Toxicity and Characterization of Cotton Expressing <I>Bacillus thuringiensis</I> Cry1Ac and Cry2Ab2 Proteins for Control of Lepidopteran Pests

Abstract

Cry1Ac protoxin (the active insecticidal toxin in both Bollgard and Bollgard II cotton [Gossypium hirsutum L.]), and Cry2Ab2 toxin (the second insecticidal toxin in Bollgard II cotton) were bioassayed against five of the primary lepidopteran pests of cotton by using diet incorporation. Cry1Ac was the most toxic to Heliothis virescens (F.) and Pectinophora gossypiella (Saunders), demonstrated good activity against Helicoverpa zea (Boddie), and had negligible toxicity against Spodoptera exigua (Hübner) and Spodoptera frugiperda (J. E. Smith). Cry2Ab2 was the most toxic to P. gossypiella and least toxic to S. frugiperda. Cry2Ab2 was more toxic to S. exigua and S. frugiperda than Cry1Ac. Of the three insect species most sensitive to both Bacillus thuringiensis (Bt) proteins (including H. zea), P. gossypiella was only three-fold less sensitive to Cry2Ab2 than Cry1Ac, whereas H. virescens was 40-fold less sensitive to Cry2Ab2 compared with CrylAc. Cotton plants expressing Cry1Ac only and both Cry1Ac and Cry2Ab2 proteins were characterized for toxicity against H. zea and S.frugiperda larvae in the laboratory and H. zea larvae in an environmental chamber. In no-choice assays on excised squares from plants of different ages, second instar H. zea larvae were controlled by Cry1Ac/Cry2Ab2 cotton with mortality levels of 90% and greater at 5 d compared with 30-80% mortality for Cry1Ac-only cotton, depending on plant age. Similarly, feeding on leaf discs from Cry1Ac/Cry2Ab2 cotton resulted in mortality of second instars of S.frugiperda ranging from 69 to 93%, whereas exposure to Cry1Ac-only cotton yielded 20-69% mortality, depending on plant age. When cotton blooms were infested in situ in an environmental chamber with neonate H. zea larvae previously fed on synthetic diet for 0, 24, or 48 h, 7-d flower abortion levels for Cry1Ac-only cotton were 15, 41, and 63%, respectively, whereas for Cry1Ac/Cry2Ab2 cotton, flower abortion levels were 0, 0, and 5%, respectively. Cry1Ac and Cry2Ab2 concentrations were measured within various cotton tissues of Cry1Ac-only and Cry1Ac/Cry2Ab2 plants, respectively, by using enzyme-linked immunosorbent assay. Terminal leaves significantly expressed the highest, and large leaves, calyx, and bracts expressed significantly the lowest concentrations of Cry1Ac, respectively. Ovules expressed significantly the highest, and terminal leaves, large leaves, bracts, and calyx expressed significantly (P < 0.05) the lowest concentrations of Cry2Ab2. These results help explain the observed differences between Bollgard and Bollgard II mortality against the primary lepidopteran cotton pests, and they may lead to improved scouting and resistance management practices, and to more effective control of these pests with Bt transgenic crops in the future.

Demographic Performance of Helicoverpa zea Populations on Dual and Triple-Gene Bt Cotton

“…Significant mortality from Cry1Ac+Cry1F was observed only in the Santa Rosa population. Cry toxins have been expressed in Bt cotton cultivars since its first commercial release, and the first report of H. zea Cry1Ac resistance was documented 15 years later in the U.S., and recently the widespread resistance to Cry2Ab [6,20,46]. These may be a contributing factor to the considerable rates of larval survival on Cry1Ac+Cry2Ab and Cry1Ab+Cry2Ae cultivars.…”

Performance of cotton expressing Cry1Ac, Cry1F and Vip3Aa19 insecticidal proteins against Helicoverpa armigera, H. zea and their hybrid progeny, and evidence of reduced susceptibility of a field population of H. zea to Cry1 and Vip3Aa in Brazil

“…The relative high mortality levels observed for Helicoverpa species from laboratory-reared colonies and hybrids on leaves of DAS-21023-5 × DAS-24236-5 × SYN-IR102-7 cotton indicate that Cry1Ac, Cry1F and Vip3Aa19 are effective in protecting cotton plants against damage from these target pests. This 3-gene Bt cotton technology also improved protection against square damage caused by Helicoverpa species and hybrids compared to 2-gene Bt cotton and should improve yield protection, since larvae may prefer to attack reproductive structures of cotton [ 37 , 38 ]. According to Rios et al [ 7 ], there is chance of interspecific crosses in Helicoverpa species, with interspecific crossing probably occurring when pest populations are at high densities, suggesting that populations of these pests should be maintained low levels in the field.…”

Expression of Bt Protein in Transgenic Bt Cotton Plants and Ecological Fitness of These Plants in Different Habitats

“…Cotton plants were reported to be most resistant to insects in the early stages of their growth as the concentration of Bt toxin declined during the growing season (Sivasupramaniam et al, 2008;Brevault et al, 2013). Cot102 insect-resistant cotton exhibited the best resistance to H. armigera in the early growth stage; with the development of insect-resistant plants, the resistance to phytophagous insects showed a decreasing trend (Llewellyn et al, 2007).…”