Figure 21. Grabbing and pulling the whorl by hand to detach it from the plant for examination (M. E. Rice).

Figure 22. Unwrapping and carefully examining whorl leaves for fresh feeding injury and live larvae (M. E. Rice).

When moth activity is evident, sampling usually begins when the earliest fields reach the early whorl stage of plant development. Knowing the number of egg masses is not necessary for whorl-stage processing sweet corn because the plants have a higher infestation tolerance level. During the whorl stage, inspect the whorl leaves of 100 plants per 20-acre field and record the number of plants showing light, moderate, and heavy (see next paragraph for definitions) feeding by early instars (less than 1/2 inch in length). Samples should be taken in 10 locations in the field and at least 100 feet inside the field to avoid border area influence. Sampling should consist of examining five consecutive plants in one row, then five more plants in the adjacent row. It is important to determine if larvae are present and actively feeding. Pull out and carefully unroll (Figures 21 and 22) the whorl leaves and tassels of the first two plants showing damage at each of the 10 field locations; then note the number of live larvae found. If fall armyworm (photo), Spodoptera frugiperda, is present, add the number of plants infested with this insect to the European corn borer assessment; a management decision must be based on the combined infestations of both insect pests.

Light whorl feeding is defined as superficial 'windowpane' damage, with a few scattered shot holes on less than half of the leaves. Injury of this type usually is associated with average infestations of less than two young larvae per infested plant. Moderate damage is indicated by notable feeding holes and some midrib injury on more than two-thirds of the leaves, with 10 to 50 percent of the leaf area affected. Moderate whorl injury produces many patterns of feeding holes and midrib tunneling, which are associated with average infestations of two to five young larvae per infested plant. Heavy whorl injury involves extensive feeding on all leaves, with greater than 50 percent of the leaf area affected.

Beyond late whorl stage for sweet corn that will be processed, plants should be sampled for egg masses and active larvae. Assessments of egg and larval stages during pretassel (tassel visible within the whorl), green tassel (tassel visible but not yet pollinating), and silking stages can be made by inspecting 100 plants per 20-acre field. Samples should be taken in 10 locations in a field and at least 100 feet inside the field to avoid border area influence. Sampling should consist of examining five consecutive plants in one row, then five more plants in the adjacent row. The object is to determine the percentage of plants carrying egg masses and the percentage of primary ears infested with eggs or young larvae of a damaging insect. Treatment after larvae have infested ears is much less effective than earlier treatment. Consequently, detecting damaging insects early is critical for preventing serious ear quality loss. For sampling egg masses during silk stage, direct attention to the leaves in the ear region (four leaves above the primary ear leaf, primary ear leaf, and one leaf below the primary ear leaf, Table 14). The ear flag leaves, if prominent on the shucks, also should be examined for egg masses.