Management Of Soybean Aphids And Two Spotted Spider Mites In Soybean: Scouting, Thresholds, And Insecticide / Miticide Considerations

By Robert Koch and Bruce Potter

Scouting of soybean for soybean aphids and twospotted spider mites should be underway. Some fields are approaching treatable (threshold) levels for soybean aphid. If drought conditions persist, spider mite outbreaks could occur in some areas. 

Soybean aphid. Photo: Bob Koch

However, do not just assume you have significant infestations of either or both of these pests. This year, infestations of these pests are variable across the state, due to variable plant development and weather conditions. In addition, populations of both pests can change (increase or decrease) very rapidly. This is why scouting is particularly important. Below, we provide an overview of scouting and thresholds for these pests in soybean and a table listing the insecticides/miticides available for management of these pests, along with notes to help determine which products to use for each pest alone or in combination.

Soybean aphids

To determine when to apply insecticides for soybean aphid, count or estimate the number of aphids on plants selected throughout the field. Relate the average number of aphids per plant for that field to the economic threshold of an average of 250 aphids per plant with more than 80 percent of the plants infested with aphids, and the aphid population appears to be increasing. Please recall that the economic threshold is the trigger point for scheduling an insecticide application to prevent populations from reaching an economically damaging level, which is called the economic injury level.

For scouting, you should pay particular attention to fields that were treated prophylactically with an insecticide mixed in with a fungicide or herbicide, because that insecticide might have eliminated the natural enemies (predators and parasitic wasps) which could allow populations of later colonizing aphids to increase rapidly. Also keep in mind that as the percentage of plants infested with aphids in a field increase from about 50% to 100%, the number of aphids per plant tends to increase more rapidly.

Management of soybean aphids is complicated because many populations of soybean aphid are resistant to pyrethroid insecticides. In addition, crop tolerances for chlorpyrifos (e.g., Lorsban, Cobalt, etc.) have been revoked, so products containing this insecticide cannot be used any longer for this pest. The table below summarizes the insecticide options available for management of soybean aphid.

Twospotted spider mites

To determine when to apply miticides/insecticide for twospotted spider mites, begin scouting on the edges of fields. If the mites or the symptoms of mite infestation (stippling, yellow or death of leave) are found on the edges, start sampling field interiors by inspecting multiple plants spread throughout the field. When active infestations (mites still present) are found, use the following rating scale to determine if a miticide/insecticide is needed. Implementation of this rating scale focuses on protecting the upper two-thirds of the plant canopy, with the action threshold being at level 3.

Rating scale for twospotted spider mites in soybean:

0. No spider mites or injury observed.
1. Minor stippling on lower leaves, no premature yellowing observed.
2. Stippling common on lower leaves, small areas on scattered plants with yellowing.
3. Heavy stippling on lower leaves with some stippling progressing into middle canopy. Mites present in middle canopy with scattered colonies in upper canopy. Lower leaf yellowing common and some lower leaf loss. (Spray threshold)
4. Lower leaf yellowing readily apparent. Leaf drop common. Stippling, webbing and mites common in middle canopy. Mites and minor stippling present in upper canopy. (Economic loss)
5. Lower leaf loss common, yellowing or browning moving up plant into middle canopy, stippling and distortion of upper leaves common. Mites present in high levels in middle and lower canopy.

Although twospotted spider mite infestations can sometimes be managed with application of insecticide to just the field borders, such treatments require careful scouting to ensure the field interior has not been infested.

Management of twospotted spider mites is complicated by the inability to use chlorpyrifos-containing products. Also, recall that most pyrethroid insecticides (except bifenthrin) should not be used against these mites because they can flare (increase) the infestation. The table below summarizes the insecticide options available for management of twospotted spider mites.

Considerations for combined infestations

Some fields have infestations of both pests. Here are some considerations for such situations:

1. Which is your current problem: soybean aphids or twospotted spider mites? Make your choice for insecticide/miticide based on that.
2. If one of these pests is at an economic/action threshold and the other species is present but below action levels, choose a product that is least likely to flare (make worse) the secondary pest infestation.
3. Do not skimp on insecticide/miticide rates. Some premixes contain lower rates of one or both insecticides compared to products with just one of the insecticides. Bifenthrin has continued to be effective against spider mites in Minnesota, but lower rates in some premixes may be below that needed for consistent mite control.
4. Some insecticides may not be plentiful and as easy to obtain as in previous years. Plan ahead and don't waste insecticide on non-problems.

Table 1. Insecticides/miticides available for management of soybean aphids and twospotted spider mites. This list of products was compiled primarily from NDSU and Purdue extension publications. Insecticide group number refers to resistance management classification where cross-resistance is likely if resistance develops to one insecticide in the group (e.g., avoid using a 1A after applying a 1B insecticide).

To hear the July 27 Field Notes podcast with Bob Koch and Bruce Potter, visit Getting ready for