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Western Corn Rootworm and Northern Corn Rootworm

Corn rootworms can cause yield loss and lodging in corn.  Immature larvae feed on corn roots while the adults feed on leaves, pollen, and silks. 

Rootworm control is primarily achieved through GMO Bt-corn and crop rotation.  In other states, over reliance of the common Cry3Bb1 gene has shown the resistance problems.  It can be challenging to rotate between stacked traits.  Generally, use a specific trait for up to 3 years and then rotate to another gene.  Corn refuges slow down development of resistance and should always be used.

Early planting of corn allows for better root development prior to the late June hatch of rootworm eggs. 

Threshold

The decision to rotate from corn or to use an insecticide may be based on field scouting for adult beetles during a three week period after pollination. Record the number of corn rootworm beetles on the foliage and silk of 100 plants. When the adult population averages 1 beetle per plant in continuous corn or 0.5 beetles per plant in first-year corn fields, the potential for larval root damage the next summer is sufficient to rotate from corn or to apply an insecticide.

When an average of 5 or more beetles per silk mass are found during the first week of pollen shed, control may be necessary. Another management threshold uses silk clipping. When silk clipping is occurring on 25% to 50% of the plants during pollen shed, control would be justified.

 

 

Source: North Dakota Field Crop Insect Management Guide (E-1143)

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