Characterizing the sublethal effects of SmartStax PRO dietary exposure on life history traits of the western corn rootworm, Diabrotica virgifera virgifera LeConte

Abstract

The western corn rootworm (WCR), Diabrotica virgifera virgifera LeConte, is an economically important pest of field corn (Zea mays L.) across the United States (U.S.) Corn Belt. Repeated use of transgenic hybrids expressing Bacillus thuringiensis (Bt) proteins has selected for field-evolved resistance to all current rootworm-active Bt proteins. The newest product available for WCR management is SmartStax® PRO, a rootworm-active pyramid containing Cry3Bb1, Cry34/35Ab1 [now reclassified as Gpp34Ab1/Tpp35Ab1] and a new mode of action, DvSnf7 dsRNA. Understanding the fitness of adult WCR after dietary exposure to SmartStax® PRO will identify potential impacts on WCR population dynamics and inform efforts to optimize resistance management strategies. Therefore, the objective of the present study was to characterize the effect of SmartStax® PRO dietary exposure on WCR life history traits. Adult WCR were collected during 2018 and 2019 from emergence tents placed over replicated field plots of SmartStax® PRO or non-rootworm Bt corn at a site with a history of rootworm-Bt trait use and suspected resistance to Cry3Bb1 and Cry34/35Ab1. Adult survival was reduced by 97.1–99.7% in SmartStax® PRO plots relative to the non-rootworm Bt corn plots during the study. Individual male/female pairs were fed different diets of ear tissue to simulate lifetime or adult exposure. Life history parameters measured included adult longevity, adult head capsule width, lifetime female egg production, and egg viability. Results indicate that lifetime or adult exposure to SmartStax® PRO significantly reduced adult longevity and lifetime egg production. Larval exposure to SmartStax® PRO significantly reduced WCR adult size. Results from this study collectively suggest that SmartStax® PRO may negatively impact WCR life history traits, which may lead to reduced population growth when deployed in an area with WCR resistance to Bt traits.

Effects of SmartStax® and SmartStax® PRO maize on western corn rootworm (Diabrotica virgifera virgifera LeConte) larval feeding injury and adult life history parameters

“…Because egg viability remained very high (≥95%) across all adult diets in both Bt-resistant and Bt-susceptible populations (Figs 2D and 3D ), data suggest that even though large reductions in egg production can occur after dietary exposure to SmartStax ® or SmartStax ® PRO, most eggs that are laid will be viable. This trend was consistent with results from previous Bt-resistant WCR dietary exposure experiments with SmartStax ® PRO [ 63 ].…”

“…WCR eggs were subsequently washed onto a milk filter (KenAg, Ashland, OH) and counted using a stereo microscope. After counting, eggs from each WCR population × adult diet combination were combined and stored in Petri dishes (Thermo Fisher Scientific, Waltham, MA) according to the methods outlined in Reinders et al [ 63 ] until the following spring at which time egg viability was determined. Fifty random eggs from each WCR population × adult diet combination were transferred to a Petri dish containing a moistened Whatman™ Qualitative Filter Paper: Grade 1 Circle (Thermo Fisher Scientific, Waltham, MA).…”

“…Comparative data are also lacking when WCR–Bt pyramid interactions take place in a Bt-resistant versus Bt-susceptible population. Results of initial field trials indicated that SmartStax ® PRO significantly reduced WCR root injury [ 62 ] and adult emergence [ 62 , 63 ] in several areas of the Corn Belt with suspected or confirmed Bt resistance, suggesting the inclusion of DvSnf7 dsRNA may provide significant IRM value to the pyramid. In addition to larval mortality, SmartStax ® PRO negatively affects various adult life history parameters.…”

“…In addition to larval mortality, SmartStax ® PRO negatively affects various adult life history parameters. Previous research documented significant sublethal effects of adult dietary exposure to SmartStax ® PRO ear tissue on adult longevity and lifetime egg production in a WCR population from Colfax County, Nebraska, with confirmed resistance to SmartStax ® [ 63 ]. These results suggested one or more components of this pyramid contributed to the negative effect on WCR life history parameters and population growth.…”

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

“…Another consideration in the continuous maize system is the recently commercialized triple pyramid Cry3Bb1 + Gpp34Ab1/Tpp35Ab1 + DvSnf7 dsRNA [53]. This technology reduced WCR survival to the adult stage by 97-99% in a recent field study conducted where WCR exhibited high levels of resistance to Cry3Bb1 and Gpp34Ab1/Tpp35Ab1 [88]. Use of this technology to greatly lower density followed by rotation to a Bt pyramid would help slow WCR resistance evolution to the RNAi trait and reduce potential injury to the Bt pyramid.…”