Wireworm Scouting: The Shovel Method and the Modified Wireworm Solar Bait Trap

Wireworm Scouting: The Shovel Method and the Modified Wireworm Solar Bait Trap

FS059E
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Aaron Esser, County Director, WSU Extension Adams County, Ritzville, WA
Wireworms (Lumonius spp) can damage cereal grain crops, resulting in increased weed pressure and reduced stands, yields, and profits. Since crop damage is not detected until after planting, wireworm scouting prior to planting is essential. This publication details several methods of scouting for wireworms, plus thresholds for risk of economic damage and treatment recommendations.
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Wireworms (Lumonius spp) can damage cereal grain crops, resulting in increased weed pressure and reduced stands, yields, and profits. Wireworms are the immature larval stage of click beetles, and these beetles can spend several years in this larval stage feeding on germinating seeds and young seedlings, resulting in thin crop stands and lower yields. Crop damage is not detected until after planting when it is too late to make preventive pest management decisions. This situation makes wireworm scouting prior to planting essential.

Spring arrives quickly in the dryland cropping region of the Pacific Northwest, so every day is critical. Fall cereal grain seeding conditions can also change quickly, most often due to precipitation. A delay in planting can be costly but so can an infestation of wireworms. Consequently, taking time to properly scout for wireworms can provide an excel­lent return on investment.

Identifying a Wireworm

The first requirement when scouting for wireworms is to be able to correctly identify them. Wireworms are ¼ to ¾ inch long, have hard, slender, semi-cylindrical bodies, and are white, yellowish, or coppery color. They have 3 pairs of short legs located behind the head (Figure 1).

Figure 1. Wireworms vary in size.

Where to Start Scouting

Wireworm scouting should start in fields that historically have had excessive weed pressure and disappointing grain yields. Sampling should begin when soil temperatures reach a minimum of 45°F in the spring and less than 80°F in the fall. Sampling should be completed prior to plant­ing, so rates of seed-applied insecticide can be adjusted.

Shovel Method

The shovel method is the quickest and easiest way to sample for wireworms, but it may be the least accurate (Figure 2). To take samples, follow these steps:

Step 1. Dig down about 10 inches and lift the shovel of soil for examination.

Step 2. Round off the soil sample to approximately 6 inches in diameter.

Step 3. Sift through at least 20 shovels of soil from different locations in the field. This is extremely important because wireworm distribution is usually patchy or irregular.

A suggested threshold for determining the level of eco­nomic injury is an average of 4 or more wireworms per 20 shovels of soil. At this level, it may be profitable to use a preventive treatment (Gesell 1983).

Modified Solar Bait Trap Method

The modified solar bait trap method requires additional time and is more difficult to use, but it is also the most accurate method for wireworm sampling. To take samples, follow these steps:

Step 1: Monitor soil temperature in the field until it nears or reaches 45°F at a depth of 4 inches.

Step 2: Mix equal parts untreated wheat and corn seed. Pour ½ cup of the wheat-corn mixture into a nylon stocking and tie off the end with string (Figure 3). Soak the filled stocking in water for 24 hours. Soak­ing the seed mixture is crucial because it starts the germination process. Because wireworm locations can be patchy, a minimum of 10 traps should be used per field.

Step 3: Dig a hole in the soil approximately 3–5 inches deep and 8–10 inches wide. Place the bait trap in the hole and spread the grain mixture across the bottom of it. Leave the string outside the hole to help relocate the trap (Figure 4).

Step 4: Cover the bait trap with sufficient soil to create a mound over the bait, but do not pack the soil.

Step 5: Cover the soil with a piece of black plastic approximately 1 to 3 feet square, and then cover this with a piece of clear plastic that is the same size or a little larger. This helps warm the soil, which helps germinate the bait that will attract wireworms. Cover the edges of the plastic with soil to keep it from blowing away. Place a flag in one corner of the plastic to make it easier to relocate the site (Figure 5).

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Copyright 2012 Washington State University

Published January, 2012

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