Timely scouting for beetles in the cornfield, beginning soon after first beetle emergence in early July, is necessary to make decisions regarding insecticide treatment the following spring. Rootworm beetle scouting should continue as long as beetles are present. However, scouting can be discontinued whenever the beetle population reaches or exceeds the threshold level and therefore categorizes that field with the potential for economic damage the following year. If this occurs, apply a soil insecticide the next year or rotate to a different crop. If beetles are not found the first few times a cornfield is scouted, continue weekly field checks until late August for beetles that may move in from surrounding cornfields.
Various factors affect the accuracy of beetle counts.
Competence and diligence when making beetle counts are essential. Sample size is also important. In general, as the number of sample sites increases, so does the accuracy of the estimate of the beetle population. The larger the field, the more sample sites are needed to accurately estimate the beetle population density. Conversely, the smaller the field, the fewer the sample sites needed.