Soybean aphid and two-spotted spider mite management considerations

Figure 1. Two-spotted spider mites (red arrows) and eggs (blue arrows) on a soybean leaf. Note the 'stippling' injury symptoms on the leaf which result when a spider mite sucks the green contents out of individual cells when feeding. Stippled cells will no longer contribute to the plant's growth and development. Photo: Angie Peltier, UMN Extension

Spider mites

Two-spotted spider mites (Figure 1) are currently being observed in some northwest Minnesota soybean fields that missed some of this summer's rains and are suffering from water stress. This is because the vegetation on field edges on which the spider mites begin the growing season feeding, decreases in quality as the summer progressed and dry conditions set up. Initial spider mite infestations in soybean fields begin on field edges. 

Spider mites suck the contents out of individual plant cells as they feed leading to an injury symptom called 'stippling' (Figure 1).  Symptoms of severe spider mite infestations, in which spider mites have moved into the upper crop canopy to feed, can be observed at highway speed as heavily stippled plants have a duller, more matte appearance than uninfested plants. However, the crop will have lost significant yield potential if an infestation progresses to this point, and scouting and treating the field is recommended when treatment thresholds, which occur before highway-visible spider mite infestations,  have been reached (see below).

Spider mite infestation scale: 0 to 5

This spider mite infestation scale, which was developed in the later 1980's by UMN Extension IPM specialist Bruce Potter and Extension corn entomologist Ken Ostlie, is designed to both delay a treatment until there is a real risk of lost yield potential, and yet not too late so that yield potential due to a spider mite infestation has been reached:

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 with yellowing on scattered plants.
3: Spray threshold: Heavy stippling on lower leaves with some stippling progressing into the middle canopy. Mites present in the middle canopy, with scattered colonies in the upper canopy. Lower leaf yellowing is common, and there’s some lower leaf loss.
4: Economic loss: Lower leaf yellowing is readily apparent. Leaf drop is common. In the middle canopy, stippling, webbing, and mites are common. Mites and minor stippling present in the upper canopy.
5: Lower leaf loss is common, with yellowing or browning moving up the plant into the middle canopy. Stippling and distortion of the upper leaves are common. Mites are present in high levels in the middle and lower canopy.

There are several active ingredients with miticidal properties, but several insecticide active ingredients used to treat soybean aphid (SBA) and other insects can actually flare spider mite infestations and so caution when selecting a pesticide is warranted. Specifically, the neonicotinoid active ingredients thiamethoxam and imidacloprid and pyrethroid insecticides other than bifenthrin can flare two-spotted spider mite (TSSM) populations. Additionally, some active ingredients only have activity against either TSSM adults or eggs and so scouting fields after an application to see if an additional treatment with a different active ingredient may be warranted is recommended.  

Visit this webpage with an article from 2021 to find a table with various pesticides labeled for TSSM.

Soybean aphid

Although we have not had problems with SBA in the last couple of years, treatment threshold-level SBA infestations are now being observed in some areas of northwest Minnesota; SBA growth and development is favored by the more moderate temperatures that we have had recently.  As a reminder, similar to treatment thresholds for TSSMs, SBA treatment thresholds are designed to occur before the point in the infestation when population densities have risen enough to be yield limiting. Briefly, there are three elements that must be present for the treatment threshold to have been reached:

• An average of 250 soybean aphids per plant 
• More than 80 percent of plants are infested with soybean aphids
• Soybean aphid populations are rising

Be sure and scout whole plants (upper and lower leaf surfaces and stems) from multiple areas of the field, estimating the total number of SBA per plant, calculating a field-wide average. Re-scouting the field several days later can help to determine that the population density is increasing. This is an important step because there are several natural enemies of the SBA in Minnesota, including the Asian lady beetle (and their larvae) and parasitic wasps that can help to keep SBA populations in check. Additionally, if there is a large number of winged SBA present, they may be getting ready to move on to another field. 

Pyrethroid insecticides    

Several conversations with northwest Minnesota crop advisors and farmers alike have led me to begin to worry that some may be sorely disappointed in their level of control provided by their chosen pesticide. It seems that many are considering applying a pesticide with the active ingredient bifenthrin, an active ingredient in the pyrethroid (3A) class of insecticides. While bifenthrin will not flare two-spotted spider mite infestations, there is a good possibility that it will also not manage many of our SBA infestations.

The issue of pyrethroid-resistant soybean aphid may have fallen to 'below the fold' of several ag press publications, but research that was done in the lab of UMN Extension soybean entomologist Bob Koch on pyrethroid-resistant SBA populations, suggests that the resistance genes are likely still prevalent, meaning that those fields sprayed with pyrethroid insecticides including the active ingredients alpha-cypermethrin, beta-cyfluthrin, bifenthrin, cyfluthrin, deltamethrin, esfenvalerate, gamma-cyhalothrin, lambda-cyhalothrin, permethrin and zeta-cypermethrin. 

The survey that informed information regarding conditions on the ground was sponsored by the Minnesota Soybean Research & Promotion Council.