Now is a great time to get out and take a look at your soybean fields and do some scouting for soybean aphids. If they are present, you will see very small pale green insects on the underside of the soybean leaves. Look at the top newer leaf growth first. Check the end of this article for threshold levels.
Soybean aphids have been around now for a few years. We have learned a lot about them since then. They were particularly devastating in 2003 when they had their first real year of infestation in south-central Minnesota. Since then we have been developing strategies to control them. However, research continues to see what we can do to control them without having to use as much insecticide as we have in the past.
Soybeans originated in China and other parts of Asia. They were imported here many years ago because of their versatility and many uses both as livestock feed and human food. For many years soybean plants were used as hay on area farms. For the past 40 years or so they have become an important crop and most farms have significant acreages of soybeans.
Soybean aphids made the jump from China within the past ten or so years. No one is absolutely sure just how they made this jump. The problem here is that while the aphids were able to make the transition, the insect predators that help control them in their native land were not able to make the jump and thus soybean aphids have been able to cause a lot of problems and have not had as many predators to help control population as are needed. In the Asian countries that raise soybeans, soybean aphids are almost none existent or at very low numbers due to these predators. One predator that is here (in the U.S.) in great numbers is Asian Lady Beetles. These insects look like and are related to the common Lady Bug. They are more aggressive and when we have a year where we have a lot of soybean aphids, we will see a lot of these insects. Most homeowner, especially in rural areas can tell you that they can be a real nuisance especially in the fall of the year when they come out of the fields and are looking for a place to over winter. They will look for a place to spend the winter and often times will come into your house and make a mess.
So where does that leave us in 2011? Well, the aphids are definitely here. If you haven't checked your fields to determine whether or not you have aphids, now is the time to do so. The threshold that we are looking at is 250 aphids per plant. This may be less (or more) than you think. It is not necessary to count each one. If there are a lot of aphids scattered around the plant, it is probably time to spray. Scouting is necessary because the kind of typical summer weather that we have is not always conducive to aphid survival. Aphids need warm weather and moisture, but they have a fairly narrow range of temperatures before funguses can set in and kill most or all of the population. Aphids are incredibly resilient. As a matter of fact, aphids are all females and are born pregnant. They can have many babies in a few days and can double and quadruple their populations in just a few days. However, this can also turn around just as quickly as the weather changes and temperatures and moisture levels allow aphid levels to plummet. That is why you need to spend some time in your fields looking at plants and seeing what is actually out there.