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).
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.
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).