HORTICULTURAL ENTOMOLOGY Field Efficacy of Sweet Corn Hybrids Expressing a Bacillus thuringiensis Toxin for Management of Ostrinia nubilalis (Lepidoptera: Crambidae) and Helicoverpa zea (Lepidoptera: Noctuidae) ERIC C. BURKNESS,1 W. D. HUTCHISON,1 PATRICIA C. BOLIN,2 DAVID W. BARTELS,3 4 5 D. F. WARNOCK, AND D. W. DAVIS J. Econ. Entomol. 94(1): 197Ð203 (2001) ABSTRACT Field studies were done in 1995Ð1996 to assess the efÞcacy of three sweet corn hybrids that express the Bacillus thuringiensis (Bt) toxin, Cry1Ab, against two lepidopteran pests, Ostrinia nubilalis (Hu¨ bner) and Helicoverpa zea (Boddie). The Bt hybrids tested were developed by Novartis Seeds, using the event BT-11, which expresses Bt toxin in green tissue as well as reproductive tissues including the tassel, silk, and kernel. Bt hybrids were compared with a standard non-Bt control or the non-Bt isoline for each hybrid; none of the hybrids were treated with insecticides during the study. Hybrid efÞcacy was based on larval control of each pest, as well as plant or ear damage associated with each pest. In both years, control of O. nubilalis larvae in primary ears of all Bt hybrids was 99Ð100% compared with the appropriate non-Bt check. Plant damage was also signiÞcantly reduced in all Bt hybrids. In 1996, control of H. zea in Bt hybrids ranged from 85 to 88% when compared with the appropriate non-Bt control. In 1996, a University of Minnesota experimental non-Bt hybrid (MN2 ϫ MN3) performed as well as the Bt hybrids for control of O. nubilalis. Also, in 1996, two additional University of Minnesota experimental non-Bt hybrids (A684su X MN94 and MN2 ϫ MN3) performed as well as Bt hybrids for percent marketable ears (ears with no damage or larvae). In addition, compared with the non-Bt hybrids, percent marketable ears were signiÞcantly higher for all Bt hybrids and in most cases ranged from 98 to 100%. By comparison, percent marketable ears for the non-Bt hybrids averaged 45.5 and 37.4% in 1995 and 1996, respectively. Results from the 2-yr study strongly suggest that Bt sweet corn hybrids will provide high levels of larval control for growers in both fresh and processing markets. SpeciÞcally, Bt sweet corn hybrids, in the absence of conventional insecticide use, provided excellent control of O. nubilalis, and very good control of H. zea. However, depending on location of speciÞc production regions, and the associated insect pests of sweet corn in each area, some insecticide applications may still be necessary. KEY WORDS Ostrinia nubilalis, Helicoverpa zea, Bacillus thuringiensis, transgenic sweet corn CONTROL OF EUROPEAN corn borer, Ostrinia nubilalis ear tip (Flood et al. 1995). Given the larval behavior (Hu¨ bner) and corn earworm, Helicoverpa zea (Bod- of each species, imperfect insecticide coverage, and die), is a major concern for producers of fresh-market limited residual activity of insecticides (Rinkleff et al. and processing sweet corn, Zea mays L., in Minnesota 1995), growers must use multiple applications to (Gingera et al. 1993, Bartels and Hutchison 1995). achieve marketable produce. Until recently, control options have been primarily A novel approach to reducing insecticide use in limited to foliar insecticide applications. Insecticidal sweet corn is the development of transgenic sweet control, however, is often inadequate because of im- corn where a cry gene from Bacillus thuringiensis perfect timing of applications (e.g., Bartels and (Berliner) (Bt) is transferred to the genome of sweet Hutchison 1995). This is primarily due to the rapid corn inbreds to create transgenic Bt hybrids (Ostlie et movement of neonates, soon after egg hatch, into the al. 1997, Lynch et al. 1999a). This transfer of the cry Bt gene allows for the production of Bt toxins in several 1 University of Minnesota, Department of Entomology, 1980 Fol- tissues of the sweet corn plant. Novartis Seeds well Avenue, St. Paul, MN 55108Ð6125. (Nampa, ID) developed the Þrst sweet corn hybrids 2 Oklahoma State University, Integrated Pest Management Pro- using the 35S promoter (Armstrong et al. 1995), and gram, Department of Entomology, Noble Research Center, Stillwater, OK 74078. the gene for Cry1Ab Bt protein (Lynch et al. 1999a). 3 USDA-APHIS-PPQ, Mission Plant Protection Center, Moore Air This event, BT-11, which provides high Bt expression Base, Building S-6414, P.O. Box 2140, Mission, TX 78573. levels in Þeld corn (Ostlie et al. 1997), also provides 4 University of Illinois, Department of Natural Resources and En- high Bt concentrations in green leaf, silk, and kernel vironmental Sciences, 1029 Plant Science Laboratory, MC-634, 1201 South Darner Drive, Urbana, IL 61801. tissue of several sweet corn hybrids (Lynch et al. 5 University of Minnesota, Department of Horticultural Science, 1999a). Lynch et al. (1999a) found that the Novartis 1970 Folwell Avenue, St. Paul, MN 55108Ð6125. sweet corn hybrids Btk-0906, Btk-0907, Btk-0908, and 0022-0493/01/0197Ð0203$02.00/0 ᭧ 2001 Entomological Society of America 198 JOURNAL OF ECONOMIC ENTOMOLOGY Vol. 94, no. 1 Btk-0909 provided high levels of control against H. zea portion of each plot and from 14 to 42 primary ears in and fall armyworm, Spodoptera frugiperda (J.E. the naturally infested portion of each plot. Total num- Smith). They showed that a high level of plant resis- ber of O. nubilalis larvae, larval instar, location of tance was evident in leaf and silk tissue for control of larvae and presence of feeding damage were recorded H. zea. Although efÞcacy against S. frugiperda was less for each ear. Presence of larvae and/or feeding dam- striking, these authors concluded that the use of Bt age caused by O. nubilalis on the primary ear was used sweet corn would signiÞcantly reduce conventional to determine the percentage of marketable ears (i.e., insecticide use for both insect pests. percentage of ears without larvae or damage). On 6 The purpose of this study was to document the September, stalk dissections were performed on Þve impact of several BT-11 sweet corn hybrids, expressing arbitrarily selected plants per plot. For each stalk, data Cry1Ab toxin, by comparing them with convention- were recorded for number of tunnels, length of tun- ally bred insect-resistant and susceptible hybrids for nels, and whether the tassel had been broken because control of O. nubilalis and H. zea and their associated of O. nubilalis larval feeding. plant damage. In 1996, plots were also established at the Rose- mount Experiment Station. All sweet corn hybrids were planted 24 June in a randomized complete block Materials and Methods design with four replicates. Plot size was increased Sweet corn plots were established in 1995 and 1996 from 1995 to 4 rows because of an increase in seed at the Rosemount Experiment Station, Rosemount, availability. Rows were nine m long with 76-cm row MN. Each year, O. nubilalis moth ßights were moni- spacing and 3.04-m alleys separated replicates. Plant tored using a blacklight trap and moth population spacing within each row was 30 cm. Because of high dynamics were summarized using a 5-d moving aver- natural O. nubilalis populations in 1996 artiÞcial in- age (e.g., Bartels and Hutchison 1995). Treatments in festations were unnecessary. Hybrids were evaluated 1995 consisted of three sweet corn hybrids expressing 17 September by harvesting 25 primary ears per plot Cry1Ab, a non-Bt hybrid (Apache) produced through and recording total number and ear location of O. conventional breeding with O. nubilalis resistance, nubilalis and H. zea larvae, instar of O. nubilalis larvae, and a standard O. nubilalis susceptible non-Bt hybrid and presence of ear damage. As in 1995, the percent- (Jubilee) (see also Bolin et al. 1996). The 1995 Bt age of marketable ears was recorded. On 20 Septem- hybrids included Rogers-8274, Rogers-8264 and ber, Þve plants were arbitrarily selected from each plot Rogers-8277. In 1996, three improved Bt hybrids were in the Þrst three replicates for stalk dissection. Num- evaluated, reßecting additional backcrosses of the pre- ber of tunnels, length of tunnels, and broken tassels vious hybrid material, but still based on the BT-11 caused by O. nubilalis larval feeding were recorded for event. The Bt hybrids, 95-0937, 95-0941, and 95-0943 each stalk. were compared. The respective O. nubilalis suscepti- Data for each year were analyzed separately using ble non-Bt isolines of Jubilee, Empire, and Heritage a one-way analysis of variance and the Ryan-Einot- were included as non-Bt standards. In addition, the Gabriel-Welsch multiple range test (P ϭ 0.05) for non-Bt hybrid Apache and three experimental Uni- mean separation (SAS Institute 1995). Insect counts versity of Minnesota O. nubilalsis resistant non-Bt hy- were transformed using a square root (͌x ϩ 0.5) and brids (A684su ϫ MN94, MN96 ϫ A685su, and MN2 ϫ data recorded as proportions were transformed using MN3) were included in the 1996 trial. The Minnesota arcsine and square-root transformation (SAS Institute hybrids were developed using lines A684su and 1995). A685su (Davis et al. 1993). No insecticide treatments were made to any of the hybrids during either year of Results the study. In 1995, sweet corn was planted 6 June and plots Initiation of the Þrst-generation O. nubilalis moth were arranged in a randomized complete block design ßight was very similar for 1995 and 1996, with peak with four replicates (see also Bolin et al. 1998). Plots ßight occurring Ϸ22 June (Fig. 1). Phenology of the consisted of single rows 15 m long, separated by 76 cm, second-generation O. nubilalis moth ßight was also with replicates separated by 3.04-m alleys. Plant spac- similar for both years; peak moth ßight occurred Ϸ3 ing was 30 cm.