread economic losses. We also favor consideration of policies and programs to counteract economic forces that are contributing to rapid resistance evolution.

The western corn rootworm, Diabrotica virgifera virgifera LeConte (Dvv) is a significant insect pest of maize in the United States (U.S.). This paper reviews the history of insecticide use in Dvv management programs, Dvv adaptation to insecticides, i.e., field-evolved resistance and associated mechanisms of resistance, plus the current role of insecticides in the transgenic era. In the western U.S. Corn Belt where continuous maize is commonly grown in large irrigated monocultures, broadcast-applied soil or foliar insecticides have been extensively used over time to manage annual densities of Dvv and other secondary insect pests. This has contributed to the sequential occurrence of Dvv resistance evolution to cyclodiene, organophosphate, carbamate, and pyrethroid insecticides since the 1950s. Mechanisms of resistance are complex, but both oxidative and hydrolytic metabolism contribute to organophosphate, carbamate, and pyrethroid resistance facilitating cross-resistance between insecticide classes. History shows that Dvv insecticide resistance can evolve quickly and may persist in field populations even in the absence of selection. This suggests minimal fitness costs associated with Dvv resistance. In the transgenic era, insecticides function primarily as complementary tools with other Dvv management tactics to manage annual Dvv densities/crop injury and resistance over time.

Although Diabrotica pest species have been well studied in field corn, Zea mays L., relatively little is known about adult habitat use by pest and no

Repeated use of field corn (Zea mays L.) hybrids expressing the Cry3Bb1 and mCry3A traits in Nebraska has selected for field-evolved resistance in some western corn rootworm (WCR; Diabrotica virgifera virgifera LeConte) populations. Therefore, this study was conducted to characterize spatial variation in local WCR susceptibility to Cry3Bb1 and mCry3A traits in Keith and Buffalo counties, Nebraska, and determine the relationship between past management practices and current WCR susceptibility. Adult WCR populations were collected from sampling grids during 2015 and 2016 and single-plant larval bioassays conducted with F₁ progeny documented significant variation in WCR susceptibility to Cry3Bb1 and mCry3A on different spatial scales in both sampling grids. At the local level, results revealed that neighboring cornfields may support WCR populations with very different susceptibility levels, indicating that gene flow of resistant alleles from high trait survival sites is not inundating large areas. A field history index, comprised of additive and weighted variables including past WCR management tactics and agronomic practices, was developed to quantify relative selection pressure in individual fields. The field history index-Cry3 trait survivorship relationship from year 1 data was highly predictive of year 2 Cry3 trait survivorship when year 2 field history indices were inserted into the year 1 base model. Sensitivity analyses indicated years of trait use and associated selection pressure at the local level were the key drivers of WCR susceptibility to Cry3 traits in this system. Retrospective case histories from this study will inform development of optimal resistance management programs and increase understanding of plant-insect interactions that may occur when transgenic corn is deployed in the landscape. Results from this study also support current recommendations to slow or mitigate the evolution of resistance by using a multi-tactic approach to manage WCR densities in individual fields within an integrated pest management framework.