Update from the Field: Boring Insects are not Boring
Posted by David Dyson, Agronomist on July 23, 2018
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Soybean fields are starting to become overrun with weeds that are herbicide-resistant. This phenomenon is not new and will continue to get worse over time. The reason glyphosate-tolerant crops became so popular in 1996 was that the weeds at the time were almost impossible to kill with existing herbicides.
Today, the main resistant weed that is a problem in soybean fields is marestail, which we have been battling for over ten years. Unfortunately, every time we leave plants alive after an application of herbicide, we are selecting for herbicide resistance. With the release of the dicamba-tolerant soybeans, we have turned a corner in the fight against herbicide resistance, much like what occurred in 1996 with glyphosate. A high dose of dicamba at planting, in combination with glyphosate and a good residual, will control marestail. I believe once the HPPD-tolerant chemistry gets approved, we’ll have another arrow in our quiver to control marestail.
The pigweed family of weeds can also be very difficult to control. This family not only contains redroot pigweed, but other nasty weeds such as waterhemp and Palmer amaranth. When I see waterhemp or Palmer amaranth in a field, I inevitably know I am dealing with glyphosate- and possibly glufosinate-resistant weeds. Why does this family seem to breed resistance so fast into their population? The answer is that unlike most plant species where the male and female flowers are on the same plant, waterhemp and Palmer amaranth have male and female flowers on separate plants. This makes for a more diverse genetic profile and leads to rapid adaptation to their environment. Unfortunately, these weeds also germinate later in the season, so an in-crop application must be made to control this family.
The weed species that’s worrying me the most is the ragweed family, common and giant ragweed. This family is a newcomer onto the resistance scene. Every year I am finding more and more acres of glufosinate- or glyphosate-resistant ragweed. As with the pigweeds, ragweed will germinate late in the season, so an in-crop application of herbicides is needed to control this pest. Planting multiple tolerant soybean genetic packages will give you options for in-crop spraying. Controlling ragweed early, before the weed gets bigger than a pencil in diameter, will prevent boring insects from “hollowing” out the ragweed. As figures 1 and 2 show, European corn borer can burrow inside the giant ragweed and prevent herbicides from translocating to the roots. These weeds will be impossible to kill once boring insects make a home inside the stalk.
Figure 1: European corn borer inside a giant ragweed plant. Picture taken near Kokomo, IN, July 2018.
Figure 2: European corn borer insect just removed from a giant ragweed plant. Picture taken near Kokomo, IN, July 2018.
In conclusion, starting out clean and keeping your fields clean will prevent water and nutrients from being diverted to weeds instead of crops, and you will also be reducing the resistant weed population.
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Dave Dyson is a regional agronomist for The Andersons’ Farm Centers which are located throughout Ohio, Michigan, and Indiana. He is an Indiana native and grew up on a dairy farm in Miami County. A graduate of Purdue University with a degree in Crop & Soil Science, Dave has a deep knowledge of various agronomic topics and is committed to helping growers improve their crops. If you have any questions, Dave can be reached at david_dyson@andersonsinc.com.
