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Title: Cross-Resistance Responses of Cry1Ac-Selected Heliothis virescens (Lepidoptera: Noctuidae) to the Bacillus thuringiensis Protein Vip3A

item Jackson, Ryan

Submitted to: Journal of Economic Entomology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/27/2006
Publication Date: 2/1/2007
Citation: Jackson, R.E., MarcusM.A., Gould F., Bradley, J. R. Jr., and Van Duyn J.W. 2007.cross-resistance responses of cry1Ac-selected Heliothis virescens (Lepidoptera: Noctuidae) to the Bacillus thuringiensis protein Vip3a. J. Econ. Entomol. 100: 180-186.

Interpretive Summary: Laboratory experiments demonstrated that three tobacco budworm populations with moderate-to-high levels of resistance to a protein, Cry1Ac, that is produced in all commercially available Bt cottons did not exhibit any increased tolerance to the Vip3A protein. This information is important as the Vip3A protein will be the primary method of plant protection in certain cotton lines in which registration is pending. Therefore, these new cotton lines that produce the Vip3A protein will be effective in controlling tobacco budworm populations in the event of resistance development to current Bt cotton lines.

Technical Abstract: One susceptible and three Cry1Ac-resistant strains of tobacco budworm, Heliothis virescens (F.), were used in laboratory studies to determine the level of cross-resistance between the Bacillus thuringiensis (Berliner) toxins Cry1Ac and Vip3A using concentration-mortality and leaf tissue experiments. Concentration-mortality data demonstrated that the three Cry1Ac-resistant H. virescens strains, YHD2, KCBhyb, and CxC, were at least 215- to 316-fold resistant to Cry1Ac compared to the susceptible strain, YDK. Results from Vip3A concentration-mortality tests indicated that mortality was similar among all four H. virescens strains. Relative larval growth on Cry1Ac reflected concentration-mortality test results, as YHD2 larval growth was mostly unaffected by the Cry1Ac concentrations tested. Growth ratios for KCBhyb and CXC indicated that they had a more moderate level of resistance to Cry1Ac than did YHD2. Relative larval growth on Vip3A was highly variable at lower concentrations but was more consistent on concentrations of Vip3A above 25 'g/ml. Differences in larval growth among strains on Vip3A were not as pronounced as seen in Cry1Ac experiments. Mortality and larval growth was also assessed in leaf tissue bioassays in which YDK, CxC, and KCBhyb neonates were placed onto leaf disks from non-Bt and Bt cottons for 5 d. Three Bt lines were used in an initial bioassay and consisted of two Vip3A-containing lines, COT203 and COT102, and a Cry1Ac-producing line. Mortality of KCBhyb and CXC was lower than that of YDK larvae in the presence of leaf tissue from the Cry1Ac-producing line. Additionally, increased larval growth and leaf tissue consumption on Cry1Ac-containing leaf disks was observed for KCBhyb and CXC. Mortality and larval weights were similar among strains when larvae were fed leaf tissue of either non-Bt, COT203, or COT102. A subsequent leaf tissue bioassay was conducted that evaluated 4 cotton lines: non-Bt, Cry1Ab-expressing, Vip3A-expressing, and pyramided-toxin plants that produced both Cry1Ab and Vip3A. Mortality levels were similar among strains when fed non-Bt, Vip3A-expressing, or pyramided-toxin leaf tissues. Mortality was higher for YDK than for KCBhyb or CXC on Cry1Ab-expressing leaf tissues. No differences in larval weights were observed among strains for any genotype tested. Results of these experiments demonstrate that cross-resistance is non-existent between Cry1Ac and Vip3A in H. virescens. Thus, the introduction of Vip3A-producing lines could delay Cry1Ac-resistance evolution in H. virescens, if these lines gain a significant share of the market.