Corn Rootworm (Diabrotica spp.) and Bt Corn: Effects on Pest Survival, Emergence and Susceptibility

Corn rootworms (Diabrotica spp.) are one of the most destructive pests of corn in the United States. Bt corn or corn that has been genetically modified to produce toxins from Bacillus thuringiensis has been rapidly adopted by growers to manage the feeding of these pests. We examined how the western ( Diabrotica virgifera virgifera) and northern (Diabrotica barberi) corn rootworm react to selection on Bt corn hybrids. The treatments were YieldGard VT Triple (Cry3Bb1), SmartStax (Cry3Bb1 and Cry34/35Ab1), SmartStax with a blended refuge of non-Bt corn and pure non-Bt corn. We also examined how the progeny of western corn rootworm react to selection on these hybrids using an F1 screen. The data show that the pyramided hybrid was the most effective at controlling rootworm and that exposure to Bt corn typically delayed the mean emergence time of these beetles compared to beetles reared on non-Bt corn. The data also show that after beetles were selected on Bt, the progeny exhibited no difference in survival when screened on Bt plants. The progeny also did not experience any delays in development on Bt corn after selection although genetic variation for survival was higher for the single toxin hybrid. Finally, there were significant correlations for survival on both the Bt hybrids in the screen when the insects were selected on YieldGard VT Triple and SmartStax with a blended refuge. This information is important when considering the insect resistance management of these species on Bt corn

Field-Evolved Resistance to Bt Maize by Western Corn Rootworm

Crops engineered to produce insecticidal toxins derived from the bacterium Bacillus thuringiensis (Bt) are planted on millions of hectares annually, reducing the use of conventional insecticides and suppressing pests. However, the evolution of resistance could cut short these benefits. A primary pest targeted by Bt maize in the United States is the western corn rootwormDiabrotica virgifera virgifera (Coleoptera: Chrysomelidae). We report that fields identified by farmers as having severe rootworm feeding injury to Bt maize contained populations of western corn rootworm that displayed significantly higher survival on Cry3Bb1 maize in laboratory bioassays than did western corn rootworm from fields not associated with such feeding injury. In all cases, fields experiencing severe rootworm feeding contained Cry3Bb1 maize. Interviews with farmers indicated that Cry3Bb1 maize had been grown in those fields for at least three consecutive years. There was a significant positive correlation between the number of years Cry3Bb1 maize had been grown in a field and the survival of rootworm populations on Cry3Bb1 maize in bioassays. However, there was no significant correlation among populations for survival on Cry34/35Ab1 maize and Cry3Bb1 maize, suggesting a lack of cross resistance between these Bt toxins. This is the first report of field-evolved resistance to a Bt toxin by the western corn rootworm and by any species of Coleoptera. Insufficient planting of refuges and non-recessive inheritance of resistance may have contributed to resistance. These results suggest that improvements in resistance management and a more integrated approach to the use of Bt crops may be necessary

Corn Rootworm (Diabrotica spp.) and Bt Corn: Effects on Pest Survival, Emergence and Susceptibility

Corn rootworms (Diabrotica spp.) are one of the most destructive pests of corn in the United States. Bt corn or corn that has been genetically modified to produce toxins from Bacillus thuringiensis has been rapidly adopted by growers to manage the feeding of these pests. We examined how the western ( Diabrotica virgifera virgifera) and northern (Diabrotica barberi) corn rootworm react to selection on Bt corn hybrids. The treatments were YieldGard VT Triple (Cry3Bb1), SmartStax (Cry3Bb1 and Cry34/35Ab1), SmartStax with a blended refuge of non-Bt corn and pure non-Bt corn. We also examined how the progeny of western corn rootworm react to selection on these hybrids using an F1 screen. The data show that the pyramided hybrid was the most effective at controlling rootworm and that exposure to Bt corn typically delayed the mean emergence time of these beetles compared to beetles reared on non-Bt corn. The data also show that after beetles were selected on Bt, the progeny exhibited no difference in survival when screened on Bt plants. The progeny also did not experience any delays in development on Bt corn after selection although genetic variation for survival was higher for the single toxin hybrid. Finally, there were significant correlations for survival on both the Bt hybrids in the screen when the insects were selected on YieldGard VT Triple and SmartStax with a blended refuge. This information is important when considering the insect resistance management of these species on Bt corn.</p

Durability of Corn Expressing Bacillus thuringiensis Insecticidal Proteins in Single and Stacked Events

Western corn rootworm is an important agricultural pest of corn. Transgenic corn producing toxins derived from the bacterium Bacillus thuringiensis Bt has been rapidly adopted by farmers. These Bt crops control many key agricultural pests such as corn rootworm. Thus, it is important to understand the durability of Bt crops. In order to delay pest resistance, farmers must plant a refuge (Box 1). Non-Bt corn is used with Bt corn as part of a refuge strategy (Figure 1). This study focuses on studying the durability of Bt corn as either a stacked event (SmartStax) or a single event (VTTriple Pro) in controlling the Western corn rootworm.</p

Field-Evolved Resistance to Bt Maize by Western Corn Rootworm

Crops engineered to produce insecticidal toxins derived from the bacterium Bacillus thuringiensis (Bt) are planted on millions of hectares annually, reducing the use of conventional insecticides and suppressing pests. However, the evolution of resistance could cut short these benefits. A primary pest targeted by Bt maize in the United States is the western corn rootwormDiabrotica virgifera virgifera (Coleoptera: Chrysomelidae). We report that fields identified by farmers as having severe rootworm feeding injury to Bt maize contained populations of western corn rootworm that displayed significantly higher survival on Cry3Bb1 maize in laboratory bioassays than did western corn rootworm from fields not associated with such feeding injury. In all cases, fields experiencing severe rootworm feeding contained Cry3Bb1 maize. Interviews with farmers indicated that Cry3Bb1 maize had been grown in those fields for at least three consecutive years. There was a significant positive correlation between the number of years Cry3Bb1 maize had been grown in a field and the survival of rootworm populations on Cry3Bb1 maize in bioassays. However, there was no significant correlation among populations for survival on Cry34/35Ab1 maize and Cry3Bb1 maize, suggesting a lack of cross resistance between these Bt toxins. This is the first report of field-evolved resistance to a Bt toxin by the western corn rootworm and by any species of Coleoptera. Insufficient planting of refuges and non-recessive inheritance of resistance may have contributed to resistance. These results suggest that improvements in resistance management and a more integrated approach to the use of Bt crops may be necessary.This article is from PLoS ONE 6 (2011): 1 doi:10.1371/journal.pone.0022629.</p

Correlation analysis for corrected survival of western corn rootworm.

<p>Correlations are shown for A) survival on Cry3Bb1 maize and Cry34/35Ab1 maize and B) survival on Cry3Bb1 maize and number of years Cry3Bb1 maize was planted in a field. Symbols in the graphs correspond to <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0022629#pone-0022629-t001" target="_blank">Table 1</a>, which lists corrected survival for populations on Bt maize and the cultivation history of fields. For (A), no significant correlation was present between survival on Cry3Bb1 maize and Cry34/35Ab1 maize (r = 0.068; df = 6; P = 0.87). For (B), a significant positive correlation was present between corrected survival on Cry3Bb1 maize and the number of years Cry3Bb1 maize had been grown in a field (r = 0.832; df = 7; P = 0.005).</p

Distribution of sites sampled within Iowa during 2009.

<p>Sites beginning with C are control fields and were not associated with feeding injury to Bt maize, and sites beginning with P were problem fields, which were associated with farmer complaints of severe injury to Cry3Bb1 maize by corn rootworm. Codes correspond to <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0022629#pone-0022629-t001" target="_blank">Table 1</a> where a field history is provided along with the corrected survival for these populations of western corn rootworm on Cry3Bb1 maize and Cry34/35Ab1 maize.</p

Sampling date in 2009, corrected survival in bioassays, and history of planting in problem fields (P1–P4) and control fields (C1–C5) from 2003 to 2009.

a<p>Field history indicates the crop that was planted in a field each year: 1 = soybean, 2 = maize lacking rootworm active Bt, 3 = Cry3Bb1 maize, 4 = Cry34/35Ab1 maize, 5 = combination of Cry3Bb1 maize and Cry34/35Ab1 maize, 6 = research plots with non-Bt maize and several Bt maize hybrids (mCry3A <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0022629#pone.0022629-Walters1" target="_blank">[46]</a>, Cry3Bb1, and Cry34/35Ab1).</p

Survival of western corn rootworm on Bt and non-Bt maize.

<p>Data are shown for A) Cry3Bb1 maize and B) Cry34/35Ab1 maize. In both cases, survival also is shown for a non-Bt near isogenic hybrid. Bar heights are means and error bars are the standard error of the mean.</p