Farm Progress

Soybean Source: If hot dry weather persists, spider mite populations may continue to build.

Clarke McGrath

July 20, 2017

6 Min Read
SPIDER MITES: Typically this insect starts to become a problem at field edges and can disperse with wind to quickly develop a field-wide infestation. Field scouting is necessary to detect early outbreaks and for effective early treatments and control.

An article I wrote a month or so ago hit on a lot of pests to watch for in soybeans, focusing on the more common ones. I didn’t spend much time on pests that can hit when hot summer temps combine with extended stretches of dry weather; we were getting good moisture at the time.

With that dynamic changing in much of the state, it’s time to share a “heads up” and get out in front of dry weather issues. We are getting early reports of spider mites in parts of Iowa. Maybe by talking about dry weather and spider mites, they’ll both go away for the rest of the summer, and you can ask, “Whatever happened with the dry weather and spider mites in that article I read?”

The two-spotted spider mite can be a problem in soybean fields, and sometimes in cornfields, in hot and dry conditions. Mid-July through August is typically when they hit economically damaging levels, since they seem to do so well as daily high temps rise above the mid-80s if combined with a stretch of dry weather and lower-than-normal humidity.

Scouting for spider mites
We typically see this insect start to become a problem at field edges, and they can disperse with wind to develop a field-wide infestation pretty quickly. ISU recommendations start with checking the edge rows first to see if mites can be found. If you find them, you’ll want to estimate populations throughout the field by walking a "Z" or "W" pattern.

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TINY TROUBLEMAKERS: Spider mite populations can explode if conditions are favorable. The entire life cycle can be completed in five to 14 days; mites can produce multiple generations per year. 

Early symptoms of two-spotted spider mite injury are little yellow dots (often called stipling) on the lower leaves of the plants. The mites typically start their feeding at the bottom of soybean plants and move up the plant after wrecking the lower leaves.

Since they pierce bean leaves and draw plant sap out, the damage they do to the soybean leaves exacerbates water loss and plant stress under hot and dry conditions.

If left unmanaged, infested leaves turn yellow, then brown, and eventually can die and fall from the plant; it isn’t a pretty sight. Sometimes we can find webbing on the edges and underside of leaves, but I haven’t found that to always be a reliable and timely scouting key. Mites can reduce soybean yield by 40% to 60% when left untreated; drought-stressed plants could experience even more yield loss. Shatter loss can also increase significantly in fields that had spider mite pressure.

To spray or not to spray?
If you see spider mites in your field, should you spray an insecticide? I wish I could give a solid set of criteria, but spider mite treatment economics depend on a lot of dynamic factors.

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LEAF DAMAGE: Spider mites pierce plant leaves and feed on the juices. Mites suck from the underside of soybean leaves, removing moisture and nutrients.

ISU Extension entomologists say "the decision to treat should take into consideration how long the field has been infested, mite density including eggs, mite location on the plant, moisture conditions and plant appearance. A general guideline for soybeans is to treat the crop between R1 to R5 stage of growth (bloom through beginning seed set), when most plants have mites, and if there is heavy stipling and leaf discoloration apparent on lower leaves."

Having fought spider mites a few times, here are some things to think about if we have to tackle them this season:
• We don’t have a lot of variety in chemistry to battle spider mites. Organophosphates are the primary insecticides for two-spotted spider mite control (dimethoate and chlorpyrifos), with only one pyrethroid (bifenthrin) that is very effective.
• Don’t treat too early or try to do a “preventive” treatment for spider mites. By eliminating natural enemies that can help suppress spider mite populations, these types of applications could cause more problems.
• Insecticides may not kill the eggs, so scout your fields five to seven days after applications to determine if a second application is necessary.
• If we have to make a second application, if at all possible, rotate to the other class of chemistry.

For the most part, the insecticides need direct contact to kill mites. With the little critters spending most of their time on the underside of soybean leaves, coverage is key to getting the best results. By coverage, I mean a lot of water — like 20 gallons per acre through a ground rig. Sure, there are cases where 15 gallons per acre or whatever has done the job. On the other hand, hauling water to a ground rig and loading a few extra times is a good trade-off for better control and reducing the odds of a second application. So I rarely will back off that 20 GPA recommendation.

The finer points of spraying spider mites
When it comes to aerial applications, it gets more “complicated.” A lot of experts recommend 5 GPA through a helicopter or airplane, and that would be ideal. Getting pilots to run (OK, fly) 5 GPA isn’t an easy sell, and can mean higher application fees. I’ve seen good results at lower aerial GPA, but I’ve also seen fields with lower GPA need a second treatment a few times where others in the area at higher GPAs didn’t. How many GPA to run via air often comes down to what your chemical supplier, the pilot and you can all agree upon and see as the best risk-reward scenario — and there isn’t always much flexibility.

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SCOUTING: Look on the underside of affected soybean leaves for mites, eggs and webbing. Mites are tiny and almost impossible to see with the naked eye. 

I used to think maybe we could scout field margins and spray them earlier to try to avoid spraying the entire field. It only worked a few times in years where we didn’t end up spraying a lot of fields for spider mites, since pressure was low and treating was questionable anyway. If you have a significant spider mite outbreak, and they are in your field borders, I’d recommend treating the entire field.

Watch and follow the preharvest intervals that are spelled out on the insecticide labels. These label restrictions specify the number of days after spraying that you must wait before you can legally harvest the beans.

McGrath is the On-Farm Research and Extension coordinator for the Iowa Soybean Research Center at ISU. Contact him at [email protected].

 

About the Author(s)

Clarke McGrath

Clarke McGrath is the On-Farm Research and Extension coordinator for the Iowa Soybean Research Center at ISU.

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