April's Ag Advice by April Borders If you weren't busy enough out in your fields monitoring your crops, you better get ready for more. Wet spring weather allowed diseases to raise their ugly heads, the bean leaf beetles have been seen along with the alfalfa weevils, and then we have had to deal with Mother Nature pouring down 3+ inches of rain across our county. Okay I think that we have had enough for a while! But somehow I don't have much say in the program.
The latest pest that is coming our direction is the soybean aphid. The soybean aphid appeared last summer in three South Dakota counties: Brookings, Moody and Minnehaha. By September of 2002, soybean aphids had been confirmed in 20 South Dakota counties. Well guess what � they are back!
The soybean aphid is currently the only aphid species that infests and multiplies on soybeans. The aphid is about 1/16 inch long when full grown and may be yellow to yellow-green in color. The soybean aphid has two black-tipped "tailpipes" or cornicles that can be easily seen with a hand lens. The aphid appears both as a winged and a wingless form.
Mike Catangui, SDSU Extension entomologist, says that we should be out in the fields scouting for the soybean aphid now. They have already been found on soybeans in the V3 stage in Brookings and Minnehaha County. He said that we should look at the edges of the fields especially by plantings of buckthorn.
He also said that we should look for populations of ants on the soybeans as an indicator insect as the ants are there to harvest the honeydew that the aphids secrete.
Soybean aphids can be found at different stages on the plant depending on the season and plant condition. Winged aphids are commonly seen in early spring or late summer as they migrate between soybean and buckthorns. Once on a soybean plant, overcrowding also may force the soybean aphid to produce winged forms, perhaps to enable it to colonize other less-infested soybean plants in the field.
Scouting for soybean aphids should start as soon as the soybean plants produce two trifoliate leaves (V2) and continue until the beginning seed (R5) stage of soybeans development. The insects can be found mainly on the young leaves and stems of the soybean plant early in the season.
Count soybean aphids from at least 10 plants at 10 representative locations in the field. The entire plant needs to be examined as the aphids could be in the stems, growing points, flowers, pods, or underneath the leaves.
The plant appears to be most susceptible to aphid injury if infestations start early in the vegetative stage (V2) of the plant and continue on through the flowering and beginning seed stages (R1-R5) of the soybean. The economic threshold is an average of 200 or more soybean aphids per plant.
Spraying with an insecticide during the flowering stages of the soybean plant (R1-R2) appears to produce the most benefit or yield advantage, according to research. Spraying too early may result in re-infestations of the sprayed field, while spraying too late may be unnecessary because the aphid population tends to decline during the pod-fill stages.
Plant injury is caused by the insects sap feeding which can cause yellowing, cupping and wilting of soybean leaves. Plant growth may also be stunted. Soybean aphids are also capable of transmitting mosaic viruses that can cause soybean diseases, such as soybean mosaic virus, bean yellow mosaic virus and alfalfa mosaic virus.
One thing to remember about the soybean aphid is that it can potentially produce 15 generations of aphids on soybeans per growing season. Careful monitoring is necessary since population explosions are possible with this insect.
If you have questions about the soybean aphid and would like more information about its lifecycle, injury and control, stop by the Extension office and pick up Fact Sheet 914 � Soybean Aphid in South Dakota.
Another insect that we need to be on the lookout for is the corn borer. Catangui said that this insect is also starting to show up in our corn fields. He also said that producers should make sure which type of Bt corn with Yield Guard that they have planted to make sure that their crop is protected against the correct insect.
For more information on any crop questions or problems contact the Extension Office at 677-7111. Also remember that our Clay County Master Gardeners are available on Monday mornings from 8 to 10 a.m. to answer your gardening questions.