Soybean aphids are small soft-bodied insects, yellow-green in color, and may be winged or wingless. The winged versions may appear when overcrowding occurs, and are slightly darker in color.  Aphids inhabit the leaves and stems near the top of the plant.

According to Iowa State University, the aphids lay a cold-hardy egg on buckthorn (Rhamnus spp.), which is the only known overwintering host. Winged females move into the crop and have live 3-8 young every few days. The young are all female and born pregnant. This results in a rapid reproduction rate, especially under the ideal climatic conditions of 72-77 degrees F and humidity less than 78%. Populations can double every 2-3 days.

Because of this rapid growth potential, careful scouting and timely treatment is required. As with greenbugs, there are parasitic insects which can control aphid populations.  As aphid populations increase, winged versions appear, allowing the aphids to disperse to other fields. Treatment level recommendations are sketchy, but 50-250 per plant, depending on stage, is a number that has been published. The KSU download on soybean aphids is available here as a PDF file.

Asian Soybean rust is another soybean threat which has received much recent publicity. From a geographical standpoint, our threat appears to relatively low at this time, but it is difficult to predict how much rust will spread under favorable conditions. The rust is caused by a fungus (Phakopsora pachyrhizi).which destroys photosynthetic tissue causing defoliation, early maturation and lowered yields. First found in  the US in November, 2004, it can infect many common plants but not corn or other grain crops.

Asian soybean rust begins with small, water-soaked lesions, which gradually increase in size, turning from gray to tan or brown. As the plant matures and sets pods, the symptoms spread rapidly to the middle and upper parts of the plant, especially the leaves.. Other diseases which look similar include brown spot, bacterial blight, and Cercospora leaf spot.

Until resistant varieties become available, timely usage of fungicides is the only defense. Some reports indicate that aerial application is superior to ground applied fungicides, because the wider droplet spectrum gives better leaf coverage, and because the disease is not spread mechanically.

Careful monitoring of crops and and adjacent areas is required to ensure timely applications. Like most diseases, rust cannot be removed, only prevented.

More Reading: You can read more about soybean insects by  downloading the KSU Soybean Insect Management Guide. Please click here. (PDF,  335 Kb.)  The Soybean Quick Reference Insect Guide is here.

You may also be interested in the KSU Soybean Production Handbook: get it here. (PDF,  254 Kb.)