By Tom Hunt, Extension Entomology Specialist
and Keith Jarvi,Wayne Ohnesorg
Scouting Methods
There are two methods to scout and determine if an insecticide treatment is warranted: a conventional scouting method using the 250 aphid/plant economic threshold and a speed scouting method.
The Conventional, 250 Aphids/Plant Method
Begin scouting soybean fields once or twice a week in late June/early July. Check 20 to 30 randomly selected plants in various areas of each field. Aphids are most likely to concentrate at the very top of the plant, although they will move onto stems and within the canopy as populations grow and/or the plant reaches mid to late reproductive stages. If a tree line or woodlot is adjacent to the soybean field, include a few sampling locations near these areas. Often soybean aphids are found first in parts of the field near wooded areas.
Counting aphids is not as difficult as it may seem at first. First, walk to a random spot in the field. Pull a plant and turn it upside down and give it a quick scan to see where the aphids are located. Get a feel for what 10 or 20 aphids look like and count by 10s or 20s.
The current threshold for late vegetative through R5 stage soybean is 250 aphids/plant with 80% of the plants infested and populations increasing. Thresholds for early R6 have yet to be determined, but are likely in the 400-500 aphids/plant range. Insecticide treatment during or after mid to late R6 has not been documented to increase yield.
The Speed Scouting Method
This method uses a spreadsheet adapted from a sampling plan developed by the University of Minnesota (Hodgson, E.W., et al.). It is different from conventional scouting in that it relies on the number of “infested” plants. Plants are considered “infested” when there are 40 or more aphids on a plant. The scout does not have to count or estimate the number of aphids on a plant to determine whether it has reached the threshold of 250 aphids per plant. You simply determine if a plant is infested and enter this in the spreadsheet. The spreadsheet then recommends further scouting or treatment options based on the number of “infested” plants in a given area.
This tool is free on the web (UNL Extension Circular EC1582 Aphid Speed Scout to be used in the field on mobile devices (e.g., smart phone, computer, iPad) and as an iPad App in the iTunes store at Aphid Speed Scout .
Soybean aphids on a single leaf, 2002. In Minnesota, as many as 13,000 soybean aphids have been found on a single plant.
General Management Guidelines
- Look for the presence of aphid natural enemies such as lady beetles, green lacewings, insidious flower bugs, aphid mummies, fuzzy aphids, and other insect predators. Predators and parasitoids may keep low or moderate aphid populations in check. Often you can find soybean aphids by examining plants where lady beetles are observed.
- Take note of winged aphids or “broad-shouldered” nymphs. Nymphs with broad or squared-off shoulders will become winged adults. A magnifying glass is helpful to see the “broad-shouldered” nymphs, but the winged adults are easy to see with the naked eye. If the majority of aphids are winged or developing wings, the aphids may soon leave the field and treatment can be avoided.
- If the plants are covered with honeydew or sooty mold, or stunted, and aphids are still present at threshold levels, an insecticide treatment may still be of value, but the optimum time for treatment has passed.
- Good insecticide coverage and penetration is required for optimal control of soybean aphid because aphids feed on the undersides of the leaves and within the canopy. For ground application use high water volume (≥15 gallons/acre) and pressure (≥30 psi). Aerial application works well when high water volume is used (≥3 gallons/acre).
Insecticides
Several insecticides are labeled for the soybean aphid. A list of registered insecticides, rates, preharvest intervals, etc. can be found at http://entomology.unl.edu/instabls/soyaphid.shtml. Pyrethroids have a relatively long residual, and work best at temperatures below 90º F. Organophosphates have a fuming action, and may work well in heavy canopies or high temperatures. Dimethoate is least effective.
Caution
Spraying flowering soybean poses a threat to honey bees. Communicate treatment plans to nearby beekeepers and follow label precautions to minimize honey bee kills. When there is concern about honey bees, pyrethroids are the better insecticide choice and spraying late in the day is preferred.
Host Plant Resistance
Plant resistance is another soybean aphid management strategy. Certain soybean cultivars have genetic qualities that prevent them from being heavily damaged by the soybean aphid when compared to other soybean cultivars.
Some commercial varieties of resistant soybean have been developed and use the Rag genes. Soybean aphids feeding on varieties with this resistance reproduce at a drastically slower rate and are less healthy. However, it is possible for this resistance to be circumvented by the soybean aphid. In fact, resistance has already been documented to some of the earliest deployed genes (e.g., Rag1 gene) in states to the east. In light of this, no single method of pest management should be relied upon solely and scouting of fields will still be necessary.
Source : unl.edu