Spatial variation in western corn rootworm (Coleoptera: Chrysomelidae) susceptibility to Cry3 toxins in Nebraska

Abstract

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.

Western corn rootworm abundance, injury to corn, and resistance to Cry3Bb1 in the local landscape of previous problem fields

“…This would explain why, in our experiment, previous management among focal fields and surrounding fields differed but rootworm resistance to Cry3Bb1 did not ( Table 2, Fig 6). However, it is worth noting that an experiment in Nebraska, which also examined resistance to Cry3 in local landscapes, concluded that individual field-level selection was a major contributor to resistance levels [34]. Indeed, most fields in our experiment, focal fields and surrounding fields, had experienced some level of selection to Cry3Bb1.…”

“…For example, computer modeling work on Helicoverpa zea, has shown that clusters of up to 20 contiguous fields where Bt use was high led to rapid evolution of resistance due to pest movement [49]. Additionally, Cry3Bb1-resistant western corn rootworm populations in Nebraska were found in cornfields with no history of Cry3Bb1 selection [34]. Like many biological 2016, and C) 2017.…”

“…However, there is also indirect evidence that western corn rootworm transport resistance alleles via dispersal. In Nebraska, it was found that populations from fields where Cry3 traits had never been grown still displayed adaptation to Cry3Bb1 in plant-based bioassays, suggesting that the alleles for resistance were transported by resistant individuals from other populations in the landscape [34]. Thus, western corn rootworm movement in the landscape likely influences the spatial structure of populations and resistance to Bt in the landscape, an important factor in considering mitigation tactics [35,36].…”

Field efficacy of soil insecticides on pyrethroid‐resistant western corn rootworms (Diabrotica virgifera virgifera LeConte)

“…For example, in the US western Corn Belt, maize production under irrigation has often been more profitable than other available crops, which discourages the adoption of crop rotation. The demand for maize is high for livestock operations and ethanol production, leading to higher adoption of continuous maize (maize planted consecutively for two or more years) . This can lead to build‐up of WCR densities, making the annual management of this pest a considerable challenge.…”

Optimizing Egg Recovery From Wild Northern Corn Rootworm Beetles (Coleoptera: Chrysomelidae)

“…However, for varying reasons, not every grower can utilize this management practice. The use of corn hybrids expressing Bacillus thuringiensis (Bt) Berliner has been widely used against corn rootworms since 2003 (Vaughn et al 2005) and resistance evolution to commercially available Bt traits by the western corn rootworm has been documented in the field (Gassmann et al 2011, Wangila et al 2015, Jakka et al 2016, Zukoff et al 2016, Ludwick et al 2017, Reinders et al 2018. Recently, field-evolved resistance of northern corn rootworm to Bt corn has been reported (Calles-Torrez et al 2018.…”