Field studies were done in 1995-1996 to assess the efficacy of three sweet corn hybrids that express the Bacillus thuringiensis (Bt) toxin, CrylAb, against two lepidopteran pests, Ostrinia nubilalis (Hubner) 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 efficacy 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 99-100% compared with the appropriate non-Bt check. Plant damage was also significantly 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 x 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 X 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 significantly 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. Specifically, 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 specific production regions, and the associated insect pests of sweet corn in each area, some insecticide applications may still be necessary.
Field studies were conducted near Rosemount, MN, during the years 1991–1994, 1996–1997, and 1999–2003, to assess the diversity of and parasitism by larval and pupal parasitoids of cabbage looper, Trichoplusia ni (Hübner), imported cabbageworm, Pieris rapae (L.), and diamondback moth, Plutella xylostella (L.). Species composition, parasitism rate, and species abundance (Hill's N1) were determined for each pest species. Voria ruralis (Fallen), Copidosoma floridanum (Ashmead), and Compsilura concinnata (Meigen) were the dominant parasitoids of T. ni; Phryxe pecosensis (Townsend), Cotesia glomerata (L.), and Pteromalus puparum (L.) are the dominant parasitoids of P. rapae; and Diadegma insulare (Cresson) is the dominant parasitoid of P. xylostella. In addition, the first records of the introduced parasitoid, C. concinnata, parasitizing T. ni and P. rapae, and Cotesia rubecula (Marshall) parasitizing P. rapae are documented for Minnesota.
This test was conducted on the University of Minnesota, Rosemount Experiment Station in corn planted 1 Jun. Plots consisted of single rows on 30" centers, 30 ft in length, and separated by untreated border rows and 4 ft alleys between replicates. Treatments were arranged in a randomized complete block design with 4 replications. All plots were infested 27 Jul with approximately 25 neonate larvae/plant; larvae were applied to the whorl using ‘bazooka’ applicators. Plant growth stage at the time of infestation was primarily VI2-V13 and 95% pre-tassel (i.e., green tassel visible within the whorl). A single application of each treatment was made 4 Aug; the majority of larvae were second-instars. A medium-clearance Spirit sprayer was used to apply all materials. Liquid formulations were applied with one TX-10 nozzle/row at 40 psi, for a final spray volume of 30 gal/acre. Granular formulations were applied using a Gandy applicator. For all materials, applicator ground speed averaged 2 mph, wind speed 2 mph and ambient temperature 59°F. All treatments were evaluated 23 Aug by dissecting 5 plants per plot, from the tassel to the soil surface.
This test was conducted on the University of Minnesota, Rosemount Experiment Station, in snapbeans planted 8 Jul. Plots consisted of three 30 ft (9.14 m) rows on 30 inch (0.76 m) centers. Two untreated border rows and 7 ft (2.13 m) alleys separated the plots. Treatments were arranged in a randomized complete block design with 4 replications. Treatments were applied 13 Aug using a moderate-clearance Spirit sprayer with 7 TX-10 hollowcone nozzles delivering 27 gpa (252.5 liters/ha) at 50 psi (3.51 kg/ cm2) and 3 mph (4.8 km/hr). All treatments included the sticker/extender Bond at 0.10 fl oz/gal (0.79 ml/liter). During application, wind speed averaged 4-5 mph (6.4-8.1 km/hr) and the temperature was 72°F (22°C). Plots were artificially infested using a ‘bazooka’ applicator on 10, 16 and 20 Aug. Plants were at 20% bloom, 25% pin-bean (beans ≤ 1 inch) and 95% pod growth stages, respectively. On each infestation date, 15-20 consecutive plants in one row of each plot were infested with approximately 25 first-instar ECB larvae/plant. Infestations were designed to evaluate the activity of each material 3-d pretreatment, 3-d posttreatment and 7-d posttreatment. Feral ECB moth flights during this time were low (18/night/light-trap at peak). All treatments were evaluated 16 Sep by selecting five consecutive plants from the center of each infested row. Total pods, pods with feeding damage and total ECB larvae were recorded. Because of poor survival at the 10 Aug infestation, and high-survival in most treatments for the 20 Aug infestation, data only for the 16 Aug infestation are summarized.
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