Tuesday, June 19, 2012

Watch For Two-Spotted Spider Mites Damaging Soybeans

Spider mites are a common pest during dry conditions. This pest prefers dry dusty conditions.  Under a severe drought, mite populations can double every 5 to 7 days.  Rains can cause the mite populations to decline but fields should continue to be scouted.
Spider mites are first found along field borders or waterways.  Mite damage first appears as speckling or spots on the soybean foliage. Later injured leaves turn yellow then brown and die.  Damage moves across the field as mites infest additional soybean plants. 
You may need a hand lens to see mites.  Also, a white sheet of paper to shake the mites on will help you see them.  The mites once shaken on the paper will crawl about.
Generally, spider mites occur later in season.  However with the dry conditions, damage can be found on small soybeans.  A general recommendation is to control this pest when the foliage yellowing reaches 20% before pod set and when foliage turns 10% after pod set.
Heavy infestations of this pest can cause severe damage.  In my experience in the 80’s, spider mites would begin as a small circle in the field.  Then the wind would catch them and move them across the field.  There would be small circles of damage from the pest, and then the circles would enlarge causing damage.  It is important to note that the yellowed areas are already damaged and the pests are moved onto undamaged plants to feed.
Scouting is critical.  Since wind picks up this pest, one should watch the entire field.  Plants may show damage symptoms for up to 7 to 10 days following control of spider mites.
Dr. Bailey reports spider mites found along the Iowa and Missouri borders.  At the time of printing of this news article, I have not had any local reports into the Extension office.  With dry weather, however, it is a matter of time before we have this problem.
For more information, contact Wayne Flanary at 660-446-3724 or Heather Benedict at 660-425-6434, Regional Agronomists, University of Missouri Extension.

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