It’s been 20 years since soybean aphids were first reported in Ontario soybean fields. Over the years, growers and researchers have learned that no two years seem to be the same and populations of the yield-robbing pests vary from year-to-year.
There has, however, been significant changes to aphid behaviour over the years and this evolution continues to impact control strategies. Winchester, Ont., agronomist Gilles Quesnel has been following these pests since they first arrived in the province. In recent years, he notes that soybean aphid populations have developed differently throughout the growing season compared to when they first arrived in eastern Ontario in the early 2000s.
On this episode of the RealAgriculture Soybean School, Quesnel shares what he’s observed during 20 years of aphid monitoring. “When they first came in they had few natural predators. And if you had an infestation, it wasn’t unusual to see that population just pop up to 20,000 aphids per plant — your pants would be all sticky and gooey walking in.” These high populations often stayed on the plant until the start of leaf drop at R6.
Back then it was easier for the growers to diagnose an aphid problem. “Over time, more beneficial insects —not just insects but predators of all kinds — have established. So the [aphid] infestations nowadays are at lower levels, they come in slower, they build up slower and they crash earlier in the season,” says Quesnel.
Based on observations of current aphid population dynamics, Quesnel notes that populations often remain just below the original economic injury level — 660 aphids per plant. This is when yield loss equals the cost of control. But even populations at 2,500 to 3,000 aphids per plant for a relatively short period of time can have a significant impact on yields. (Story continues after the video.)
Quensel says cost of aphid control depends on a number of factors. Application cost and tramping may sometimes be partially or entirely associated with other field operations. For example, sometimes the insecticide is tank mixed with other herbicide and fungicide applications and tramp lines are already present in the field following other applications.
What’s the return on aphid control with soybeans at $18 to $20 bu/ac? “If the timing is right, a grower can say his only cost to going in is that extra insecticide, maybe $14, $15 an acre. So that’s basically one bushel or just a little less, and you’ve paid for it.” So that means [that aphid counts] don’t need to be really high (between 300 to 400 per plant) for a long period of time to pay for itself.”
If growers need to make a specific field pass for aphid control, the application will need to protect three bushels per acre. In this case, Quensel says growers need to see about 400 to 500 aphids per plant, but there are other factors to consider such as whether the field is experiencing drought conditions.
In the video, Quensel also comments on whether growers can learn anything from the past year to help predict or forecast aphid levels for 2023. He says there really are no shortcuts. The key is be aware of the growing environment and growing conditions, scout early and often and effectively use the tools available.
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