Resistance management and integrated pest management insights from deployment of a Cry3Bb1+ Gpp34Ab1/Tpp35Ab1 pyramid in a resistant western corn rootworm landscape.

In Nebraska USA, many populations of western corn rootworm (WCR), Diabrotica virgifera virgifera LeConte, now exhibit some level of resistance to all corn rootworm-active Bacillus thuringiensis Berliner (Bt) proteins expressed in commercial hybrids. Therefore, a study was conducted in northeast Nebraska from 2020-2022 to reevaluate current corn rootworm management options in continuous maize (consecutive planting for ≥2 years). Results from on-farm experiments to evaluate a standard soil-applied insecticide (Aztec® 4.67G) in combination with non-rootworm Bt or rootworm-active Bt pyramided maize (Cry3Bb1 + Gpp34Ab1/Tpp35Ab1) are reported within the context of WCR Bt resistance levels present. Corrected survival from Bt pyramid single-plant bioassays (<0.3, 0.3-0.49, >0.5) was used to place populations into 3 resistance categories. Variables evaluated included root injury, adult emergence, proportion lodged maize, and grain yield. Key results: A composite analysis of all populations across resistance levels indicated that addition of soil insecticide to Bt pyramid significantly reduced adult emergence and lodging but did not significantly increase root protection or yield. Within and among resistance category analyses of root injury revealed that the Bt pyramid remained highly efficacious at any non-rootworm Bt root injury level when resistance was absent or low. When corrected survival was >0.3, mean Bt pyramid root injury tracked more closely in a positive linear fashion with mean non-rootworm Bt root injury (rootworm density x level of resistance interaction). Similar trends were obtained for adult emergence but not yield. Mean Bt pyramid root injury rating was <0.75 in most populations with Bt resistance, which contributed to no significant yield differences among categories. Results are discussed within the context of IPM:IRM tradeoffs and the need to reduce WCR densities in this system to decrease the impact of the density x resistance interaction to bridge use of current pyramids with new technologies introduced over the next decade.

Characterization of western corn rootworm (Coleoptera: Chrysomelidae) susceptibility to foliar insecticides in northeast Nebraska.

Foliar-applied insecticides are commonly used for adult western corn rootworm (WCR), Diabrotica virgifera virgifera LeConte (Coleoptera: Chrysomelidae), control in Nebraska but little efficacy data is available. Anecdotal reports of reduced efficacy in areas of northeast Nebraska led to the conduct of this study. Objectives were to (i) evaluate the efficacy of commercial applications of commonly used formulated insecticides (bifenthrin, lambda-cyhalothrin, chlorpyrifos, or tank mixes) for WCR control in 7 northeast Nebraska counties during 2019 and 2020 and (ii) conduct adult WCR concentration-response vial bioassays with bifenthrin, chlorpyrifos, and dimethoate active ingredients on a subset of field populations. Whole plant counts (WPC) were used to measure WCR densities in insecticide-treated and untreated maize fields before and after insecticide application. Field control was excellent with organophosphate/pyrethroid tank mixes as proportional change in mean WPC of treated fields was significantly reduced (>0.90) versus untreated fields where little change in WPC occurred. The exception was one treated Boone County field where proportional reduction in WPC was ≤0.78. Bioassays revealed LC50s and resistance ratios of most populations exposed to bifenthrin and dimethoate were not significantly different than the susceptible control. Most populations exhibited a low level of chlorpyrifos resistance when compared to the susceptible control. Field and lab data suggest the local onset of practical WCR field-evolved resistance to bifenthrin in Boone County and chlorpyrifos in Boone and Colfax counties. Results of this study will increase our understanding of WCR resistance evolution, serve as a comprehensive baseline for future research, and inform WCR management programs.

Genetic variability of the european corn borer, Ostrinia nubilalis, suggests gene flow between populations in the Midwestern United States.

The European corn borer, Ostrinia nubilalis (Hübner) (Lepidoptera: Crambidae), is a widely distributed and serious economic pest to corn production in the U.S. Genetic variability of O. nubilalis was studied in 18 sub-populations in the upper Midwestern United States using amplified fragment length polymorphism. The relatively low GST values indicate that more variation exists within populations than between populations. High gene flow (Nm) values were indicated across the entire O. nubilalis population; the lowest degree of gene flow was in the northern samples (Nm = 1.96) and the highest degree of gene flow was in the southern samples (Nm = 2.77). The differences observed in the respective regions (north vs. south) may be explained by the voltinism patterns (univoltine vs. multivoltine, respectively) of O. nubilalis: southern multivoltine populations have opportunities for multiple matings for the duration of the year, further mix alleles. AMOVA results also indicated that most of the genetic variation was within sub-populations ( approximately 81% of total variation); less variation ( approximately 13%) was detected among populations within each of the three regions as designated for this study. However, the most striking and unexpected result was the low percentage of variation between all groups ( approximately 6%), further supporting implications of a high degree of gene flow. These results provide support for current requirements of refugia corn planting in Bt-corn management. These results also indicate that if resistance to Bt were to evolve in O. nubilalis, quick action would be necessary to deter the rapid spread of the gene for resistance.

Population variation of the fall armyworm, Spodoptera frugiperda, in the Western Hemisphere.

Spodoptera frugiperda (J.E. Smith) (Lepidoptera: Noctuidae), the fall armyworm is the most economically important maize pest in the western hemisphere. This research focused on the genetic variability of the maize host strain because there is a lack of information in this area of S. frugiperda research. Amplified fragment length polymorphism (AFLP) was used to assess the genetic variability of S. frugiperda over a large geographic area. Twenty populations were collected from the maize, one population was collected from princess tree, one population was collected from lemon tree, and one population was collected from bermudagrass. The 23 populations were from Mexico, the continental United States, Puerto Rico, Brazil, and Argentina. The objective of this research was to evaluate whether the majority of genetic variability was within populations or between populations. The AFLP results showed that the majority of the genetic variability is within populations and not between populations, indicating minor gene flow and suggesting that S. frugiperda in the Western Hemisphere are an interbreeding population.