Don't rush out and spray soybean aphids early, just because the soybean price is high, Kelley Tilmon, South Dakota State University Extension entomologist.
"For soybean aphids in particular, a lot of people are asking this question, 'if the action threshold was 250 when soybeans were $6/bushel, shouldn't it be half that, now that prices are in the teens?' But the answer is no,"' Tilmon said. "This is because of how the action threshold and the economic injury level are related."
Tilmon explains that the economic injury level is the point where the insect injury justifies the control cost. This does vary with both commodity value and control costs. The economic threshold, or action threshold, is a lower value - not when economic loss is occurring, but when to take action to keep a pest population from climbing to the economic injury level. It builds in time to react to an insect population before it becomes a problem, and is based on how quickly the population can be expected to grow, relative to the economic injury level.
"When soybean values were lower the economic injury level was around 675 aphids/plant; we recommended treatment at 250 aphids/plant to give a 7-day lead time," Tilmon says. "We can re-calculate the economic injury level for current values. At $15/bushel and $8/acre for treatment, the economic injury level is 460 aphids/plant. This means that if you continue to use an action threshold of 250 aphids/plant, you have about four days of lead time to line up treatment, based on what we know about how quickly aphid populations build. So if you can treat in this window, this is still a solid threshold to prevent economic loss."
Tilmon says you are unlikely to see a positive yield return at lower treatment levels than 250 aphids/plant. This is because of the damage boundary, which is the minimum number of insects that it takes to cause yield loss that can even be detected.
"In most trials conducted in South Dakota and throughout the region, 250 aphids/plant is definitely below the damage boundary," Tilmon said. "In other words, we see the same yield when we treat at 250 aphids as we do when we treat at five aphids - treating at lower levels gains you nothing because it just takes more aphids than that before we can reliably detect even a small yield loss."
Tilmon also notes that there are drawbacks to prophylactic or "insurance" treatment, even when the cost of treatment is low.
"We often see pest resurgence or secondary pest outbreaks in treated fields that wouldn't have occurred otherwise, because the natural enemies - the beneficial predatory insects that were helping keep pests in check - are gone," Tilmon says.
Furthermore, Tilmon adds that many products, particularly pyrethroids, can flare spider mite populations, which is especially a concern in hot, dry weather.
"The bottom line is that a single, well-timed application for soybean aphids using a guideline of 250/plant can still save you time and money - even when crop values are high and plants are stressed," Tilmon said. "And we're seeing so few aphids this summer anyway."