NDSU Crop Pest Reports
Issue #9, July 22, 2009

• Soybean Aphid

• Grasshoppers

• Silver Maple Galls

• Corn Borer

Soybean Aphid

    It is time to start monitoring for soybean aphid.  Usually the highest numbers are found next to shelterbelts.  The economic threshold for soybean aphid is 250 aphids/plant in 80% of the field and with soybean aphid numbers increasing.  
    A common question that occurs is "how are economic thresholds determined"?  Economic thresholds are determined by the number of insects that will cause a certain amount of damage to justify control based on the price of the commodity.  As for soybean aphid, the economic threshold was developed over research conducted over a three year period at 19 locations in the northern US including ND.  The 250 aphid/plant was established to give growers a seven day lead time for application before populations reach a level where economic loss actually occurs.
   Some growers like to put an insecticide in with their last gyphosate application .  If the 250 aphid/plant threshold has not been reached, this application might be too early and lead to an additional insecticide application.  Early applications will kill natural enemies of soybean aphid such as lady beetles and damsel bugs.  The lack of these beneficial insects can cause re-invading aphid populations to increase rapidly and lead to a second application.  While some chemical companies will guarantee the chemical if a second application is needed, the producer still has the application cost.  Early applications might also open up the possibility of spider mites due to the lack of beneficial mites.  So in conclusion, conservation of beneficial insects is important in controlling soybean aphid.  That is why to monitor and treat only when you have reached the 250 aphid/plant threshold.  Don't spray unless it is necessary.

Grasshoppers

    The dry conditions in Griggs county has cause the observance of more grasshoppers then in past years.  Monitor field margins and fields next to CRP or haylands to see if treatable thresholds have been reached.  The following table from the 2009 North Dakota Field Crop Insect Management Guide, will show the economic thresholds for grasshoppers.  For products cleared for grasshopper control can be found on page 154 of the 2009 North Dakota Field Crop Insect Management Guide assessable at the link above.

Silver Maple Galls

    There are two galls showing up on silver maple trees in our area.  They are the following.
Maple bladder gall:  This gall causes small red growths on the upper part of the maple leaf.  Go to the following link for more information.  Maple Bladder Gall.
Erineum Gall:  This gall causes the bottom of the leaf to take on a red fuzzy appearance.  Go to the following link for more information:  Erineum Gall

Corn Borer

    Adult corn borer moths are starting to emerge and lay eggs.  Now is the time to start scouting for egg masses to determine possible thresholds that warrant control strategies.  For more information, click on the following publication "European Corn Borer Management in North Dakota".

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