Preplant Wireworm Assessments for Corn and Soybean

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Figure 1. Wireworms present near a corn seedling.

• Wireworms are occasional early season pests of corn and soybean
• Wireworm damage is favored by pasture, hay, sod, or small grains rotated with corn or soybeans and early planting into cool, moist soils
• Bait stations can be used to determine preplant risk 
• Several seed treatment, soil, and hopper box treatments can be used to control wireworms in high risk fields

Click beetles are not of any importance to cropping systems; however, their larvae, better known as wireworms, can occasionally be of economic significance in corn and soybean fields.  For example, in 2020 several corn fields in Southern Illinois experienced losses associated with this pest.  Luckily, proper pre-plant scouting can help ensure that management can be implemented on the appropriate acres, improving yields and ROI.  This article will discuss wireworms, scouting, and potential management strategies.

What are wireworms?
Wireworms are thin, yellow to brown, hard-bodied larvae ranging from ½ to 1 ½ inches in length and can have a larval stage that lasts 4-7 years.  Wireworms tend to be most problematic in situations where fields of sod or pasture are being put back into corn/soybean production or in systems where small grains are rotated with corn.  These pests are most active in cool, moist environments; therefore, early plantings may be at greater risk for damage.  Conversely, under warm, dry conditions, the larvae move deeper into the soil and do not damage germinating seedlings and developing plants.  This behavior is why we typically see wireworm issues in early planted fields.  Wireworms feed on the germ of corn and soybean seed and drill into the base of the corn stem, destroying the growing point.  Symptoms of damage include hollowed out seeds, skips and gaps in rows, and stunted plants that prematurely wilt.  In corn the whorl leaves typically are affected first. 

Preplant scouting for wireworms
There are no rescue treatments for wireworms; however, there are thresholds based on pre-plant bait traps.  A typical recommendation is to space out 10 traps across every 40 acres of field.  Traps are constructed by digging a hole approximately 4 inches deep and 9 inches wide. Then, add a mix consisting of ½ cup untreated corn and ½ untreated wheat seed, or a cup of all-purpose flour.  Fill the hole and mound the dirt to prevent water from pooling.  Cover the mound with a square piece of black plastic to warm the soil and help the seeds germinate.  Cover the black plastic with a larger piece of clear plastic and tuck the edges into the soil to hold it in place.  Flag each trap so you can easily find them in the future.  

Approximately a week before planting, dig up the traps and count the number of wireworms found at each station.  The threshold for using a seed treatment or soil insecticide is an average of at least one wireworm per trap.  There are several soil insecticides, seed treatments, and planter box treatments labeled for wireworm control in corn and soybean.  Make sure that the rate used is sufficient for wireworm suppression, and of course that wireworm control is listed on the label.  For example, you would need to use Poncho 500 or more to adequately control this pest.