Model of Corn Earworm (Lepidoptera: Noctuidae) Development, Damage, and Crop Loss in Sweet Corn

Phenology models of development of com earworm, Helicoverpa zea (Boddie), and processing sweet corn, Zea mays L. (‘Jubilee’) were combined to predict pest infestation and economic loss in western Oregon. Cohorts of com earworm larvae feeding on newly silking corn and artificial diet were monitored in the field. Degree-days for larval development (threshold = 12.5°C) were 185 ± 54 (SD) and were represented in the model by varying degree-day totals for six larval subpopulations. Oviposition of moths closely followed corn silking and was normally distributed for degree-days. Harvest-time distributions of larvae combined with ear damage tunnel lengths for each instar were used to estimate cullage and economic loss. The model was validated by comparing predictions with damage and economic loss in three randomly selected fields from each of 4 yr. The predicted versus actual damage distributions matched more closely than did larval distributions at harvest; predicted and actual economic losses differed by an average of $0.88/ha ± $4.15 (SD), or 8.2% ± 17.7%. A sensitivity analysis was conducted to examine the change in economic loss caused by varying harvest date using one planting from 1990 that had earworm-induced losses of $22.16. Harvest 7 days early decreased loss to $18.79 (–19%). Harvest 7 days late increased loss to $25.39 (+10%), indicating that a delay in harvest has less effect on loss than an early harvest. Simulations were run to assess economic loss potential of an infestation level of 41% using 21 yr of temperature data and four silking dates. Analysis showed decreased averages and increased variation in loss for later silking dates. Late-season plantings generally had less economic loss than early-season plantings because of declining average temperatures, but year-to-year late-season temperature variations increased the uncertainty of losses.