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Virginia Cooperative Extension -
 Knowledge for the CommonWealth

Scouting for Corn Rootworm Equals Big Savings for Growers

Crop and Soil Environmental News, April 1999

Curt Laub, Rod Youngman, and Tom Kuhar
Department of Entomology, Virginia Tech

Some Virginia corn silage growers will save about $11 per acre by not having to treat for corn rootworm in 1999, while others will treat knowing that insecticide is being applied only in fields where it is needed, AND that silage yields will not be reduced by rootworm damage. These growers participated in Virginia Cooperative Extension's corn rootworm scouting program in 1998. Of the almost 800 acres of field corn enrolled, only two fields, totaling 17 acres, exceeded the threshold for rootworm beetles. To avoid economic injury by rootworms this year, these two fields will need to receive an insecticide application or be rotated out of corn. The other 772 acres scouted were well below the economic threshold, and may be planted to corn this year without an expensive insecticide application to control rootworms.

Rootworm populations occur in continuous corn but in any given year they may not be large enough to warrant treatment with a granular insecticide. Research in Virginia from 1993 to 1994 on 32 long-term continuous cornfields that were treated each year indicated that 72% of those fields were below threshold. The rootworm scouting program, which was developed based on this research, identifies fields at risk to root damage and advises growers in the fall whether a granular insecticide is needed next year for rootworm control.

Scouting program results from 1993-98 indicate that almost 75% of first and second year cornfields, and just under 50% of fields that were in corn for three or more years, were below threshold (Table 1). The net benefit to growers with fields that are below threshold is about $11 per acre ($15 per acre savings on insecticide minus the scouting cost).

Table 1. Fields above and below threshold for corn rootworm, 1993-98.

Year in Corn

No. Fields
No. Fields
Pct. Fields


Fourth or More95494651.6%


Growers whose fields are above threshold and who apply insecticide at planting will usually prevent a serious yield loss. Assuming they prevent a 20% yield loss, and using a silage value of $27 per ton and a yield of 12 tons per acre, this means a net savings of $46 per acre in improved yield (AFTER accounting for scouting and insecticide costs).

How Corn Rootworm Scouting Works

Yellow sticky traps are installed in cornfields in early July. Trained field scouts monitor the traps through August to determine rootworm adult (beetle) populations, which correlate with the following year's damage by rootworm larvae. Control recommendations for the following year are based on whether the threshold of 20 beetles per trap per week is exceeded. If a field is below this threshold, there is a low risk of root damage by rootworm larvae to corn next summer, and we recommend that the field can safely be planted to corn without applying an insecticide to control rootworms. If a field is above threshold, there is a good probability of moderate to severe root damage by rootworm larvae next summer, and we recommend rotation or application of a granular insecticide at planting. See the current edition of the Pest Management Guide for Field Crops (Virginia Cooperative Extension Publication 456-016) for recommended materials and rates.

In 1999, the charge for rootworm scouting is $3.50 per acre (this covers field scouts' salaries, travel, and supplies). This program has potential for large savings to growers in reduced insecticide costs and improved yield. For information on enrolling in the corn rootworm scouting program, contact your county Extension office, or call Peter Warren, area IPM Agent (540-672-1361), Curt Laub, IPM Research Associate (540-231-7311), or Rod Youngman, Extension Entomologist (540-231-9118)

Contact:   Curt Laub, Rod Youngman
Department of Entomology
Virginia Tech (0319)
Blacksburg, VA 24061
540-231-7311, 540-231-9118

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