with Lepidopteran resistance traits have been available since at least 1902 when Burpee Seeds released “Golden Bantam.” Varieties with antibiotic properties have been developed by selecting for high levels of the toxin 2,4-dihydroxy-7-methoxy-1,4-benzoxazin-3-one (DIMBOA) and related hydroxamic acids, which vary widely in concentration in different corn plant tissues (Houseman et al. 1992). Structural traits such as highly lignified stalks have also been selected to serve for increased pest resistance. However, many of these varieties are variably or moderately resistant and may have relatively lower yields than susceptible corn lines. For example, DIMBOA levels in initially resistant varieties tend to decrease with plant age (Barry et al. 1994, Frey et al. 1997). Thus, some conventionally bred varieties resistant to ECB have been replaced by higher-yielding varieties with increased total yields despite heavier losses from ECB (Gianessi and Carpenter 1999).
Transgenic insect pest resistance can be obtained by transferring part of a bacterial genome (from various strains of Bacillus thuringiensis) into a plant genome. The genetic sequence encodes production of a toxic protein so that plant tissues become lethal to target caterpillar pests feeding on those tissues. At least three major opportunities are afforded by transgenic insect resistance traits in crops: (1) pest resistance in crops whose close relatives are not resistant to the target pests, (2) more effective pest control than pesticides and/or conventionally available resistance, and (3) the trait can be transferred into varieties showing excellent agronomic performance (e.g., Wiebold et al. 2000), thus avoiding the yield drag that sometimes accompanies conventional breeding efforts. For caterpillar pests of corn, all of these factors can be important. Therefore, starting in the 1980s, transgenic Bt corn lines were developed and field-tested for efficacy against target Lepidopteran pests, including ECB, SWCB, and, when toxins are expressed sufficiently in the corn ears, CEW.
Other crops have shown promise for Bt-based insect resistance. By October 2000, 25 petitions for nonregulated status (from Monsanto, Northrup-King, Ciba-Geigy, DeKalb, AgroEvo, and Calgene) had been submitted to APHIS for Bt crops (corn, potato, tomato, and cotton). Sixteen were approved for deregulation of transgenic crops with Bt-based insect pest resistance, seven petitions were withdrawn, and two were pending a decision. Field trials of dozens of other transgenic Bt-based insect-resistant plant varieties (including cotton, potato, rapeseed, poplar, broccoli) have been conducted (see “Field Test Releases in the U.S.,” Information Systems for Biotechnology online database: www.nbiap.vt.edu).
For comparison, one of the earliest Bt corn petitions for deregulation by APHIS and one of the most recent are included in this case study (see “Current Status of Petitions,” www.aphis.usda.gov/biotech/petday.html). Transformation Event 176 corn (Maximizer), with marker genes for resis-