Kentucky Pest News Newsletter


Number 1037_________September 20, 2004




By Doug Johnson

Soybean aphid I am not sure of the outcome, but I have had phone conversations with producers / consultants in Jefferson and Oldham Counties concerning populations of soybean aphid. It appears that soybean aphid numbers are great enough in these areas to be noticed by individuals that were NOT looking for them. In addition, I have received word from a colleague at the University of Tennessee Experiment Station in Jackson that they are seeing large populations of soybean aphid in their area. Also, Ron Hines at the University of Illinois Experiment Station in Dixon Springs reports some increase in aphid populations, although they are not at threshold in his samples. It doesn't take much geographic knowledge to know that if noticeable populations of aphids are in Oldham Co. KY and Jackson, TN, then every area in between must be suspect.

All the KY cases have been very late planted beans. These are the plantings that I have often stated will be the "most at risk" for soybean aphid in Kentucky. The questions to answer when making management decisions remain the same: "What is the average number of aphids per plant?", and "In WHAT DEVELOPMENTAL STAGE ARE THE PLANTS?" This presumes that theses late planted beans still have good yield potential.

The bottom line is this. The economic threshold for soybean aphid is 250 aphids or more per plant in the R1 (Beginning Bloom) to R4 (Full Pod) stages. At R 5 (Beginning seed) to R6 (Full seed), the aphid population must be much higher, around 1,000 aphids per plant. All available data indicates that no return will be gained for treating plants after R6.

If you are unfamiliar with identifying soybean aphid, you can look at the Soybean Aphid web pages to find general information on the soybean aphid and perhaps of most interest right now a pictorial depiction of soybean plant stages. Go to the IPM main web pages at:
Then click on for the soybean aphid at the bottom of the page.

To find the photographic illustrations of soybean plant stages, look about halfway down the page for "soybean plant stages". Clicking here will take you to full color pictures for the plant stages at most risk.

For scouting and plant stage information see IPM B 3, KY Integrated Crop Manual for Field Crops: Soybean. This may be found on the web at:

You should examine at least twenty plants per field, selected from across the field. Count the total number of aphids on all parts of the plants. For plants in R1-R4 stop counting aphids at 250, for plants in the R5-R6 stages stop at 1,000 aphids per plant.

Insecticides for use against this pest can be found in ENT- 13 Insecticide Recommendations for Soybeans, which may be found on the web at: With good application technique any of the insecticides should provide adequate control at this time of year. However, you may want to pay particular attention to the "days to harvest" restrictions.

Besides plant stage and aphid number, consider yield potential, drought conditions, and how many beans are going to be run over during treatment. No one will get 100% control. However, if an application is needed, a good job of applying should reduce the population well below the threshold.

I realize that there is a very large chance that I am crying wolf, but "to be forewarned is to be forearmed"

NOTE: Trade names are used to simplify the information presented in this newsletter. No endorsement by the Cooperative Extension Service is intended, nor is criticism implied of similar products that are not named.

Lee Townsend
Extension Entomologist