Wireworms 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 wire worm scouting prior to planting essential.
Wireworms are 1/4 to 3/4 inch long and have slender, semi-cylindrical bodies. They are a white, yellowish, or coppery color and have three pairs of legs behind the head.
When to begin scouting:
Wireworm scouting should start in field that historically have had excessive weed pressure and disappointing grain yields. Ideally sampling should be done:
- When soil temperatures reach a minimum of 45°F in the spring or less than 80°F in the fall.
- 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.
- 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.
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.
- 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 1/2 cup of the wheat-corn mixture into a nylon stocking and tie off the end with string. 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.
- Step 4: Cover the bait tap 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 what will attract wireworms. Cover the does 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. Another method of covering the trap is to staple both the black and the clear plastic to wood lath and drill a hole for a flag that will keep the plastic in place and make it easier to relocate the site.
- Step 6: Seven to ten days later, remove the bait traps and be careful to collect any wireworms that may fall out when removing the bait traps from the soil. Place the bait traps in a small bucket or a resealable plastic bag. Wireworms may be found in the grain and/or may be caught up in the mesh of the stocking.
- Step 7: Cut the bait trap open and examine the grain inside. Then count the number of wireworms in each trap. Record the average number of wireworms per bait trap for fields and field site to determine which fields and field portions warrant treatment. Table 1 provides recommendations based on the average number of wireworms per trap.
- Step 8: Determine what level of control is needed. If wireworms are detected, several integrated pest management options can be used. These include incorporating fallow, treating with seed-applied insecticide, and/or delaying planting time.
Today nearly all spring cereal crop acres in eastern Washington are treated for wireworm control with seed-applied neonicotinoid insecticides. These insecticides are under the trade names Cruiser® (thiamethoxam) and Gaucho® (imidacloprid) and are traditionally applied at rates between 0.190-0.315 oz/cwt.
At these levels, the neonicotinoids are toxic at sub-lethal doses to wireworms, or in other words, they repel or provide only seedling protection.