Soybean aphid scouting and management
Should you adjust your soybean aphid threshold for high crop values?
Many producers may wonder what higher crop values mean for the insect treatment thresholds they’ve used in the past. It’s logical to assume that climbing commodity prices mean lower treatment thresholds, but that’s not always the case. For soybean aphid thresholds in particular, 250 aphids/plant is still a valid guideline for decision-making. This has to do with the relationship between the decision (aka ‘economic’) threshold, the economic injury level, and the damage boundary. The economic injury level is the point where the insect injury justifies the control cost. This does vary with commodity value and control costs. The decision threshold is a lower value – not when economic loss is occurring, but when to make a decision take action to keep a pest population from climbing to the economic injury level. It builds in time to react before the population becomes a problem, and is based on how quickly the population can be expected to grow. The damage boundary is the lowest insect pressure/damage level where yield loss can be detected. It is not logical to have an economic injury level lower than the damage boundary, because no economic injury can occur when no yield is being lost. The average damage boundary for soybean aphid is greater than 4000-5000 cumulative aphid days [corresponding on average with a population greater than 485-600 aphids/plant].
When soybean values were lower the economic injury level ~675 aphids/plant, and the recommended decision threshold was 250 aphids/plant to give a 7-day lead time. We can recalculate economic injury levels for more current values. For example at a value of $12/bushel and a $13/acre treatment cost with a yield expectation of 50 bushels, the economic injury level can be calculated theoretically as 356 aphids/plant. However, this is below the damage boundary where yield loss begins – typically in excess of 500 aphids/plant), and thus is not a valid economic injury level [there is no economic injury when yield is not being lost]. Using a conservative damage boundary of 485 aphids/plant, a decision threshold of 250 aphids/plant can still be used and gives an average lead time of 5 days to arrange treatment before the populations reaches 485 aphids/plant. So if treatment can be made in this window, this decision threshold will still prevent economic loss. The data behind these guidelines were collected over three years in 19 locations under a wide variety of conditions, including moisture stress. In most trials in the region, including threshold field trials in South Dakota, we have not seen a yield return when using a threshold lower than 250 aphids/plant.