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Fall armyworm, Spodoptera frugiperda (J. E. Smith), and corn earworm, Helicoverpa zea (Boddie), are the most important lepidopteran pests of corn in the southeastern United States. Fall armyworm often infests whorl stage plants causing leaf injury. Corn earworm often infests ears causing direct loss of grain, but fall armyworm also may infest ears especially during large infestations. Insecticidal control to prevent ear damage in field corn is difficult and generally not cost effective. Typically, early planting times are recommended in the Southeast partly to avoid damaging levels of both insects, which often occur later in the season (Buntin 2007). Transgenic corn hybrids expressing the insecticidal Cry protein from Bacillus thuringiensis (Bt) has been available in the Southeast since 1998, and this technology offers the potential for reducing losses by fall armyworm and corn earworm in field corn (Buntin et al. 2001, 2004). Several events of transgenic Bt corn have been developed with different modes of toxin expression (Ostlie et al. 1997). The MON810 event (Monsanto Co., St. Louis, MO) and a similar event Bt11 (Syngenta Crop Sciences, RTP, NC) contain the Cry1Ab gene. These events are marketed as YieldGard[R] corn borer (YGCB) corn, and express endotoxin in vegetative and reproductive structures throughout the season (Armstrong et al. 1995; Williams et al. 1997). More recently, a new transformation event TC 1507 expressing a Bt-derived insecticidal protein Cry1F is being marketed as Herculex[R] I Insect Protection (HX) (Dow AgroSciences, Indianapolis, IN). Both MON810 and TC1507 events are very effective against the European corn borer, Ostrinia nubilalis (Hubner), and southwestern corn borer, Diatraea grandi-osella Dyar (Williams et al. 1998; Graeber et al. 1999; Lauer & Wedberg 1999; Archer et al. 2000; Abel & Pollan 2004; Allen & Pitre 2006; Siebert et al. 2008). European corn borer occurs throughout most of Georgia but usually is not an important pest of field corn. Southwestern corn borer does not occur in the coastal plain region of the southeastern United States but is present in northwestern Georgia and northern Alabama where it can cause economic damage (Buntin 2007). Laboratory feeding trials and small controlled field trials have shown that hybrids containing Cry1Ab endotoxin reduced fall armyworm and corn earworm growth and survival (Williams et al. 1997, 1998; Bokonon-Ganta et al. 2003; Abel & Pollan 2004). However, fall armyworm is less susceptible to Cry1Ab endotoxins than southwestern corn borer (Williams et al. 1977; Abel & Pollan 2004). Storer et al. (2001) showed that corn containing Cry1Ab (Bt11) stunted growth of H. zea larvae, reduced H. zea adult emergence from Bt-corn fields by about 75% and delayed adult emergence by 6-12 d. Furthermore, kernel damage was reduced an average of 80% in Bt hybrids. Buntin et al. (2001, 2004) conducted a series of trials in 1998-2000 in Georgia with corn planted at the recommended time and one and two months later, which showed that MON810 and Bt11 events prevented whorl damage, kernel damage, and yield loss by lepidopterans, primarily fall armyworm and corn earworm, in later plantings at all locations. Bt traits generally did not improve the performance of corn planted at recommended times (March and April depending on location), because these plantings mostly escaped severe lepidopteran damage. However, Bt traits prevented yield loss of 50% or more in some later plantings. Nevertheless, field observations indicate that hybrids containing the MON810/Bt11 events can still suffer substantial whorl damage by fall armyworm when serve infestations occur (GDB, personal observation). More recently, Siebert et al. (2008) showed that Cry1F provided a high level of protection in corn against fall armyworm leaf feeding and whorl damage. Adapted field corn hybrids expressing Cry1F have only become available in the last few years in the southeastern United Sta

Science & Nature
December 1
Florida Entomological Society

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