Soybean aphids ready for flight. Image: Erin Hodgson. University of Iowa.
By Kelly vanFrankenhuyzen
Farmers and researchers are increasingly using a faster way to decide when to combat wimpy sapsuckers that are destroying soybeans
Erin Hodgson and her research team at the University of Minnesota published a “speed scouting” technique in 2004 to help farmers decide when to kill invasive soybean aphids. Using her method, farmers inspect the soybeans and tally the amount of aphids on a certain number of plants.
That number is important because in great numbers an insect that is “wimpy” by itself can devastate soybeans.
It is a fast way to decide if the infestation is bad enough to spray the crops with insecticides, Hodgson said.
Hodgson, an associate professor and extension entomologist at Iowa State University, began her research as an undergraduate student in 2001.
“Soybean aphids were confirmed in North America in 2000,” Hodgson said. The following year they were found in Minnesota.
Soybean aphids are native to China and unknowingly hitched a ride upon Chinese traded goods, according to Christian Krupke, professor of entomology and field crops at the University of Purdue.
Minnesota and New York are two Great Lakes states with the highest soybean aphid infestations, John Tooke said, an associate professor of entomology at Pennsylvania State University. Ontario also has high infestation rates.
Aphid colony. Image: Erin Hodgson. University of Iowa.
And the Great Lakes region has a lot of soybeans for them to eat.
Minnesota, Illinois, Indiana and Ohio rank in the top 10 states with the highest yield of soybeans, according to a 2014 United States Department of Agriculture report.
“The best time to start scouting is when the soybean plant emerges, in late May or June,” Hodgson said. This is the colonization stage. Scouting continues until the first or second week of October.
Typically, scouting begins when the plants start to bloom and when they are at full flower. By the time the pods begin to turn yellow, most aphids are gone and counting is ineffective.
Hodgson’s speed scouting model examines 11 soybeans plants, checking if each plant has 40 or more aphids. Finding 40 plus aphids per plant means the crop is infested and should be treated, otherwise loss of the crop is probable. A more common method is scouting for 250 aphids per plant. Hodgson’s method is faster by counting smaller populations on plants. Both methods evaluate the health of the crop to determine treatment.
“Speed scouting is a tool that some farmers use and some don’t,” Bruce Potter said, an integrated pest management specialist at the University of Minnesota.
“Minnesota farmers with high infestation rates are hesitant to get on board with this model,” Potter said. They have used their own methods since the soybean aphids first came to Minnesota.
“The soybean aphids are part of the invasional meltdown,” Matthew O’Neal said, associate professor of entomology at Iowa State University. That’s where one invasive species in a new environment facilitates the invasion of another non- native species. For instance, Buckthorn is an invasive plant that came from Europe. Once it was established in North America, it provided optimal overwintering home for the aphids that came later.
Full leaf. Image: Erin Hodgson. University of Iowa.
O’Neal suggests that farmers should scout weekly because populations increase rapidly.
That could get time consuming. And many farmers already use their own techniques. Farmers are slow to accept this method because it was created a couple years after the invasion, Hodgson said.
“Speed scouting was introduced after farmers had been learning to deal with this pest,” Potter said.
A number of Great Lakes states have controlled the soybean aphids with natural predators. Hodgson and O’Neal create podcasts for farmers, researchers and entomologists to use as educational tools. Podcast topics include information about environmental factors that affect soybean aphids and crops, pests affecting crops and basic speed scouting techniques.
Test your scouting skills!
Think you could spend a day in the field successfully identifying the number of aphids on soybean plants? Take this quiz to find out.
The images associated with these questions train people to estimate the number of aphids per plant using the 250 count method. Knowing how to identify aphids on the plants is an important skill before speed scouting. Absolute counts are not needed when making a sound decision, Potter said.