Sampling Transgenic Corn Producing Bt Toxins for Corn Earworm Injury.

Transgenic corn, Zea mays L., hybrids expressing insecticidal proteins from Bacillus thuringiensis (Bt) and non-Bt near isolines were sampled for injury from Helicoverpa zea (Boddie) in North and South Carolina from 2012 to 2017. A total of 7,260 ears were sampled, with an average kernel injury from H. zea feeding of 1.22 ± 0.02 (SEM) cm2. The χ2 statistics for observed and Poisson predicted distributions of the area of kernel injury indicated nonrandom distributions for all hybrids (P < 0.0001), which were confirmed by all variance to mean ratios (ID) being significantly greater than one. Slopes b of Taylor's power law and β of Iwao's patchiness regressions for the area of kernel injury were all either not significantly (P > 0.05) different from a value of one or significantly (P < 0.05) less than a value of one. Within each family of hybrids, relationships between proportions of ear samples with injury and area of kernel injury were similar among Bt and non-Bt hybrids. For the same level of injury, to reach a population estimate within 10% of the mean (Dx = 0.1), the number of sample units required was large (>100), particularly at low levels of injury. Sample sizes for estimates within 30% of the mean (Dx = 0.3) were considerable smaller. Sample size for Bt hybrids relative to non-Bt hybrids varied with the levels of injury and with family. Our study has provided the first sampling recommendations for population estimates of H. zea injury to corn ears.