Daniel L. Frank

Mortality Impact of Bt Transgenic Maize Roots Expressing eCry3.1Ab, mCry3A, and eCry3.1Ab Plus mCry3A on Western Corn Rootworm Larvae in the Field

ABSTRACT Mortality of the western corn rootworm, Diabrotica virgifera virgifera LeConte, larvae due to feeding on maize, Zea mays L., expressing Bacillus thuringiensis Berliner (Bt) was evaluated in five Missouri sites in 2007, 2008, and 2009. Specifically, eCry3.1Ab (5307), mCry3A (MIR604), and eCry3.1Ab plus mCry3A proteins relative to survivorship on maize with the same genetic background without these genes (isoline maize) was evaluated. An average of 890.8 ± 152.3 beetles emerged from isoline plots, whereas average beetle emergence from 5307, MIR604, and 5307 × MIR604 was 1.9 ± 0.6,19.3 ± 6.3, and 0.8 ± 0.3, respectively, when averaged across 22 replications in five environments. Overall, 66, 50, 61, and 51% of beetles recovered from 5307, MIR604, 5307 × MIR604, and isoline maize, respectively, were female, and there was no significant difference between the number of male and female beetles that emerged from any of these treatments. Mortality due to 5307, MIR604, and 5307 × MIR604 was 99.79, 97.83, and 99.91%, respectively. There was an 8.0-d delay in time to 50% beetle emergence from 5307 compared with isoline maize, which was significantly later than to the other three maize lines. The average delay to 50% emergence from MIR604 and 5307 × MIR604 averaged 4.1 and 4.6 d, respectively later than 50% emergence from isoline maize. Female beetles had a significant delay in time to 50% emergence compared with male beetles from all treatments with the exception of 5307 × MIR604. Data are discussed in terms of insect resistance management in relation to other control measures for western corn rootworm.

Development of resistance to eCry3.1Ab-expressing transgenic maize in a laboratory-selected population of western corn rootworm (Coleoptera: Chrysomelidae).

ABSTRACT A laboratory colony of western corn rootworm, Diabrotica virgifera virgifera LeConte, was selected for resistance to transgenic maize expressing the eCry3.1Ab protein. The selected colony was developed by rearing larvae on nonelite noncommercial Bt maize expressing the eCry3.1Ab protein. After four generations, selected and control colonies were screened on eCry3.1Ab-expressing and isoline maize using greenhouse experiments. There was a significant colony × maize pedigree interaction in terms of the number of larvae recovered. There was no significant difference in the number of larvae recovered from eCry3.1Ab-expressing and isoline maize for the selected colony, whereas this difference was significant for the control colony. There was not a significant colony × maize pedigree interaction in terms of root damage, or the number of beetles recovered, but the effect of maize pedigree was significant. After four and eight generations of selection, seedling bioassays were performed. Again, there was a significant colony × maize pedigree interaction in terms of the number of larvae recovered. After 11 generations of selection, larvae from the selected colony had higher LC50 values than the control colony when exposed to increasing concentrations of the eCry3.1Ab protein. The resistance ratio of the selected colony was 2.58. These data provide necessary information for understanding the potential for Bt resistance by western corn rootworm and underscores the need for insect resistance management plans for this pest.

Direct and indirect plant defenses are not suppressed by endosymbionts of a specialist root herbivore.

Insect endosymbionts influence many important metabolic and developmental processes of their host. It has been speculated that they may also help to manipulate and suppress plant defenses to the benefit of herbivores. Recently, endosymbionts of the root herbivore Diabrotica virgifera virgifera have been reported to suppress the induction of defensive transcripts in maize roots, which may explain the finding of another study that once attacked plants become more susceptible to subsequent D. v. virgifera attack. To test this hypothesis, we cured D. v. virgifera from its major endosymbiont Wolbachia and tested whether endosymbiont-free individuals elicit different defense responses in maize roots. The presence of Wolbachia did not alter the induction of defense marker genes and resistance in a susceptible maize hybrid and a resistant line. Furthermore, attacked maize plants emitted the same amount of (E)-β-caryophyllene, a volatile signal that serves as foraging cue for both entomopathogenic nematodes and D. v. virgifera. Finally, the effectiveness of the entomopathogenic nematode Heterorhabditis bacteriophora to infest D. v. virgifera was not changed by curing the larvae from their endosymbionts. These results show that the defense mechanisms of maize are not affected by Wolbachia. Consequently, D. v. virgifera does not seem to derive any plant-defense mediated benefits from its major endosymbiont.

Effect of Seed Blends and Soil-Insecticide on Western and Northern Corn Rootworm Emergence from mCry3A + eCry3.1Ab Bt Maize

ABSTRACT Seed blends containing various ratios of transgenic Bt maize (Zea mays L.) expressing the mCry3A + eCry3.1Ab proteins and non-Bt maize (near-isoline maize) were deployed alone and in combination with a soil applied pyrethroid insecticide (Force CS) to evaluate the emergence of the western corn rootworm, Diabrotica virgifera virgifera LeConte, in a total of nine field environments across the Midwestern United States in 2010 and 2011. Northern corn rootworm, Diabrotica barberi Smith & Lawrence emergence was also evaluated in four of these environments. Both western and northern corn rootworm beetle emergence from all Bt treatments was significantly reduced when compared with beetle emergence from near-isoline treatments. Averaged across all environments, western corn rootworm beetle emergence from 95:5, 90:10, and 80:20 seed blend ratios of mCry3A + eCry3.1Ab: near-isoline were 2.6-, 4.2-, and 6.7-fold greater than that from the 100:0 ratio treatment. Northern corn rootworm emergence from the same seed blend treatments resulted in 2.8-, 3.2-, and 4.2-fold more beetles than from the 100:0 treatment. The addition of Force CS (tefluthrin) significantly reduced western corn rootworm beetle emergence for each of the three treatments to which it was applied. Force CS also significantly delayed the number of days to 50% beetle emergence in western corn rootworms. Time to 50% beetle emergence in the 100% mCry3A + eCry3.1Ab treatment with Force CS was delayed 13.7 d when compared with western corn rootworm beetle emergence on near-isoline corn. These data are discussed in terms of rootworm resistance management.

Effect of MIR604 transgenic maize at different stages of development on western corn rootworm (Coleoptera: Chrysomelidae) in a central Missouri field environment.

ABSTRACT The establishment and survival of western corn rootworm, Diabrotica virgifera virgifera LeConte, was evaluated on transgenic Bacillus thuringiensis Rerliner maize, Zea mays L., expressing the mCry3A protein (MIR604) and non-Bt maize with the same genetic background (isoline maize) at different stages of development in 2007 and 2008. Overall, western corn rootworm larval recovery, root damage, and adult emergence were significantly higher on isoline maize compared with MIR604. The number of larvae and adults collected from MIR604 did not significantly differ among egg hatch dates from each maize developmental stage evaluated in either year. In 2007, damage to isoline maize roots was lower than expected and never exceeded 0.24 nodes of damage. In 2008, over 0.60 nodes of damage occurred on isoline maize roots. The mean weight and head capsule width of larvae and adults recovered from MIR604 and isoline maize were generally not significantly different. Results are discussed in relation to insect resistance management of western corn rootworm.

Development and Characterization of MIR604 Resistance in a Western Corn Rootworm Population (Coleoptera: Chrysomelidae)

Abstract mCry3A is one of only four proteins licensed for commercial use in Diabrotica control. Utilizing a colony of western corn rootworm, Diabrotica virgifera virgifera LeConte, selected for resistance to mCry3A, we evaluated how mCry3A resistance was inherited and whether fitness costs were associated with mCry3A resistance. Reciprocal crosses between a selected colony and a control colony were performed; resulting progeny along with parent colonies were evaluated in dose toxicity assays, greenhouse assays, and seedling assays. Dose toxicity assay results were inconclusive, as the highest dose of protein tested did not produce sufficient mortality for accurate LC50 calculation. In whole-plant greenhouse assays on mCry3A-expressing corn, larval relative survival of the selected female × control male reciprocal cross was similar to that of the selected colony, while that of the control female × selected male was intermediate between the mCry3-selected colony and the control colony. However, when adult relative survival in whole-plant greenhouse assays was examined, no significant difference between the reciprocal crosses and the two parent colonies was detected. Heritability calculations based on both larval (0.66) and adult (1.03) survival data indicate that resistance to mCry3A is not inherited in a recessive manner. The selected colony was removed from selection pressure and evaluated after three or eight generations of removal. At three generations of removal from selection, a slight decrease in larval relative survival was detected compared with the selected colony. At eight generations of removal from selection, larval relative survival was comparable with that of the selected colony.

Management of the Western Corn Rootworm, Diabrotica virgifera virgifera LeConte, Using Transgenic Bt Maize

The western corn rootworm, Diabrotica virgifera virgifera LeConte, is an important pest of maize, Zea mays L., in the United States and Europe. In the United States, transgenic maize hybrids that express Cry endotoxin proteins from the soil bacterium Bacillus thuringiensis Berliner (Bt) have been developed to limit feeding damage caused by corn rootworm larvae.