August 28th 2020
by Andrew Blomme on 8/28/2020
August 28th, 2020
Every week is different during the growing season. This week we will talk about a newer insect pest that is starting to be a more common problem and the dry, arid conditions we have been experiencing and its effect on crop progression.
Soybean Gall Midge
The soybean gall midge is an insect pest that has found its way to western Iowa in the last couple of years. Below is a map from Justin McMechan at the University of Nebraska that show the soybean gall midge's movement over the last couple of years. It was first identified in our area in 2018.
Lou and I have been able to find gall midge larvae infested plants in several fields over the last couple weeks. The larvae are extremely small, orange worms that are difficult to see with the naked eye. They tend to burrow into the base of soybean stems. Below is a picture of a larva I found this week in northern Audubon county.
This next picture is from Iowa State and shows a heavily infested plant. The pale larvae are also soybean gall midge just at a younger stage. Luckily, I have not come across populations that were this high this year.
Soybean Gall Midge Damage
Soybean gall midge larvae are hard to see but their damage is much more noticeable. Larvae bore into the lower stem of soybean plants and destroy the vascular tissue of that stem. This results in the disruption of nutrient and water uptake which kills the plant. Damage appears to be more common in earlier planted fields. Below are pictures of the interior of a healthy soybean stem on the left and a soybean gall midge damaged stem on the right.
Damage varies on the population of the larvae present. The damage shown above is rather mild, caused to only a few larvae. On a field level, midge damage can be found as dead plants randomly scattered on the edge of a field. Higher midge populations lead to more dead plants. When you find a dead plant, you can typically find more larvae in the stems of plants that are immediately adjacent to it.
The soybean gall midge has the potential to be a serious agronomic pest in our area in the future. As populations grow in the coming years, we need to keep and eye out for this pest and be prepared to take action to suppress them if necessary.
You can click on this link to learn more about the soybean gall midge from Iowa State University: Iowa State ICM Soybean Gall Midge
This week has been one of the most hot and dry of the year. Temperatures regularly reached above 90 degrees while the humidity stayed relatively low. The crop has had to respond to these conditions for the last 5-7 days. In the areas that are lacking water the crop has started to shorten its grain fill period. A shorter grain fill period limits the starch content of a kernel. Less starch in a kernel results in a lighter test weight. Lighter test weights can be expected in fields that are experiencing this stress during grain fill. It is also important to watch the milk line of a corn crop to estimate harvest timing. Milk line recession is occurring at a much faster pace this year compared to last year. The ear in the picture below is about 1/2 milk line.
Picture of the Week
Mike found this ear that was 22 kernels around and weighed like a couple bricks earlier this week. It was off the outside row with plenty of sunlight and water, but it is still cool to see a kernel set like this. It wasn't that long ago that finding an ear that was 18 kernels around was exciting and now that can be common throughout a whole field.