Browsing by Subject "Bt"

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    Detecting Corn Rootworm (Diabrotica spp.) Resistance to Transgenic Bt-RW Traits: Emergence, Root Injury, and Species Interactions
    (2020-09) Benkert III, Edwin
    Western (Diabrotica virgifera virgifera LeConte) and northern (D. barberi Smith and Lawrence) corn rootworms remain economically important pests of field corn (maize; Zea mays L.) in the U.S. Corn Belt. With western corn rootworm resistance documented against all commercially available transgenic corn rootworm-active Bacillus thuringiensis Berliner (Bt-RW) traits, and northern corn rootworm resistance documented against two Bt-RW traits, corn rootworm continues to be a challenge for corn growers to manage. Methods to detect early resistance development to Bt-RW traits would enable growers to change management tactics earlier and allow for appropriate resistance mitigation to be practiced. This research proposes two methods to aid in the field detection of corn rootworm Bt-RW resistance development, an emergence model for the pyramid of Cry3Bb1 + Cry34/35Ab1 and a more sensitive pruning criteria for rating nodal injury. A greater understanding of interspecific competition under field conditions between western and northern corn rootworm, and implications for management is also discussed. Historically, scouting adult beetles has been an effective method to estimate the oviposition potential of a field population and its ability to injure corn roots the following growing season. Timing of adult scouting is critical to successfully estimating the egg-laying potential of a population, but because of protracted emergence of corn rootworm beetles, successful timing can be difficult. Corn rootworm adult emergence models have been developed and successfully used to determine the appropriate time for scouting adult beetles, but current models do not account for delays in emergence that occur when a population is exposed to a corn rootworm-active transgenic proteins derived from Bacillus thuringiensis Berliner (Bt-RW). This paper presents an emergence model developed to predict corn rootworm emergence after exposure to a hybrid expressing a pyramid of two Bt-RW proteins: Cry3Bb1 + Cry34/35Ab1. A logistic regression model of the form y = 0.94 / 1 + e (595.29 – x) / 43.22 was chosen to model adult emergence. Cumulative degree days were calculated using 11 and 18C as lower and upper temperature thresholds, and accumulations starting January 1 of each year. The predicted model estimated a scouting window of 35.5 days for Cry3Bb1 + Cry34/35Ab1, and the window began 13.1 days after the non-Bt-RW hybrid. Predicted emergence was significantly different from actual emergence for decreased efficacy sites and sites with suspected resistance to Cry3Bb1 + Cry34/35Ab1. The lack of statistical differences between actual emergence delays from decreased efficacy sites and suspected resistant sites suggests that emergence delays decrease drastically once a population falls below 94.5% efficacy. This model has important implications for corn rootworm scouting and resistance management. If actual emergence occurs earlier than predicted, the population may be undergoing resistance development to the Bt-RW protein, necessitating a change in management. Rating the injury corn rootworm causes on corn roots has long been used to evaluate not only the efficacy of different management tactics, but also the feeding intensity of the larval population. The current 0-3 nodal injury rating (NIR) utilizes 3.8 cm pruning criteria to evaluate root injury. Although sufficient for evaluating the efficacy of soil insecticides, the current pruning criteria is unable to detect lower levels of root feeding that could reveal developing Bt-RW resistance. We propose using pruning criteria of 7.6 cm to evaluate corn rootworm injury in populations that may be undergoing early resistance development to transgenic Bt-RW traits. We compared the relationship between root injury using both the 3.8 cm criteria and the 7.6 cm criteria from a non-Bt-RW hybrid and a Cry3Bb1 expressing hybrid with lodging, proportional yield, and beetles emerged per hectare. The 7.6 cm criteria provided three advantages: 1) the 7.6 cm criteria detected increased root feeding at lower injury levels; 2) these levels were associated with greater sensitivity to lodging susceptibility and yield reductions; and 3) were indicative of increased beetle emergence and reduced trait efficacy. For these reasons, the 7.6 cm criteria should be used when investigating developing resistance to transgenic Bt-RW traits. Despite differences in evolutionary history, host preferences, environmental tolerances, and resistance to management tactics, corn growers embrace the same management approach for both species. Previous studies have suggested western corn rootworm may out-compete northern corn rootworm under lab and greenhouse conditions. But do these results hold true under field conditions? These studies, conducted from 2017-2019 near Rosemount, MN, explored the competitive impacts of western and northern corn rootworm in mixed populations under field conditions. Differing ratios of western to northern corn rootworm eggs were used to infest seedling corn in these species’ competition field studies. Resulting impacts on species survival and corn root injury provide insights into interspecific competition. Emerged beetles were heavily skewed toward western corn rootworm; e.g., western corn rootworm comprised over 50% of emerging beetles when constituting only 25% of infested eggs. Root injury increased with higher western corn rootworm egg ratios. Patterns in root pruning lengths between treatments suggested western corn rootworm prefer to feed closer to the plant base while distal pruning increased with higher northern corn rootworm egg ratios, but statistical evidence for these differences was not found in this study. These results have important implications for the management of both species and provide a better understanding of how changing production situations (climate, weather, crop and rootworm management, insect resistance) could affect the competitive outcome between these species particularly in regard to Bt-RW resistance monitoring.
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    Effect of nitrogen on Bt gene expression in corn roots, resulting trait performance against corn rootworms (Diabrotica spp.), and transgenic hybrid performance
    (2013-01) Franz, Trisha Marie
    This study examined the impact of nitrogen rates on the expression of corn rootworm (Diabrotica spp.) resistant traits in transgenic corn engineered to express one or more of Bt (Bacillus thuringiensis Berliner) Cry protein(s). The resulting protection against corn rootworm, and the comparative performance of hybrids containing zero, one or multiple (pyramided) corn rootworm traits were evaluated. Recommended nitrogen application rates have been suggested based on economics, but not for optimal expression of Bt Cry proteins. Specifically, this experiment explored the need to shift N rates to optimize Bt trait expression, corn rootworm protection provided by different traits, and a possible revision of nitrogen requirements for optimal yield with pyramided traits. The experiment featured a factorial treatment arrangement in a split-plot randomized complete block design with six nitrogen rates as the main plots and three hybrids differing in corn rootworm traits as the sub-plots. Corn roots were sampled at the beginning of and just after peak larval feeding and run through an Enzyme-linked Immunosorbant Assay (ELISA) to determine gene expression levels. Resulting root injury and adult emergence were measured to assess impacts on larval survival and damage. Root injury generally decreased with nitrogen rate in all hybrids while Bt gene expression, for Cry3Bb1, increased. Nitrogen rate did not affect expression of Cry34 Ab1/Cry35Ab1. Below-ground biomass was found to be highest in the Round-UpTM Ready hybrid at growth stage V6, along with nitrogen uptake. Nitrogen uptake was similar in VT TripleTM to the Round-Up Ready hybrid, which were both significantly higher than the Smart StaxTM hybrid. Yield for the Smart Stax hybrid did not plateau with the nitrogen rates applied at one site in this study, while the unprotected Round-Up Ready hybrid could not take advantage of the higher N rates. Implications of these results for growers in terms of adjusting nitrogen application rates in fields to obtain better gene expression, optimizing hybrid protection from corn rootworm traits, and reducing the risk of resistance are discussed.
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    Effect of nitrogen on Bt gene expression in corn roots, resulting trait performance against corn rootworms (Diabrotica spp.), and transgenic hybrid performance
    (2013-01) Franz, Trisha Marie
    This study examined the impact of nitrogen rates on the expression of corn rootworm (Diabrotica spp.) resistant traits in transgenic corn engineered to express one or more of Bt (Bacillus thuringiensis Berliner) Cry protein(s). The resulting protection against corn rootworm, and the comparative performance of hybrids containing zero, one or multiple (pyramided) corn rootworm traits were evaluated. Recommended nitrogen application rates have been suggested based on economics, but not for optimal expression of Bt Cry proteins. Specifically, this experiment explored the need to shift N rates to optimize Bt trait expression, corn rootworm protection provided by different traits, and a possible revision of nitrogen requirements for optimal yield with pyramided traits. The experiment featured a factorial treatment arrangement in a split-plot randomized complete block design with six nitrogen rates as the main plots and three hybrids differing in corn rootworm traits as the sub-plots. Corn roots were sampled at the beginning of and just after peak larval feeding and run through an Enzyme-linked Immunosorbant Assay (ELISA) to determine gene expression levels. Resulting root injury and adult emergence were measured to assess impacts on larval survival and damage. Root injury generally decreased with nitrogen rate in all hybrids while Bt gene expression, for Cry3Bb1, increased. Nitrogen rate did not affect expression of Cry34 Ab1/Cry35Ab1. Below-ground biomass was found to be highest in the Round-UpTM Ready hybrid at growth stage V6, along with nitrogen uptake. Nitrogen uptake was similar in VT TripleTM to the Round-Up Ready hybrid, which were both significantly higher than the Smart StaxTM hybrid. Yield for the Smart Stax hybrid did not plateau with the nitrogen rates applied at one site in this study, while the unprotected Round-Up Ready hybrid could not take advantage of the higher N rates. Implications of these results for growers in terms of adjusting nitrogen application rates in fields to obtain better gene expression, optimizing hybrid protection from corn rootworm traits, and reducing the risk of resistance are discussed.
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    Utility of Scouting with Sticky Traps for Integrated Pest Management of Corn Rootworm
    (2017-12) Leaf, Trisha
    Corn rootworms (Diabrotica virgifera virgifera LeConte and Diabrotica barberi Smith and Lawrence) have been pests of economic importance on corn for over a century due to the injury they inflict on corn roots. Injury primarily takes place following larval feeding and occurs in the form of root pruning, stalk lodging, reduced nutrient up-take, and secondary attack by pathogens, but may also occur from adults feeding on silks and pollen. For the past decade, growers have been relying primarily upon corn rootworm- active Bacillus thuringiensis Berliner (Bt) transgenic hybrids to control corn rootworms, and thereby disregarding classic IPM practices due to the reliability of these hybrids alone. In 2009, however, field-evolved resistance to corn rootworm- active Bt hybrids was found in Iowa. Since then, cases of problem fields (i.e., fields where excessive lodging and root pruning occur despite the use of transgenic hybrids) are becoming more prevalent across the Corn Belt where corn-on-corn is common, causing IPM to become a renewed focus in the effort to control corn rootworms. The frequency of problem fields and high cost of corn rootworm-resistant traits is leading growers to use outdated integrated pest management (IPM) decisions to control corn rootworms and hopefully reduce the injury in inflict on the corn plant. Integrated pest management plans with corn rootworm-active Bt hybrids are a cornerstone for successful use of refuge-based Insect Resistance Management (IRM) plans as well. IRM plans combine corn rootworm biology with agronomic practices to delay population resistance to current Bt hybrids containing corn rootworm-active events. While corn rootworm phenology has been extensively reviewed in the past, the influence of Bt hybrids on current phenology is not fully understood although developmental have been observed. A delay in adult emergence has been observed in beetles emerging from Bt hybrids in comparison to non-Bt (refuge) hybrids. The delay in emergence may result in reduced efficacy of refuge corn, which is essential to the IRM plan by increasing mating between the beetles from the refuge and Bt corn. Emergence differences may cause Bt tolerance to develop more quickly. Planting date is known to play an important role in controlling the amount of damage that may occur on the corn plants as well but often coming at the expense of yield. Late planting results in a lack of food availability for corn rootworm larvae, which results in increased mortality, reduced injury, and delayed adult beetle emergence. The combination of planting date and Bt hybrid on corn rootworm adult emergence is largely unknown. This has implications for determining adult scouting windows in order to determine insect pressure the following season. Current Bt hybrids exhibit superior genetics when compared to past inbred lines; in fact, no current data exists for adult beetle thresholds in relation to injury on Bt hybrids. With the new threat of Bt-resistant corn rootworm populations becoming more prevalent, recommendations on how and when to scout for resistant populations are needed. Updated beetle thresholds and scouting windows will also help prevent the overuse of pesticides and control beetle populations the following year. Studies which try to establish economic injury levels with adult beetles as well as economic thresholds in order to aid grower’s management decisions are important for extending the shelf life of corn rootworm-active Bt hybrids as well. This study aimed to evaluate sticky traps as a scouting tool on current corn hybrids, establish precision and required sample sizes, define scouting windows based on planting date and transgenic hybrid-induced emergence delays, and determine the relationship to root injury the following season.