Corn Rootworm Scouting

FIELD FACTS CORN ROOTWORM SCOUTING

Corn rootworm larvae damage corn by feeding on roots of Scouting for corn rootworm larvae: growing corn plants, thereby reducing root mass, structural support, and the plants’ ability to absorb water and nutrients.  Begin in early to mid-June, or when the corn is in the V6 The impact of root feeding by corn rootworm larvae on corn to V12 growth stage. yield varies greatly depending on timing of rootworm  Dig up 2 plants at each of 5 locations with the soil from feeding, soil type, available moisture, the hybrids ability to 6-8" around the plant. Sift soil over a sheet of black regenerate damaged roots and the related environmental and plastic looking for 1/32" to agronomic conditions. University and USDA researchers 1/2" long larvae. have not established a relationship between rootworm feeding and yield loss; however, some studies have suggested an  There is no economic economic loss with root ratings of 2.5 on the Iowa State threshold for larvae per Scale. This fact sheet will provide guidelines for the lifecycle plant; however, some and management of corn rootworms. consultants determine the need for emergency controls with an average of 2-3 larvae per plant using a visual search, or 8 or more larvae are found using soil washing.  If average length of larvae are > ½-inches long or pupae are found, a rescue treatment may be too late. Several key facts about the corn rootworm:

 The corn rootworm overwinters as eggs in top 8-12 Scouting for adult corn rootworm activity: inches of soil, and hatch late spring.  Evaluate fields for silk clipping. If pollination is in  Their peak hatch varies 10-14 days from year to year. progress and the beetles have chewed back the silks so  Rootworm eggs will hatch when approximately 500 air that less than a 1/2-inch of the silks is exposed beyond heat units, base 50, have been accumulated. the husks, beetles should be controlled.  Larvae may move 1.5 ft for food.  To control adults before  There are three stages of development (instars). egg laying, examine 2  Adult emergence begins late July and may last several plants in 25 locations in the field. Treat if the weeks. left: adult Western corn rootworm beetle, number of beetles right: adult Northern corn rootworm beetle  Egg laying may persist into September. averages 0.75 or more  Corn rootworms are members of order Coleoptera, genus per plant and 10% Diabrotica. They are females are gravid with  not susceptible to Bt endotoxins that control eggs. lepidopteran pests. left: adult Western corn rootworm beetle, right: adult Northern corn rootworm beetle

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®, TM, SM Trademarks and service marks of Pioneer Hi-Bred International, Inc. ©2010, PHII Table 1. Comparison of Node Injury Scale to Iowa State Scale for evaluating rootworm feeding damage to corn roots.

Node Injury Scale* Hills and Peters Iowa State Scale

0 No feeding damage 1. No damage or only a few minor feeding scars

2. Feeding scars evident but no roots eaten off to within 1½ inch of the plant

3. Several roots eaten off to within 1½ inch of the plant but never the equivalent of an entire node of roots destroyed

1 One node, or the equivalent of a node, pruned within 6. One root node, or the equivalent of a node, 2 inches of the stalk completely destroyed

2 Two nodes pruned to within 2 inches of the stalk 7. Two root nodes completely destroyed

3 Three nodes pruned to within 2 inches of the stalk 8. Three or more root nodes destroyed

* Damage between complete nodes eaten is noted as the percentage of the node missing; e.g., a rating of 1.25 = one node and ¼ of the next node are pruned to within 2 inches of the stalk.

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