Walk into the field 50 feet and dig three plants, making sure to include about six inches on each side of the stalk. Remove the stalk and wash the roots to remove the soil. Repeat this step four times at representative locations in the field. Examine the root by looking for and counting pruned roots. If you count an average of three to five roots per plant that have been pruned back to within 1.5 inches of the plant, rootworms have likely caused economic damage to the plant.