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.

Fig 1. Life history parameters characterized during lifetime dietary exposure experiments in 2018 and 2019.

A) Mean female longevity (days, d), B) Mean female head capsule width (HCW; mm), C) Mean egg production per female, and D) Proportion of viable F₁ eggs. Data from 2018 and 2019 experiments were analyzed separately. Individual bars represent the mean ± standard error and include the number of male/female pairs (n) established for each lifetime diet treatment. An asterisk indicates significant differences between lifetime diet treatments within years (P<0.05).

Fig 2. Life history parameters characterized during the male dietary exposure experiment conducted in 2018.

A) Mean male longevity (days, d), B) Mean male head capsule width (HCW; mm), C) Mean egg production per UNL-ENREEC female by male diet treatment prior to mating, and D) Proportion of viable F₁ eggs from UNL-ENREEC females by male diet treatment prior to mating. Individual bars represent the mean ± standard error and include the number of male/female pairs (n) established for each male diet treatment. An asterisk indicates significant differences between diet treatments (P<0.05).

Fig 3. Life history parameters characterized during adult dietary exposure experiments in 2019 and 2020.

A) Mean male longevity (days, d), B) Mean female longevity (days, d), C) Mean male head capsule width (HCW; mm), D) Mean female head capsule width (HCW; mm), E) Mean egg production per female, and F) Proportion of viable F₁ eggs. Data from 2019 and 2020 experiments were analyzed separately. Individual bars represent the mean ± standard error and include the number of male/female pairs (n) established for each adult diet treatment. An asterisk indicates significant differences between adult diet treatments within years (P<0.05).

Fig 4. Mean larval survival (± SE) of field-collected WCR populations and susceptible lab control colonies used in plant-based bioassays.

A) 2019 bioassays and B) 2020 bioassays. Non-RW Bt = non-rootworm active Bt corn; Cry3Bb1 + Cry34/35Ab1 = SmartStax^®. Bars with the same letter were not significantly different (GLMM, P>0.05).