Moths captured in light traps or pheromone traps can provide indicators for anticipating when young larvae probably will become present in whorl-stage corn. First moth capture normally occurs mid-May in Delaware, southern Illinois, Kansas, Kentucky, and Missouri; during early June in Iowa, northern Illinois, Indiana, Maryland, Nebraska, Ohio, and southeastern Pennsylvania; and mid-June in central Wisconsin, central Michigan, southern Minnesota, North Dakota, southern Ontario, northern Pennsylvania, and South Dakota. The moths become active in late June to early July in Alberta, Manitoba, northern Minnesota, and Quebec. The first generation usually does not cause economic damage to corn in the southeastern states.

As mentioned earlier, when European corn borer larvae attack whorl-stage plants, yield can be reduced substantially. Scouting should begin either 170 degree-days after the first spring moth has been detected or 200 degree-days after plants reach the 6th-leaf stage, whichever occurs first. Moths are attracted to the earliest planted corn (usually the tallest) for egg laying. It usually is not necessary to scout smaller corn because DIMBOA in the plant prevents survival of larvae (refer to larval discussion under Life Cycle).

Accurate assessment depends on using good sampling technique to get a close representative of the true population. Where possible, samples should be taken from five representative sets of 20 plants for every 40 to 50 acres. Although less precise, a practical compromise for very large fields (80 acres or more) is limiting surveys to five sets of 25 consecutive plants per field. Chances of making a poor decision increase greatly if fewer samples are taken. Chemical control can be expensive and harmful to beneficial organisms, such as insect predators, parasites, and pollinators. If there is any doubt about the level of infestation, more samples should be collected.

European corn borer moths and egg laying may be concentrated along field edges, grass waterways, or access roads. Sampling European corn borer infestations along these edges does not give an accurate estimate of the field population. To eliminate the influence of border areas, the first sample should be taken after walking 100 feet or more into the field. Samples should represent all topographical, environmental, and cultural features that may create changes in plant height, plant maturity, and plant density. Sampling only plants that show larval injury should be avoided. Sampling only infested plants could inflate the population estimate. If more than one variety is planted in the same field, consider each variety as a separate field for scouting purposes.

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).

Treatment guidelines for corn attacked in the whorl stage are based on active whorl feeding, which is indicated by fresh leaf damage (Figure 15) and live larvae inside the whorl. Whorl feeding can be assessed by grabbing the upper five to seven leaves of the corn plant and pulling straight up (Figure 21) to detach the whorl from the basal part of the plant. Leaves should be unwrapped carefully and examined for fresh feeding injury and live larvae (Figure 22). Newly hatched larvae are very small and may be difficult to see. Pin holes and the "windowpane" characteristic (Figure 11) in young whorl leaves indicate larval feeding. Older larvae may be tunneled into the midrib of a leaf. Shot holes (Figure 15) indicate second and possibly third instar leaf feeding. The number of live larvae in each plant must be recorded to correctly calculate the average number of larvae per plant. Pin holes and shot holes in older leaves indicate that larvae were on the plant; however, the number of larvae per plant can be determined only by dissecting whorls. It is important to check for live larvae because larval mortality in the first 3 to 5 days following hatch is normally very high. Adverse environmental conditions (high temperatures, extreme dry conditions, or excessive rainfall), predatory insects, and plant resistance are major causes of larval mortality in the whorl.

Sampling a field when hatching of larvae is just beginning may produce misleading evidence that there are not enough European corn borers in the field to economically justify an insecticide application. Additional hatching may occur. Therefore, the field should be rescouted every 3 to 5 days until insecticide is applied or scouting shows that the population is not large enough to justify treatment. Similarly, uneven plant emergence following planting can cause variability in European corn borer infestations due to older plants being more attractive than younger plants. Unusual cultural or environmental events should be carefully weighed when making a treatment decision.