Quality time spent in NW MN soybean fields

It’s (obviously) still dry.

The NW Research and Outreach Center (NWROC) in Crookston has collected weather data since 1890. Our drought started in the month of September last year. From September 1, 2020 through July 31, 2021, the NWROC accumulated 5.59 inches of precipitation, or 30% of the 18.78 inch normal for that time period.

A recent tour of NW MN reveals problem areas usually hidden.

A recent tour of soybean fields in northwest Minnesota revealed all sorts of things. This year’s drought is giving us conspicuous indications of many of those things that remain hidden in non-drought years but can have a negative impact on plant growth and development and in the end, yield. SCN infestations that in a typical growing season would not result in plants showing above-ground symptoms in a drought can cause soybean leaflets to orient themselves vertically earlier in the day and may stimulate earlier than normal maturity.  We can also easily pick out field areas that are compacted because plants are more stressed and it is more difficult to get a shovel into the ground than in areas that aren’t compacted. Compaction can significantly limit water infiltration so that even when rain falls, not as much can reach the roots. We can also see considerable variation caused by tillage practices if residue from previous crops is any indication, with crops in more aggressively tilled fields struggling more than crops in fields without as much tillage.  

Not all soybean pests have six legs.

Two-spotted spider mite (TSSM) ID.

TSSM are very small (1/50^th of an inch in length), eight-legged pests that are yellow-orange colored and have a dark spot on either side of their body (Figure 1). TSSM are difficult to see without magnification and so either bring along a magnifying glass or hand lens or have a smart phone camera handy to zoom in on the underside of leaves to see them. Alternatively, one can try and shake some loose (literally) from their meal source by placing a piece of copy paper on the ground under an infested plant and shaking the plant.

Figure 1. Two-spotted spider mites (red arrows) and their eggs (blue arrows) on the underside of a soybean leaf.

How they get into soybeans.

Mites overwinter in perennial plants, like the grasses along field borders. Drought years favor TSSM reproduction, and they use air currents and the web material that they produce to hitch a ride into soybean fields. Because they are coming in from the field edge, infestations start there, progressing further into the field as favorable conditions persist. Infestations typically begin on the lower leaves, progressing into the middle and upper canopy as conditions favorable for reproduction continue.

Mites feed by piercing individual cells and feeding on the cell contents. Leaf tissue that mites have fed on has a ‘stippled’ appearance, with individual lighter colored cells next to typically green colored cells (Figure 2). 

Figure 2. Stippling symptoms caused by the feeding of two-spotted spider mites on soybean.

As feeding continues, the affected leaf tissue can begin to turn a reddish-brown color (Figure 3) and whole leaves may eventually drop from the plant (Figure 4). 

Figure 3. Reddish-brown foliar discoloration caused by two-spotted spider mites feeding on soybean leaves.

Figure 4. Soybean plants that have lost leaves due to a severe two-spotted spider mite infestation.

In multiple fields we observed large patches of plants with severe enough infestations that they were dropping leaves. These patches weren’t always along field edges, with several snaking through the middle of  what appeared to be lower-lying field areas where perhaps SCN infestations concentrated or heavy, pooled water compacted soil (Figure 5).

Figure 5. A severe two-spotted spider mite infestation running through the middle of a soybean field in NW MN in 2021.

Making a treatment decision.

Treatment thresholds are geared toward protecting the leaves in the middle and upper canopy from economic injury occurs. The decision about whether or not to treat for spider mites this year depends on a producers’ risk tolerance and whether they think that they have enough of a crop to protect should rain fall. UMN Extension IPM specialist Bruce Potter and Extension entomologists Bob Koch and Ken Ostlie recently wrote an article about spider mites. In this article they present a ratings scale that can help people to make treatment decisions and information about pesticide active ingredients (ai’s) that Minnesota TSSM populations may be to and those ai’s that can actually cause TSSM populations to flare.

Potter, Koch and Ostlie were also featured discussing TSSM and other pests common in Minnesota this year in the August 4 Strategic Farming: Field Notes podcast.