Detecting Corn Rootworm (Diabrotica spp.) Resistance to Transgenic Bt-RW Traits: Emergence, Root Injury, and Species Interactions

Abstract

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.