Western Corn Rootworm Rotation-Resistant Variant

  • The western corn rootworm (WCR) rotation-resistant variant is creating management challenges for some Midwestern corn growers.10;10; 
  • WCR variant adults lay eggs in soybeans that can hatch into larvae and damage corn roots the following season.10;10; 
  • Soybean fields should be scouted every week through the first week of September to help identify fields where WCR may attack corn next spring. Farmers are encouraged to adopt best management practices to reduce WCR damage to corn roots.10;10; 

WCR Variant Population Increasing

 
Figure 1. Female western corn rootworm adult beetle.10;10; 

Some areas of the eastern Midwest have observed a steady and substantial increase of damage in first-year corn due to a behavioral variant of the western corn rootworm (WCR) (Diabrotica virgifera virgifera) that has evolved to lay eggs in soybeans and to a lesser extent in other crops. This “rotation-resistant�? WCR variant also lays eggs in wheat stubble, alfalfa. and oats. When WCR eggs laid in non-corn fields hatch the following season, the larvae can cause corn root damage.10;10; 

WCR Identification and Scouting

 
Figure 2. Northern corn  rootworm adult beetle.10;10; 

Male and female adult WCR are yellow to green in color with a black stripe along the sides of their wing covers. They are about 5/16-inch long (Figure 1). Female wing covers usually have a regular striped pattern, while male wing covers 10;10;may be nearly completely black. Northern corn rootworm adults are tan to pale green (Figure 2), and southern corn rootworm adults are yellow to green in color (Figure 3), with 11 black spots on 10;10;their back.10;10; 

Farmers in or near areas where the WCR variant has been identified should scout soybean fields for adult WCR in August and early September to identify the potential for crop-damaging levels of WCR larvae where corn will be planted next spring. Scouting this summer allows time for planning a WCR management strategy for next spring. Soybean fields should be scouted in the morning or late afternoon, when WCR adults are most active, using either sweep nets or sticky traps. 10;10; 

Sweeping:

  • Using a 15-inch diameter sweep net, take 20 continuous sweeps in 5 different areas of a field, keeping the net closed between the 5 areas.
  •  Take sweeps from areas that represent differences in topography: high ground, low ground, waterways, wet and dry areas, dark and light soil, etc.
  •  Sample areas within 50 feet of a corn field separately.
  •  Sweep the net from left to right with the top of the net slightly above the top of the foliage.
  •  Reverse the net and sweep through the foliage in the opposite direction.
  •  Count the total number of WCR adults in 100 sweeps.
  •  Samples should be taken weekly.    
  • Sticky Traps:

  • Pherocon® AM yellow sticky traps should be mounted on stakes just above the soybean canopy.
  •  12 traps should be placed throughout the field in a grid pattern, avoiding field edges.
  •  Count and record the number of male and female adult WCR beetles captured on each trap each week.
  •  Traps should be replaced every 7 days. Continue trapping for 6-8 weeks or as long as significant beetle activity is noted.
  •  Research suggests that catches of 5 or more WCR beetles per trap per day during any trapping week indicates a potential problem with rootworm the following year.   
  • WCR Management

     
    Figure 3. Southern corn rootworm adult beetle. Capinera, J. University of Florida, Bugwood.org.10;10; 

    In fields that were planted with products that contain Genuity® VT Triple PRO® technology that experienced greater-than-expected CRW damage during this season:

    •  Rotate to Genuity® Roundup Ready 2 Yield® soybeans or another non-host crop if soybeans are not a suitable recommendation for that area.
    •  Switch to a dual-mode-of-action SmartStax® RIB Complete® corn blend or SmartStax® corn.
    •  Products with SmartStax® technology offer dual modes-of-action for above- and below-ground insect protection.
    •  If a dual mode-of-action product is not available for a specific geography, consider use of a soil-applied insecticide with non-CRW traited corn products.  Placement of VT3P corn products on fields where insufficient rootworm protection has been observed is not recommended.