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Out of Sight, Out of Mind

Thursday, March 21, 2019
filed under: Insects

       Walking out into a newly planted sunflower field, a sunflower grower was puzzled to find a low plant stand and empty hulls of seeds in the soil.  Digging around a few inches down revealed the true culprit: WIREWORMS!  
Figure 1 -- Adult clikc beetles (Photo: Kevin Wanner, MSU)
      Wireworms are the immature larval stage of click beetles in the family Elateridae. Adult beetles (Figure 1) do not cause any feeding damage to sunflower and other field crops.  In fact, only the female wireworm will go into fields to lay eggs, and they prefer grassy crops, such as wheat or barley.  Adults spent most of the time elsewhere outside of the sunflower field. 
       Wireworms (larvae) are about ?to 1-inch long, cylindrical shape, creamy white to golden brown and a hard exoskeleton “shell” with three pairs of legs behind the head (Figure 2).  The last segment is flattened and elongated, forked or notched, with short, stout appendages. 
       The invisible wireworm feeds directly on the seed and/or the seedling plant, boring into the stems, seed or root crown (Figure 3).  
Figure 2 -- Wireworm larvae (Photo: Mark Boetel, NDSU)
Damage can range from a patchy, thinning stand to complete stand loss at high populations (Figure 4).  The long life span of wireworms (three or more years) can cause damage to almost any field crop.
       Wireworm biology has not been well studied, being an underground insect pest. Soil conditions — such as temperature, moisture, texture and organic matter — affect their movements.  Their seasonal movement is largely vertical, moving deeper in the winter to avoid freezing and later in the summer to avoid hot (>80°F), dry topsoil.  In the spring, larvae move to the soil surface when soil temps are above 50°F and the topsoil is moist.  They can remain at the surface as long as soil conditions are favorable. Irrigation and no-till practices produce ideal topsoil conditions for wireworm feeding. 
Field Scouting & Baiting
Figure 3 -- Wireworm injury tunneling into stem of a sunflower seedling
       Due to their long larval lifespan and clumped distribution, wireworms tend to cause localized damage in the same field over several years, so knowledge of the sunflower field’s history is important.  During crop establishment, thin patchy plant stand areas should be scouted.  Digging around wilted or stunted plants often reveals wireworms feeding on the roots or tunneling into the seed or the plant stem. 
       Attractive baits can be placed in the soil at 10 to 20 locations throughout the field or within areas with a history of poor stand establishment.  One easy method is to fill a nylon stocking with ½ cup of grain or corn seed, soak the seed in water overnight and bury the stockings 4 to 6 inches deep.  After 10 to 14 days, look for wireworms in and around the germinating “bait bag.”  
       In general, an average of one or more wireworms per trap before planting warrants pest management.  Cold soil temperatures early in the spring can make trap catches variable. 
Figure 4 -- Wireworm stand loss in a sunflower field near Mohall, ND
Pest Management Strategies
       Chemical Control —Rescue foliar insecticide treatments do not work for wireworm management in any field crops, since larvae feed underground.  Wireworm control relies mostly on insecticidal seed treatments.  In sunflower, the neonicotinoid class insecticides, such as Cruiser 5FS (thiamethoxam active ingredient), provide stand protection, but do not significantly reduce or kill wireworm populations in the soil. 
       The decision to treat seeds is based on confirmed damage from the previous year and/or the presence of wireworms in the field prior to planting (an average of one or more wireworm per bait station).  
       Another option is applying an at-plant insecticide, such as an in-furrow spray when planting sunflower, such as Mustang Max (zeta-cypermethrin active ingredient), which belongs to the pyrethroid class of insecticide. Pyrethroid insecticides repel wireworms and again do cause significant mortality (similar to the neonicotinoids).            
       Our NDSU Extension Entomology team is evaluating new insecticide modes of actions that kill wireworms in wheat, and these new insecticides should be registered by 2022 or later.  Sunflower and other row crops will be registered after wheat.          
* Janet Knodel is professor and extension entomologist, North Dakota State Unversity.
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