Crop Scene Investigation - 16 Solved: Why are Rick's beans defoliating?

Monday, March 2, 2009

by MIKE FOLKARD

Spider mites caused Rick's soybeans to defoliate.

Also called two-spotted spider mites, the pests are tiny – approximately 0.5-1.0 mm in length  and thus barely visible to the naked eye. The eight-legged and yellowish-brown mites are rounded with two dark spots on the sides of the abdomen. They overwinter in sheltered areas, such as field margins and move into crops in the summer as their numbers build up.

Spider mites thrive in hot, dry and windy conditions. They feed on the underside of leaves by piercing and sucking plant juices, causing small yellow or white spots. As feeding activity increases, leaves become yellow, bronzed and brown, eventually dropping off. Upon close examination, fine webbing on the foliage can be seen.
As the spider mites in Rick's field were still actively feeding and more dry weather was in the forecast, I suggested that he spray a foliar insecticide around the borders of the field to stop the pests from moving any further and stealing any more yield. Rick had used a seed-applied insecticide, but none specifically target spider mites.

At harvest, Rick calculated a 10- to 15-bushel yield loss on the infected areas of the field. For future years, he can protect his yield potential and avoid a spider mite infestation by scouting the field earlier, starting the first week of July. Look for tiny whitish spots on the upper surface of the leaves in the mid-canopy. Turn these leaves over and look for webbing and slow-moving dark specks using a hand lens, if available. 

Knowing your risk of spider mite infestation is also key to prevention. Because the pests overwinter in field debris, high-risk areas are those near fields of winter wheat stubble or hay. No-till soybeans following winter wheat underseeded to red clover are also at risk.

Watching for mites in ditch banks and fencerows before they move into the field will further protect your crop and your yield potential. Congratulations to Andrew Byvelds, Williamsburg for his correct answer. BF
 
Mike Folkard, CCA, is a Field Agronomist for NK® Brand, Syngenta Seeds. He is based near Belmont.