By John Dietz
Soybean aphids have blown in on summer winds to become a serious problem a few times in Manitoba’s short history of growing soybeans. Although not known to overwinter in Manitoba, they could return this summer, says John Gavloski, entomologist with Manitoba Agriculture in Carman.
Whether they get to pest levels depends on several factors.
“We’ve had a few years where soybean aphids have been here at economic levels on a fair number of farms. It’s not an annual problem, but we certainly can get an economic level of infestation,” Gavloski said in a telephone interview. “They don’t overwinter here. Normally we don’t see them until July. Usually, it’s late July and through August when they become an issue.”
The soybean aphid was first noticed in 2000 in the U.S. Midwest. It was found in North Dakota soybeans in 2001, and was first detected in Manitoba late in the 2001 growing season.
“They potentially could land anywhere that soybeans are grown in Manitoba,” Gavloski said. “Normally, they don’t get dumped in one or two fields. There can be quite a bit of variability within a region and between fields. They can be patchy within fields.”
It’s easy to miss the early stages. At first, they probably will be on the underside of a few upper leaves, scattered across the field. After the population builds, the clusters of yellow specks on various parts of the plant will be more obvious.
The soybean aphid is a soft-bodied yellowish-green insect, less than 1/16 inch in size with a pear-shaped body and a pair of dark “tailpipes” at the end of the abdomen. The mouth parts can pierce and suck sap from the leaf. They can be winged or wingless, and they quickly form aphid colonies.
While feeding, aphids excrete sugars or “honeydew” as a sticky waste.
A few on a plant are not a problem; but several hundred per plant can reduce profit enough that control may be worthwhile. In southern Minnesota, where they can overwinter, they can produce more than a dozen generations in a growing season.
The good news is that soybean aphids also have natural enemies that also can multiply and knock down the aphid population.
Many factors will determine how serious soybean aphids become in Manitoba.
If they blow into our soybean region, the seriousness will depend on when they blow in, what kind of numbers arrive, how quickly levels of natural enemies respond to this food source, and the conditions when they get here.
Natural enemies (lady beetles, lacewings, hover flies, and minute pirate bugs) often can respond to increasing soybean aphid levels and begin to be more common in the field.
Several insecticides are registered for soybean aphid control.
However, Gavloski said, see if natural enemies can do the job. Hold off on insecticide until the soybean aphid population has gone over the economic threshold.
Growers and agronomists need to watch carefully. In a 50-acre field, pick five to 10 areas to inspect individual plants, and visit frequently after early July.
“Have a careful look. You are looking for little yellow specs on the leaves. They are much smaller than the aphids we get on cereals and peas.
“If at first you’re not sure you’re seeing an aphid, take a magnifier and have a closer look. Later, they get into clusters in big numbers, then it’s really obvious.”
Note the average number on infested plants and return in a few days to monitor the population change.
Populations may grow, remain quite stagnant, or even decrease, depending mainly on levels of natural enemies.
“We’ve seen populations start to build and then literally crash when natural enemies move in. Some species of lady beetles will eat over 100 soybean aphids per day. Lacewings and hoverfly larvae can eat quite a few, too,” he said.
A cell phone app for the decision-making process, Aphid Advisor, uses aphid and natural enemy numbers along with expected population growth rates. It can indicate whether there are enough natural enemies to keep aphid populations below action thresholds or if an insecticide application may be needed.
The app was developed in Ontario and can be used on the Prairies. The app is available at: www.aphidapp.com
“Get it, plug in your aphid count and your number of natural enemies, and the app will tell you whether you should spray or not,” Gavloski said. “The threshold for spraying is about 250 aphids per soybean plant, with the population increasing, and plants in the beginning bloom to beginning seed growth stage.”
A healthy soybean can tolerate more than 250 aphids on the leaves.
“The break-even where the aphids are costing you as much as the insecticide application, in decent growing conditions, actually is about 670 aphids per plant. But, if they’re not being controlled by natural enemies, the population can increase quickly,” he said.
“So, we don’t want people waiting until there’s that many per plant to make a spray decision. That’s why the threshold is set at a population of 250 and still increasing.”
However, the spray decision also depends on crop growth stage. The entomologist usually recommends making the spray decision between the R1 stage, when bloom is beginning, and the R5 stage, when seed growth is beginning.
“Early podding is the most susceptible stage, when you really have to keep an eye on things. Once the seeds start to firm up, you’re probably not going to gain much from an insecticide application,” he said.
One last detail. Estimate the number of aphids; don’t try to count them at the higher levels. Look at a leaf and try to estimate the number.
There’s an aid for that, too. It’s a “photo key” for what a leaf looks like at different aphid populations. Here is a link to a photo key from Ontario: http://fieldcropnews.com/wp-content/uploads/Soybean-Aphid-Scouting-Card-ENGLISH.pdf. Other guides are available from U.S. websites.
“Normally, once a person uses it for a while they become familiar with what 50 aphids on a leaf looks like, and they don’t need it. Sometimes, to get going, it’s nice to have a photo guide to train your eye to do these estimations,” he said.