Time To Start Thinking Soybean Aphid

Mike Rankin
Crops and Soils Agent
University of Wisconsin - Extension

Make no mistake……there's still a lot to learn about the soybean aphid.  However, it's clear that this is an insect that is apparently here to stay.  Up until July, 2000 the soybean aphid had never been detected anywhere other than a few Asian countries.  By now, most producers have seen fields with some degree of aphid infestation.  Here's a brief summary of information learned during the past two growing seasons and some control tips as we approach the soybean aphid season.  

Planting date:

Late-planted or double-cropped soybeans are more attractive to migrating aphids than soybeans planted at more typical dates. In both 2000 and 2001, soybeans planted after early-June developed higher aphid populations and subsequent injury symptoms than did soybeans planted between late April and mid May. Early planted soybeans are still subject to damaging soybean aphid populations and their transmitted viruses, but the severity has typically been less.

Scouting:

Avoid insecticide control measures when aphids first appear in a field.  Insecticide application to control low and non-damaging populations of aphids will also kill beneficial insects, allowing surviving aphids and migrants to more readily repopulate the field. Thus, early-season treatments can lead to higher populations than if the field had not been sprayed. 

Aphid populations begin to build about the time soybeans start flowering, however, this varies from field to field.  Begin intensive scouting when soybeans start to flower.  Pay close attention to the upper 2-3 sets of trifoliolate leaves, where the aphids first set-up residence.  Continue monitoring fields through the flowering stages.

 Economic threshold level for spraying:

Wait to treat soybean fields until aphids are abundant on the upper trifoliolate leaves, petioles, and stems but before plants show noticeable signs of damage.  Damage symptoms include leaf distortion, plant stunting, yellowing of leaves, and leaves coated with honeydew and/or sooty mold (which grows on the honeydew).  From a growth stage standpoint, soybeans are most susceptible to yield loss around the R4 stage (full pod).  This means that sprays need to be applied probably when plants are in the R2 to R3 stages (full bloom to beginning pod).  

Based on the limited research data available, University of Wisconsin entomologists are currently recommending that aphids be controlled when populations reach about 200 aphids per plant (field average).  Although lady beetles, minute pirate bugs, lace wings and other predators are an important component of soybean aphid pest management, insecticide intervention will be required if aphid populations reach the current threshold level of 200 per plant.  At this point, predators simply can't bring aphid numbers down to non-damaging levels. 

Last year, most fields were sprayed from about mid-July to early August with late July resulting in the most effective treatment timing. 

Watch for diseased aphids, winged aphids, and aphid nymphs with developing wings as we move into later flowering stages.  Nymphs with developing wings are referred to as alatoid nymphs.  If most of the aphids present are winged they will soon be leaving the plants and spraying probably is no longer warranted. Populations in such fields will decline rapidly to non-damaging levels.  This is also true of the fungal pathogens that attack the soybean aphid; they are also capable of causing aphid populations to rapidly decline. 

Insecticide options: 

Furadan 4F, Lorsban 4E, Penncap-M, Mustang, Asana, and Warrior T are currently labeled for control of soybean aphids.  Other insecticides such as Dimethoate, Pounce, and Option can be used but are not specifically labeled for soybean aphid. 

Research is being conducted on systemic seed treatment insecticides for the control of soybean aphid, but none of these are currently registered for such use. 

Potential Yield Loss: 

Yield loss caused by the soybean aphid is variable and can be influenced by a number of factors including aphid numbers, virus transmission, weather, natural enemies, and variety selection.  Research trials to date show a range of yield losses from 0 to 16 bushels per acre.  This range is very similar to that obtained in a series of side-by-side farm trials in 2001. 

Virus transmission: 

        Evidence continues to mount regarding the soybean aphid and its ability to spread plant viruses.  The aphid appears to be an efficient carrier of both soybean mosaic virus and alfalfa mosaic virus.  Both virus diseases are known to cause yield loss in soybean.  In the case of soybean mosaic virus, seed mottling (a "bleeding" of the hilum) also occurs.