August 2, 2012

Figure 1. Soybean aphid Figure 2. Soybean aphids on a single leaf, 2002. In Minnesota, as many as 13,000 soybean aphids have been found on a single plant.

Although it has been too hot for soybean aphids to thrive, we are beginning to see them in northeast Nebraska soybean fields. One could consider many fields to be “seeded” with soybean aphid. If, or should I say when, temperatures decline, soybean aphid populations could increase quickly, doubling in just two to three days. Growers should be scouting their soybean fields for aphids now.Last year we monitored a soybean field in Dixon County that was almost devoid of aphids on July 22, but by August 18 it had over 2,000 aphids per plant in areas that had not been treated.

Biology and Growth

The soybean aphid is soft-bodied, light green to pale yellow, less than 1/16th inch long, and has two black-tipped cornicles (cornicles look like tailpipes) on the rear of the abdomen (Figure 1). It feeds primarily on the undersides of leaves, but also on stems and pods, by removing plant sap with its needle-like mouthparts.

Soybean aphid populations can grow to extremely high levels under favorable environmental conditions. They favor temperatures of 70-85°F, do not do well in higher temperatures, and are reported to die when temperatures are maintained at 95°. Aphid development stops when temperatures drop below 48°.

Soybean Aphid Natural Enemies

Soybean aphids have many natural predators that can limit their populations.  The Asian lady beetle is most visible, but the tiny (1/10 inch) insidious flower bug (or Orius) is the most common and important predator. Other common predators include green lacewing, brown lacewing, damsel bugs or Nabids, and spined soldier bugs. These predators can significantly slow aphid population growth and should be considered when deciding whether to treat since most insecticides used for aphids will also kill their predators.

Other groups of natural enemies include parasitoids and pathogens. The presence of aphid “mummies” (light brown, swollen aphids) indicates the presence of parasitoids. These mummies harbor immature parasitoids, which will become adults, emerge from the mummy, and parasitize more aphids. The presence of “fuzzy” aphid carcasses indicates fungal pathogens are present, which occasionally can lead to dramatic reductions of aphid populations.

Scouting

There are two methods to scout for soybean aphids: conventional scouting and speed scouting.

Conventional Method. Begin scouting soybean fields once or twice a week in late June to early July. Check 20 to 30 randomly selected plants in various areas of each field. Aphids are most likely to concentrate at the 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 since soybean aphids are often found first in these areas. 

Counting aphids is not as difficult as it may seem. First, walk to a random spot in the field. Pull a plant, turn it upside down, and give it a quick scan to see where the aphids are located. Get a feel for what 10-20 aphids look like and count by 10s or 20s. The current threshold for late vegetative through R5 stage soybean is 250 aphids per plant with 80% of the plants infested and populations increasing. Thresholds for early R6 have yet to be determined, but are likely in the range of 400-500 aphids per plant. Insecticide treatment during or after mid-late R6 has not been documented to increase yield.

Speed Scouting Method. This method uses a spreadsheet adapted from a sampling plan developed by the University of Minnesota. (See the original resource, Enumerative and Binomial Sequential Sampling Plans for Soybean Aphid (Homoptera:Aphididae) in Soybean, written by E.W. Hodgson, E.C. Burkness, W.D. Hutchison, and D.W. Ragsdale, and published in 2004 in the Journal of Economic Entomology.) 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. One simply determines if a plant is infested and enters this in the spreadsheet. The spreadsheet then recommends further scouting or treatment options based on the number of “infested” plants in a given area.

The soybean aphid speed scouting tool is available free as a downloadable spreadsheet on the Extension Publications website (UNL EC1582 Aphid Speed Scout) and as an iPad app in the iTunes store.

Soybean Aphid Management

The current recommended economic threshold for late vegetative through R5 stage soybeans is 250 aphids per plant with 80% of the plants infested and populations increasing. Depending on economic conditions, this gives you five to seven days to schedule treatment before populations reach economically damaging levels. If populations don’t increase during this period, you may be able to eliminate or delay treatment. Determining if the aphid population is actively increasing requires several visits to the field. Factors favorable for aphid increase are relatively cool temperatures, plant stress (particularly drought), and lack of natural enemies.

Several insecticides are labeled for the soybean aphid. A list of registered insecticides, rates, preharvest intervals, etc. can be found on the UNL Department of Entomology website 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.

Tom Hunt, Extension Entomology Specialist, Haskell Ag Lab, Concord
Keith Jarvi, Extension Educator, Dakota, DIxon and Thurston Counties
Wayne Ohnesorg, Extension Educator, Madison County