Scout beans for summer insects
July can be hot, but it’s a time when scouting for insects is important. This month I’m writing about one persistent and one emerging pest in Iowa. Soybean aphid is the persistent one, while the newer pest on the Iowa scene in recent years is Japanese beetle. Scouting and making treatment decisions after soybean bloom is critical to protecting the grain filling period.
The soybean aphid is a phloem feeder; it pierces the plant and sucks out the sap. While feeding, aphids secrete a sugary excrement called honeydew. Honeydew is a good growth medium for black sooty mold and can interfere with photosynthesis. The combination of feeding aphids and sooty mold can cause a yield loss, as well as reduce soybean seed size and quality.
The temperature range for soybean aphid nymph reproduction is 47 to 95 degrees F. Soybean aphids can produce up to 15 generations each summer if conditions are right. Optimal reproduction is at 82 degrees and relative humidity is below 78%, when populations can double in six to seven days. Above that, temperature development time is lengthened. During persistently high temperatures (high 80s to 90-plus) soybean aphid reproduction declines. At 95 degrees the lifespan is greatly shortened, and no nymphs are produced.
When scouting you need to keep these facts in mind and pay attention to the weather forecast. Whether doing traditional whole-plant scouting or speed scouting, the method you use will make a difference. Speed scouting is conservative and will likely recommend spraying an insecticide treatment before whole-plant scouting will.
Keep in mind the difference between economic injury level and economic damage level. The economic injury level is the point at which economic damage can occur and is about 650 aphids per plant. The economic threshold is the point that’s set below the economic injury level in order to prevent yield loss, also sometimes called the action threshold.
Consider applying a foliar insecticide if there are 250 aphids per plant, and populations are increasing on 80% of the plants. This threshold allows a five- to seven-day lead time in order to make timely treatments to protect yield.
If you are finding even 200 aphids per plant, and the forecast is for hot weather, don’t treat. Rather, come back in three to five days and count again. Scout for soybean aphid through R5.5 growth stage. By the time beans reach R6 (full seed) there may not be a cost benefit for treatment.
The Japanese beetle is an emerging pest in Iowa field crops. While they cluster along field edges and look like an overwhelming number of beetles, oftentimes they do not venture far into the field. In soybeans they feed mainly on the leaf tissue between the veins, which gives a lace-like appearance to the affected leaves.
If you see this insect infesting a field, spot-treat border rows, if practical. The economic threshold for Japanese beetle is based on defoliation — a 30% defoliation before bloom and 20% defoliation after bloom. Most fields never reach these levels, so be careful to not jump the gun. It’s easy to overestimate the percent defoliation. See www.ipm.iastate.edu/ipm/icm/2002/7-29-2002/soydefoliation.html for more accurate estimations.
What is the harm of treating before reaching the threshold for field crop pests?
Your input costs may not be recovered and you will kill beneficial insects. Keep in mind beneficial insects control aphids at low to medium reproductive rates. They also help control other soft-bodied herbivores, such as two-spotted spider mites and caterpillars. Several beneficial insects in soybeans are natural predators of aphids. They include adult and larval lady beetles (ladybugs), damsel bug nymphs and adults, insidious flower bugs, and green lacewing larvae. Learn what these insects look like, and look for them while you scout.
This article published in the July, 2014 edition of WALLACES FARMER.
All rights reserved. Copyright Farm Progress Cos. 2014.