Here's What We Learned About the Soybean Aphid in 2000

Mike Rankin
Crops and Soils Agent
University of Wisconsin - Extension

One of the biggest stories of the 2000 crop year was the arrival of the soybean aphid.  These little insects took-up residence in virtually every soybean field in the county.  Up until last year, they had previously only been found in China and several Asian countries.  Given the surprise outbreak, little research or experience was available to establish economic threshold levels or guide control recommendations.  Although such studies are planned for 2001, there was some limited data generated this past year from field trials where insect control measures were implemented for reasons other than soybean aphid control, but because the plots were infested with the aphid, the desired data was achieved.  From these studies, we can at least get a preliminary look at the impact that aphids had on soybean yields.

 In a University of Wisconsin field trial involving two varieties (Spansoy 201 and 250) and comparing insecticide vs. check (no insecticide) treatments, significant yield differences were measured (Table 1).  S201 yielded about 8 bushels per acre more where aphids were controlled and S250 yielded about 6 bushels per acre more.  In addition, aphid feeding in the check plots significantly affected plant height.  Clearly, the insecticide treatments had merit.  This study did not give an indication of economic threshold levels or optimum timing of aphid control. 

Why did yields decrease from aphid feeding?

Aphids are not "chewers"; that is they do not actually defoliate plant tissue.  Rather, they suck sap from the leaf vascular system.  This results in increased plant stress and stunting.  Many heavily infested fields in 2000 also showed signs of potassium deficiency.  Aphids are vectors (carriers) of several viral diseases.  Field trials in 2000 often confirmed a direct relationship between aphid number, viral disease symptoms, and grain yield (Table 2).  Viral diseases apparently contribute to yield losses where heavy insect infestations occur.

Monitoring soybean fields

We really don't know when the soybean aphid will begin its growing season migration back into soybean fields.  In China, aphid numbers build and peak during the period between the late seedling stage (V2, two fully expanded trifoliolate leaves) to flowering (R1-R2).  Colonies concentrate on new terminal trifoliolate leaves and new leaves on side branches.  Once the top growing point stops initiating new leaves (usually late-July), aphids move to the undersides of leaves in the middle or lower areas of the canopy.

What's the soybean aphid outlook for 2001?

Given our inexperience with the soybean aphid, it's difficult to predict how much of a problem we will have during the upcoming growing season.  The aphid overwintering host is common buckthorn.  There is an abundance of this plant growing both in the wild and as an ornamental.  To what extent the soybean aphid survives a Wisconsin winter is yet to be determined.  The educated guess is that they will survive unless conditions are extremely harsh.  It seems likely that the aphid was introduced to the U.S. prior to last year but had never been present in high enough numbers to be recognized as a serious problem.  The one advantage we have for this year is that field scouting can begin before infestations reach critical levels.