The University of Minnesota Crookston held the final event of a series of webinars on Let’s talk Crops! this morning. Below they share what they presented in their final webinar.
Sometimes organisms in our crop fields are already helping us with pest control. Growers can take advantage of the benefits these predators and parasites provide.
The soybean aphid, a major soybean pest, invaded the U.S. from eastern Asia in 2000. Shortly after its arrival, Dr. Heimpel, a University of Minnesota entomologist, traveled to Asia to hunt for natural predators of this pest. “We looked for parasitic wasps that were attacking aphids in their native range,” he explained. They hoped that they might be able to bring them back to the United States for release in soybean fields.
They found about 20 species of tiny parasitic wasps, called parasitoids, that were attacking the soybean aphids. Through an extensive screening process in the USDA quarantine lab at the University of Minnesota campus, researchers found two species that attacked only the soybean aphid. Unfortunately, they were not able to get established in Minnesota. Other parasitoids were tested and rejected as biological control agents because they would attack our native aphid species.
Aphelinus certus (A. certus) was one of the parasitoids that had been tested and rejected in the quarantine lab. However, it was accidentally introduced in Pennsylvania around 2005. It arrived in Minnesota in 2011 and quickly became widespread and persistent.
Between 2011 and 2021, soybean aphid density over the season has generally declined. In most parts of the state, soybean aphid populations have been relatively low in the last few years. At the same time, the rate of parasitism by the wasp has increased.
The big question, according to Heimpel, was, “Is A. certus responsible for the decline of soybean aphids?” To help answer that question, his lab conducted several studies.
An early field study suggested that A. certus alone suppressed soybean aphid densities below the insecticide treatment threshold. These early results were exciting but not consistent with studies that followed.
A follow-up study showed that while aphids were suppressed, there wasn’t strong evidence for significant control. “We learned that higher parasitism rates did lead to lower aphid growth rates, but the parasitism rates just weren’t high enough,” Heimpel stated.
Their next step was to model a typical season in the field. What did they conclude? Under most normal conditions, they would expect A. certus to suppress aphids to levels below the economic injury level (EIL) – the level where yield is reduced – about 31% of the time. In the same way, aphid suppression below the economic threshold (ET) – where insecticide treatment is recommended before populations reach the EIL – might be expected about 10% of the time.
There is still much to learn about A. certus and the field practices that might favor them. For example, their overwintering habits are something of a black box. Studies did show that when mummies overwintered on the soil surface, they did a great job of emerging in the spring. However, when they were buried just under the soil surface, like in a tillage situation, they couldn’t emerge at all.
Ideally, Heimpel would like to see them overwinter in soybean fields since they could emerge in time to suppress aphids. When they overwinter in a wood lot instead, emergence is too late for them to have an effect on soybean aphids.
Because suppression can vary from year to year and field to field, growers should continue to scout for soybean aphids and use the recommended economic threshold before applying an insecticide. When combined with integrated pest management practices, growers can take advantage of the free benefits biological control agents provide.
For more information on soybean insect pests, visit the University of Minnesota Extension’s website at https://extension.umn.edu/soybean/soybean-pest-management.
We wish to thank our generous sponsors, the Minnesota Corn Research and Promotion Council and the Minnesota Soybean Research and Promotion Council.
For more news from U of M Extension, visit https://www.extension.umn.edu/news or contact Extension Communications at extdigest@umn.edu. The University of Minnesota Extension is an equal opportunity educator and employer.
