Bryan Jensen
Although soybean aphid populations have been low this growing season, I thought I would offer a quick summary of an article recently written and reviewed by several research and extension entomologist from the northern soybean producing states. In recent years, there has been a wealth of soybean aphid management information provided from Land Grant Universities which are the results of multiple studies, conducted in several environments that are science-based, statistically sound and peer-reviewed. There is also advice offered from several other sources which contradicts these findings and/or makes claims based on hunches and/or observations. Always consider the source of your information as well as the method in which it was collected.
Soybean aphids damage soybean by extracting plant sap from phloem vessels which carry the products of photosynthesis to other parts of the plant. Initially aphids may “test-probe” plants to determine if they are suitable for sustained feeding. Sustained feeding during the early reproductive stages can result in reduced growth, pod number, seed size and weight as well as oil concentration. Feeding during the later reproductive stages may only affect seed size. Making crop stage an important consideration when making treatment recommendations. Duration of feeding is also an important consideration and is why multiple scouting trips are recommended.
Neither sustained feeding or test probing causes the plant to “leak sap” nor has there been documented evidence that this feeding transmits bacterial or fungal pathogens. However, aphid feeding does transmit viral pathogens such as alfalfa mosaic and soybean mosaic viruses. These viruses are not currently considered wide spread and are not included in the economic threshold.
Understanding the soybean aphid Economic Threshold (an increasing population of 250 aphids/plant on 80% of the plants) is better understood with an appreciation of the Damage Boundary (lowest pest population which can cause measurable yield loss) and Economic Injury Level (point at which cost of control equals yield loss). The Damage Boundary is not based on economics, but rather the ability of scientists to statistically measure yield loss at a specific pest population. No economic gain would be expected by treating aphid populations which are below the Damage Boundary. The Economic Injury Level is based on economics and includes the cost of application and crop value. The Economic Threshold (250 aphid/plant, etc.) is an arbitrary and conservative number which was based on close monitoring of several multi-state research projects (including Wisconsin). It was established at this level to allow producers and crop advisors time to react and prevent an aphid population from reaching the Economic Injury Level. The soybean aphid Economic Threshold is well below the Damage Boundary. Some people question the legitimacy of the Economic Threshold because it was based on economic estimates from the mid-2000’s. Recall, the Economic Threshold is already below the Damage Boundary and no economic gain would be expected if treating below the Economic Threshold as some sources have suggested. It may help your understanding if you view the Economic Threshold as an “action” or “treatment” threshold. Furthermore, treating a population below the Economic Threshold decreases the chance that natural enemies or other environmental condition will control the aphid population for you.
There are additional costs to treating soybeans too early. New foliage that emerges after an application of a synthetic pyrethroid or organophosphate will not be protected. Other classes of insecticides may translocate upward but only a leaf or two. This is especially a concern if treatments are done early in the growing season.
Most insecticides used today are broad spectrum. Meaning they kill a wide variety of insect species not just those which are considered pests. Killing beneficial insects may allow for the soybean aphid population to resurge at a later date in the absence of these beneficial organisms. If spider mites are present at sub-economic populations and an insecticide is used that does not control mites, mite populations may flare in the absence of beneficial insects and mites.
Soybean aphid populations vary from field to field. Environmental, geographic, biological and agronomic factors can all influence soybean aphid populations. The low cost of insecticides and/or the concept of free applications costs when tank-mixed with another pesticide can lead to negative after-effects including insecticide resistance.