Wireworms

Wireworms are the soil-dwelling larval form of click beetles (Coleoptera: Elateridae). Click beetles get their name from the sharp “click” noise they produce when they flip into the air (a defensive mechanism and a means of righting themselves when turned onto their back). Adults are narrow-bodied beetles that are generally black or brown in color (Figure 1). Their larvae, wireworms, are slender and cylindrically shaped, and have shiny, slightly hard bodies and small legs (Figure 2). Newly hatched larvae are often white in coloration, but over time larvae darken to become yellow to brown. Though their size differs between species, adults range from 1/4 to 1/2 inch (6-13 mm), and larvae range from 1/2 to 1 1/2 inch (13-38 mm) in length.

Biology

Wireworms have a multiyear lifecycle, so there are often multiple generations of wireworms present in a given field at any one time. Adults and larvae both overwinter in soil, anywhere from 6-24 inches (15-61 cm) deep. When soil temperatures reach 50-55 degrees Fahrenheit (10-13°C) adults emerge, mate, and lay eggs in the soil. At this point larvae move within about 3 inches (8 cm) of the soil surface to feed. As the season progresses and soil temperatures increase to 80 degrees Fahrenheit (27°C) and above, larvae move deeper in the soil, where they will typically remain until the next spring. In irrigated potato fields the soil likely does not reach this temperature. Wireworms generally take anywhere from two to six years to mature, but eventually larvae pupate within the soil and emerge as adults.

Damage

Wireworm feeding on germinating seeds and seedlings, including potato seed pieces, can reduce seed viability, weaken stands and generally reduce seedling vigor. Feeding damage can also lead to secondary infection by pathogens. In potatoes and other tubers/roots, wireworm feeding manifests as round holes and smooth-walled tunnels (Figures 3-4). This causes surface damage and reduced quality. Significant damage is usually caused by wireworms at least two years in age.

Monitoring

As with most soil-dwelling insects, wireworms can be challenging to monitor, but soil sampling may aid in detection and assessment of damage risk. Fields that have had wireworm damage in the past, have recently been planted with grasses (e.g., cereals, corn, pasture) or that have had high grassy weed pressure should be prioritized for monitoring efforts as they are more likely to host large populations of wireworms.

To sample soil using the shovel method

The shovel method is simple, but it can be challenging to get an accurate idea of wireworm densities as they are often distributed unevenly in a field. Monitoring using this method is most effective after soil temperature at four inches deep reach 50 degrees Fahrenheit (10°C) in the spring, or when temperatures drop below 80 degrees Fahrenheit (27°C) in the fall. To monitor using this method:

• Dig up a soil sample about 10 inches deep and 6 inches in diameter
• On a sheet of plastic, sift through soil and count wireworms
• Repeat at least 30 times in different locations around the field. As wireworms are often distributed patchily, this repetition and wide sampling area is key to ensure monitoring efficacy
• If two or more wireworms are found in 30 samples, economic damage is likely and control is recommended (Table 1)

To sample using the bait trap method

Bait trapping may be more effective in estimating wireworm densities, though field conditions (i.e. soil moisture, presence of organic matter that may also attract wireworms, etc.) may affect this method’s accuracy. Bait traps should be set before soil temperatures reach 50 degrees Fahrenheit (10°C) at a 4-inch depth, preferably a month before planting (Figure 5). To monitor using this method:

• Soak untreated cereal or corn seed in water for 24 hours
• Place about 1/2 cup of seed in a hole about 3-5 inches deep and bury, but do not pack soil
• Lay a piece of black plastic (approximately 1-3 ft square) over the trap location and cover plastic with soil to keep it in place (optional, but this should increase soil temperature and encourage wireworm activity)
• Flag or otherwise mark trap
• Place at least 25 bait traps for 30 acres, and at least 4 traps in fields of less than 30 acres
• Retrieve traps in 7-14 days
• Look through bait (soaked seed) and soil immediately surrounding it for wireworms
• Record average number of wireworms per trap and consult the table below for treatment recommendations

Table 1. Economic thresholds for wireworms in potatoes (Note that wireworm distribution is often patchy, and trap captures are dependent upon field conditions, so these values should be used only for guidance. Adapted from Bechinski et al. (1994)

Management

Primary management tactics

Crop rotation is a cornerstone of effective management of wireworms. At-plant insecticides provide additional crop protection.

Cultural

Crop rotation is a cornerstone of wireworm management.

• Avoiding grasses (e.g., corn, cereals, pasture) for multiple years, especially just prior to a highly susceptible crop such as potato, can help reduce wireworm pressure in a field
• Including alfalfa, clover or mustards in crop rotations may reduce wireworm populations

Fallowing a field will subject wireworms to a dry environment lacking in food and will reduce populations, but any benefits must be weighed against the loss of production for that field for a season.

Drying the top 15 inches of soil can target eggs and larvae of sugar beet wireworm and Pacific coast wireworm (Limonius spp.), but this practice may not be compatible with maintaining crops in the field.

Thoroughly saturating the soil for at least two weeks while soil temperatures are above 68 degrees Fahrenheit can increase wireworm mortality, particularly when soil flooding and soil drying are alternated, but this practice may not be compatible with maintaining crops in the field.

Biological

• Limiting the use of broad-spectrum pesticides can protect natural predators of wireworms, such as predatory beetles (e.g., predatory ground beetles and rove beetles) and beneficial fungi and nematodes
• Birds are also a natural enemy of wireworms

Chemical

• There are multiple neonicotinoid insecticides that can help control wireworms when applied at-plant. This class of insecticides may protect the crop from damage without reducing wireworm populations in the field, which should be considered for subsequent crops in the rotation.
• Other at-plant insecticides may kill wireworms, thus reducing damage and reducing populations
• Foliar-applied, systemic insecticides targeting wireworms are available as well
• Treating after wireworm damage is observed is not effective
• Treating one crop in a rotation with a product that kills wireworms can reduce wireworm damage in subsequent years
• Recommendations for pesticides to use in the management of wireworms can be found on the PNW Pest Management Handbooks website

Pesticide warning

Always read and follow the instructions printed on the pesticide label. The pesticide recommendations in this University of Idaho webpage do not substitute for instructions on the label. Pesticide laws and labels change frequently and may have changed since this publication was written. Some pesticides may have been withdrawn or had certain uses prohibited. Use pesticides with care. Do not use a pesticide unless the specific plant, animal or other application site is specifically listed on the label. Store pesticides in their original containers and keep them out of the reach of children, pets and livestock.

Trade names — To simplify information, trade names have been used. No endorsement of named products is intended nor is criticism implied of similar products not mentioned.

Groundwater — To protect groundwater, when there is a choice of pesticides, the applicator should use the product least likely to leach.