The efficacy of nontransgenic sweet corn, Zea mays L., hybrids cross-pollinated by Bacillus thuringiensis (Bt) sweet corn hybrids expressing Cry1Ab toxin was evaluated in both field and laboratory studies in Minnesota in 2000. Non-Bt and Bt hybrids (maternal plants) were cross-pollinated with pollen from both non-Bt and Bt hybrids (paternal plants) to create four crosses. Subsequent crosses were evaluated for efficacy in the field against European corn borer, Ostrinia nubilalis (Hübner), and corn earworm, Helicoverpa zea (Boddie), and in laboratory bioassays against O. nubilalis. Field studies indicated that crosses with maternal Bt plants led to low levels of survival for both O. nubilalis and H. zea compared with the non-Bt x non-Bt cross. However, the cross between non-Bt ears and Bt pollen led to survival rates of 43 and 63% for O. nubilalis and H. zea larvae, respectively. This intermediate level of survival also was reflected in the number of kernels damaged. Laboratory bioassays for O. nubilalis, further confirmed field results with larval survival on kernels from the cross between non-Bt ears and Bt pollen reaching 60% compared with non-Bt crossed with non-Bt. These results suggest that non-Bt refuge plants, when planted in proximity to Bt plants, and cross-pollinated, can result in sublethal exposure of O. nubilalis and H. zea larvae to Bt and may undermine the high-dose/refuge resistance management strategy for corn hybrids expressing Cry1Ab.

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‘Jubilee’ was planted 25 Jun at the Uni-versity of Minnesota Agricultural Experiment Station at Rosemount, MN. Treatments were arranged in a RCB design with 4 replications. Plots consisted of 2 rows, 25 ft long with 30-inch row spacing. A single skip row separated each plot and 10-ft alleys separated the replications. Treatments were applied with a CO2-pressurized backpack sprayer using a single nozzle fitted with an XR-Teejet 8002 flat fan nozzle with no screen. The sprayer was calibrated to deliver 20 gpa at 34 psi, both rows of each 2-row plot were treated. Treatments were applied to the ear zone and began at 50% silk. Three applications were made 31 Aug, 5 and 11 Sep. Treatments were evaluated on 23 Sep by selecting 25 ears at random from each plot (100 per treatment) and recording the total number of larvae, larval size and location, and feeding damage (cm2).

Two insecticide trials were conducted in a commercially grown field of pumpkins managed by Owatonna Canning Co., in Mower County, Minnesota. The field was planted 18 May. Plots in both trials consisted of single 25 ft (7.62 m) rows on 5 ft (1.52 m) centers. Plant spacing was 18 inches (0.46 m). Both trials were arranged in a randomized complete block design. The first trial consisted of 4 replications and the second, 5 replications. All plots were separated by 1 or 2 skip rows. Treatments were applied with a CO2 pressurized backpack sprayer fitted with a XR-Teejet 8002 flat fan nozzle. The sprayer was calibrated to deliver 25 gpa (233.8 liter/ha) at 35 psi (242 kPa). Adios, a cucurbitacin bait treatment, contained 13% carbaryl. Treatments for the first trial were applied 3 Jun. Percent infestation and insect counts were taken 4 Jun (1 DAT), 6 Jun (3 DAT), and 11 Jun (8 DAT). Treatments for the second trial were applied once, 16 Jun. Percent infestation and insect counts were taken 17 Jun (1 DAT), 20 Jun (4 DAT), and 24 Jun (8 DAT). For each study, SCB was the dominant insect pest, with only a low-level population of spotted cucumber beetle, Diabrotica undecimpunctata howardi, present in the field. Count and percentage data were transformed by Vx + 0.5 and arcsin (x), respectively.

This study was conducted in a commercially grown field of pumpkin managed by Owatonna Canning Co., Dodge County, Minnesota. The cultivar ‘Buckskin’ was planted 29 May. Plots consisted of three 25-ft rows on 5-ft centers. Plant spacing was 18 inches. Treatments were arranged in a RCB design with 4 replications. Treatments were applied to all 3 rows of a plot once (25 Jun) with a CO2-pressurized backpack sprayer fitted with a DG-Teejet 8002 flat fan nozzle, and no screen. The sprayer was calibrated to deliver 25 gpa (233.8 1/ha) at 35 psi (242 kPa). Adios, a cucurbitacin bait formulation, contained 13% carbaryl. Percentage of plants infested and live as well as dead insect counts were determined for the middle row of each plot 26 Jun (1 DAT), 28 Jun (3 DAT) and 2 Jul (7 DAT).