Scout for Soybean Aphids

July 24, 2015

Scout for Soybean Aphids AND SCN!

Last week I was scouting fields for soybean aphids and made an unintended discovery. As I pulled up soybean plants to examine the undersides of the leaves for soybean aphids, I had the roots of the plants literally right under my nose. In one of my neighbor's fields, I noticed cysts from the soybean cyst nematode, or SCN, on the roots of the plants. This is significant because SCN causes more yield losses for Nebraska and U.S. soybean growers than any other pest.

So when I got back to the house, my phone call went something like this. "Hi Ralph (not his real name - LOL), I just checked your field for soybean aphids and I have good news and I have bad news. The good news is I didn't find any aphids. The bad news is, I found SCN." He knew he had SCN in other fields, but not in that one, so he thanked me and will start managing that field for SCN the next time soybeans are planted there.

This is the time of year to examine roots of soybean plants for SCN. Cysts will develop on soybean roots about a month after the soybeans emerge. It's especially important to check if you see problem areas developing in your soybean fields that you can't explain otherwise. If you have pockets in a field with sudden death syndrome, or SDS, be sure to check these areas for SCN. SDS enters the soybean plant through the roots and is more likely to occur where feeding damage from SCN has occurred.

The cyst is the only stage in the life cycle of the SCN that can be seen without a microscope. Look for a small, lemon-shaped, white to cream colored "bump" on the outside of the root. These may be confused with the larger, darker brown nodules which contain the nitrogen-fixing bacteria that are normally found on a soybean root.

The cyst is much smaller, oblong and is lighter in color than nodules. When scouting fields, if you don't find any cysts, you can not be sure that your field is SCN-free. However, if you DO find cysts, you know you have this pest and need to take action to reduce its buildup in the soil and the yield loss that can occur. That's why it's a good idea to take an extra 15 seconds to examine the roots of a soybean plant when you are checking plants for soybean aphids. It takes very little time, but could pay big rewards.

For a more definitive determination if you have SCN, you need to take a soil sample, much the same as you would sample for fertilizer recommendations. Take 15-25 cores from a field, mix them together, then take a small sample from this mixture. In fact many farmers collect a few more soil cores when sampling for their fertilizer recommendations, then split the sample and send half in for fertility recommendations and the other half for SCN analysis. The Nebraska Soybean Board sponsors a program that pays for the cost of analyzing soil for SCN ($20/sample). You can pick up bags for this free analysis from your local Nebraska Extension office.

In 2006 through 2013, the University of Nebraska conducted 29 trials looking at the yield advantage of resistant varieties over susceptible varieties in SCN-infested fields. Statewide, there was a 6 bushel yield advantage to the resistant varieties. But if you looked at the yield response in the 5 trials conducted in northeast Nebraska, the yield response was over 12 bushels/acre, double the statewide average. That is why it is so important for soybean growers to check for SCN in this part of the state.

In almost all of the plots on infested sites over the years, you could not distinguish between susceptible and resistant varieties. Susceptible varieties were not yellow or stunted, all plants looked dark green and healthy. That is why it is so important to scout for SCN now. Many producers have a 10-20% yield loss (or more!) and are not aware that they have fields infested with SCN.

With other pests, it's easy to see damage such as shot holes, plants cut off, or leaf and pod feeding. However, with SCN, you may not see any damage to the plants at all! Frequently, the first indication that something is wrong with your soybeans is at harvest. Yields hit a plateau or even drop back for no apparent reason such as weather, weeds, insect or herbicide damage, while corn yields continue to improve in the same field. This isn't a guarantee that there is SCN in the field, but it should be one of the first things you check. So you do the math, a six bushel/acre yield increase at today's prices could put a lot of money in your pocket at the end of the season by managing for SCN.

For more information on scouting for and managing SCN, contact your local Nebraska Extension Office.