Field Trial Performance of Herculex XTRA (Cry34Ab1/Cry35Ab1) and SmartStax (Cry34Ab1/Cry35Ab1 + Cry3Bb1) Hybrids and Soil Insecticides Against Western and Northern Corn Rootworms (Coleoptera: Chrysomelidae)

Abstract

Western corn rootworm, Diabrotica virgifera virgifera LeConte (Coleoptera: Chrysomelidae), and northern corn rootworm, Diabrotica barberi Smith and Lawrence (Coleoptera: Chrysomelidae), are important insect pests in corn, Zea mays L. For more than a decade, growers have been using transgenic plants expressing proteins from the bacterium Bacillus thuringiensis (Bt) to protect corn roots from feeding. In 2011, western corn rootworm populations were reported to have developed resistance to Bt hybrids expressing Cry3Bb1 and later found to be cross-resistant to hybrids expressing mCry3A and eCry3.1Ab. The identification of resistance to Cry3 (Cry3Bb1, mCry3A, and eCry3.1Ab) hybrids led to concerns about durability and efficacy of products with single traits and of products containing a pyramid of a Cry3 protein and the binary Bt proteins Cry34Ab1 and Cry35Ab1. From 2012 to 2014, 43 field trials were conducted across the central United States to estimate root protection provided by plants expressing Cry34Ab1/Cry35Ab1 alone (Herculex RW) or pyramided with Cry3Bb1 (SmartStax). These technologies were evaluated with and without soil-applied insecticides to determine if additional management measures provided benefit where Cry3 performance was reduced. Trials were categorized for analysis based on rootworm damage levels on Cry3-expressing hybrids and rootworm feeding pressure within each trial. Across scenarios, Cry34Ab1/Cry35Ab1 hybrids provided excellent root protection. Pyramided traits provided greater root and yield protection than non-Bt plus a soil-applied insecticide, and only in trials where larval feeding pressure exceeded two nodes of damage did Cry34Ab1/Cry35Ab1 single-trait hybrids and pyramided hybrids show greater root protection from the addition of soil-applied insecticides.

Keywords: Cry34Ab1/Cry35Ab1; Cry3Bb1; SmartStax; northern corn rootworm; western corn rootworm.

Fig. 1

Mean node injury scale (NIS) rating ± standard error of corn hybrids with or without soil-applied insecticide (SAI), presented by level of corn rootworm injury to Cry3-expressing corn (normal or unexpected) and under high (>2 NIS), moderate (1–2 NIS), or low (<1 NIS) root feeding pressure. NIS ratings with the same letter do not differ significantly (LSD, α = 0.05).

Fig. 2

Mean grain yield (kg/ha) ± standard error of corn hybrids with or without soil-applied insecticide (SAI), presented by level of corn rootworm injury to Cry3-expressing corn (normal or unexpected) and under high (>2 NIS), moderate (1–2 NIS), or low (<1 NIS) root feeding pressure. Yield with the same letter do not differ significantly (LSD, α = 0.05).