Corn: Corn Rootworm

Order OMAFRA Publication 811: Agronomy Guide for Field Crops

Table of Contents

1. Description
2. Life History
3. Damage
4. Scouting Technique
5. Action Threshold
6. Management Strategies
7. Updates on Corn: Corn Rootworm
8. Related links..

Description

There are two species of corn rootworm in Ontario. Western corn rootworm (WCR) adults are yellow to green with three, wavy black stripes on their wings. Northern corn rootworm (NCR) adults are uniformly green to yellowish. Larvae are white with a brown head and a distinct dark tail plate; they are approximately 1 cm in length.

Plate 13. Western corn rootworm adults are yellow to green with three wavy black stripes on their back.

Plate 14. Northern corn rootworm adults are usually green to yellow and have no stripes.

Do not confuse the western corn rootworm with the striped cucumber beetle. The striped cucumber beetle's abdomen on the underside is black, and its stripe is distinctly paralleled, not wavy.

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Life History

Both WCR and NCR are uniformly distributed across Ontario at a ratio of about 85% western to 15% northern. Both have only one generation per year. Eggs are deposited in the soil from July until a killing frost in the fall. The eggs overwinter and hatching begins in early June. Adults emerge in late July where they feed on silks and tassels. If tassels have not emerged they will feed on the leaves, stripping tissue on the underside of leaves between the veins leaving "window panes."

Damage

Larvae feed on and within the roots from mid-June to mid-July, interfering with nutrient and water uptake, causing stress to the plant. Larger larvae feed on the brace roots, reducing the stability of the plant, causing it to lodge or gooseneck. Adults feed on pollen and clip the silks, interfering with pollination. Risk factors include heavier soil (clay), high beetle populations in corn of previous season and being the latest field planted in the previous season.

Scouting Technique

Monitor 20 plants in five different locations in your field weekly from when adults emerge at the end of July to the end of August.

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Action Threshold

If there is less than one beetle per corn plant on average throughout the month of August, then no insecticide is necessary in the following corn crop. (Note: 1 WCR = 2 NCR when counting adults). Field corn can withstand heavy adult populations (more than 10 adults per ear) at pollination without economic loss. After pollination is complete, beetle feeding no longer poses a threat to yield. For seed corn, foliar insecticides for control of adults may be necessary if adult populations are high enough to cause extensive silk clipping and pollination disruption.

Management Strategies

Crop rotation is the best strategy for control. Since corn is the primary host crop, avoid planting corn on corn. Continuous corn fields produce up to 4 million beetles per hectare. Although rootworm insecticides can protect the crop from damage, on average they reduce beetle emergence by only 25%. Crop rotation is thus far superior to insecticides for reducing rootworm populations. If crop rotation is not practical, it may be necessary to treat second-year corn, especially on clay soils. If extensive goose-necking occurs or monitoring for adults in August shows significant populations, consider insecticide treatments the following spring if corn is to be replanted in that field. For soil-applied insecticide recommendations, see OMAFRA Publication 812, Field Crop Protection Guide. (Order OMAFRA Publication 812)

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Updates on Corn: Corn Rootworm

• Field Crop Protection Guide - Publication 812
  • Corn: Corn Rootworm

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Related links...

• Insects
• Corn

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