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

Abstract

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.

Fig 1. Nebraska state map showing counties in gray where on-farm research was conducted.

The expanded map of northeast Nebraska includes the unique numbers assigned to each field/WCR population used in the project.

Fig 2. Results of generalized linear mixed model analysis to investigate effect of soil insecticide * hybrid treatments on corn rootworm and agronomic variables.

(a) mean ± SE root injury rating: Proportion, average of 10 plants, normal distribution by Central Limit Theorem; (b) mean ± SE total adult emergence: Negative binomial with log link function; (c) Mean ± SE proportion lodged plants: Beta distribution; (d) Mean ± SE yield: Normal distribution. Number of sites = 34 except (d) N = 31 sites. Within a variable, LSMEANS with the same letter are not significantly different (SAS 9.4, GLMM; GLIMMIX procedure, LSMEANS with slice option; p >0.05).

Fig 3

Linear regression of non-RW Bt and Bt pyramid strip trial variables for each Bt hybrid bioassay corrected survival category (<0.3 blue dots [a, d, g], 0.3–0.49 black dots [b, e, h], >0.5 gray dots [c, f, i]). (a-c): Mean NIS; (d-f) Total adult emergence; (g-i) Mean non-RW Bt NIS x Bt pyramid yield.