Soybean Aphids

Bryan Jensen
UW Extension

I have been getting a few phone calls regarding increasing soybean aphid populations. Nothing extreme but some fields have been slightly over or under thresholds. Treatment decisions at this time of the year are very difficult to make, especially when populations hover around the economic threshold. I’ll offer a few thoughts and comments.

First a comment on crop stage. The economic threshold for soybean aphids on R1-R5 soybeans is 250 aphids on 80% of the plants and the population is increasing. Soybeans at R6 are very unlikely to suffer significant yield loss unless that field is under drought stress. In a short conversation with Shawn Conley, many fields are in the R4-R5.5 stage of development. Therefore, the window for treatment will be closing soon.

When scouting, look for “white dwarf” soybean aphids which are, of course, soybean aphids but do not live as long (50%) or reproduce as fast (70%) as the yellow-green form. White dwarfs are smaller in size and cream colored. Other species of aphids will produce these morphs in response to environment conditions. For soybean aphids, it is possible that high temperature, humidity and/or shorter day length may bring about a higher percentage of white dwarfs. Count the white dwarfs and include them in your total plant counts. However consider their presence when you are on the fence and deciding to spray or not. One might expect the percentage of white dwarfs to increase because the forecast calls for high temperatures through the weekend.

Also, consider if you plan ground application of insecticides that wheel tracks will reduce yield. That reduction is dependent on the boom width. Research by Dr. Conley and others at Purdue indicate that reduction can be as high as 4.9% (30 ft boom) to as little as 1.3% (120 ft. boom).

Predators and parasitoids can negatively influence aphid populations if timing and conditions are right. We are all familiar with the adult and larval stage of lady beetles. Both life stages will feed on aphids and can rapidly reduce aphid populations if numbers are high. When scouting, observe relative numbers of lady beetle life stages (adults, larvae and pupae) over time. This observation will also indicate if their numbers are decreasing, increasing or are stabilized. If lady beetle numbers are increasing faster than aphids it may be a good sign that treatment may not be needed for the time being. Parasitoids, indicated by the presence of aphid mummies, can also negatively affect aphid populations. Their effect, however, tends to take place over a longer period of time.

Our typical mid to late-August weather patterns favor entomopathogenic fungi. If environmental conditions (cool/humid) are expected, you may see a reduction in soybean aphids. Early symptoms of this fungus are aphids that are rose colored.

One final comment on soybean aphid management. Some areas of Wisconsin have been dry if not very dry. While scouting for aphids look for twospotted spider mites and/or signs of their damage. Choose insecticides wisely if mites are present. Use of synthetic pyrethroid insectides will have limited effects on spider mites but will kill beneficial mites and insects leading to higher mite populations. Insecticides with the active ingredients chlorpyrifos and dimethoate can control both aphids and spider mites. However, plan on revisiting sprayed fields to verify control and check on possible aphids and mite resurgence.

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