Received August 3
Current Conditions.
Rainfall has been sparse across the area for the last couple of weeks leading to dry soil conditions in area fields. Soybeans and corn can both use rainfall. Some stress is being observed in crops in areas of fields with lighter soils. Some of the heat or drought stress symptoms I have observed include leaf rolling in corn, and in soybeans, I have seen leaf flipping.
Data from the Iowa Mesonet that includes the timeframe of April 11 to Aug. 1 (for the area that I serve in North Central to Northwest Iowa) shows that we are 2-3 inches behind the climatology value. The climatology value is the average value of rainfall since 1951. Rainfall during the month of August will benefit soybeans helping them to fill pods and in corn, it will help through the later reproductive stages.
Soybean Aphid.
I have heard of sporadic reports of soybean aphids present in some fields. Scouting for soybean aphids is recommended to understand if this pest is present and at what levels. Adults are greenish and around 1/16-inch long. Soybean aphids have a pear-shaped body. When we scout for soybean aphids, we should sample at least 30 plants per field. Utilize a scouting pattern such as a W or a Z scouting pattern so that the whole field is observed. Look at the entire plant and count the number of live soybean aphids (winged and wingless) on each plant, including stems and pods.
The threshold is at least 250 aphids per plant and at least 80 percent of plants infested and aphid populations increasing. When counting, do not count the cast skins (white flakes), or the diseased and parasitized aphids. If you find that the average number of aphids across the field is currently below the threshold, following up in one week with a scouting trip is recommended to monitor levels.
Continue to return weekly to monitor aphid populations through R6 (full seed or a pod containing a green seed that fills the pod at one of the four uppermost nodes on the main stem). If the soybean field has been sprayed with an insecticide, scout the field regularly after that application to monitor for a second outbreak.
Corn Rootworm.
Some farmers have reported corn rootworm feeding on fields in Iowa. A severe corn rootworm larval infestation can destroy nodes 4-6 (the nodes closest to the soil surface). Root pruning by corn rootworm interferes with water and nutrient uptake and makes the plant unstable. When wind events occur, corn – due to this root pruning – may lodge. This makes corn difficult to pick up at harvest time.
Research predicts a 15 percent yield loss for every node pruned back to 1 ½ inches of the stalk. Corn rootworm survival is greater when soil is dry. Dry conditions can increase the yield impact of root pruning.
Right now, both northern and western corn rootworm beetles have emerged. Scouting your fields by digging roots and understanding the levels of feeding that have occurred to your plant roots and the levels of adults in the field will help with management decisions next year. If you are in a corn-on-corn rotation, those management decisions may include planting soybeans for a year to break up the cycle of corn rootworm.