The two-spotted spider mite can be a problem in soybean fields, and sometimes in corn fields, in drought years. "We recommend scouting corn and soybean fields for mite infestations this year because this insect can thrive in hot, dry conditions," says Clarke McGrath, Iowa State University Extension field agronomist at Harlan in western Iowa.
The tiny pest gathers at field edges, then can disperse with wind to develop a field-wide infestation. ISU agronomists recommend checking the edge rows first to see if mites can be found. If their presence is confirmed, then you need to estimate the populations throughout the field by walking a "Z" or "W" pattern.
Depending on what you find, you may need to spray an insecticide. McGrath says you should work with your area ISU Extension field agronomist, local crop consultant or ag chemical dealer for help with treatment decisions. McGrath writes a column, called "Corn-Soybean Insight" each month in Wallaces Farmer magazine.
Keep an eye on spider mites on soybeans, as mite populations can explode
McGrath has spent a lot of time the past two weeks checking fields and watching spider mite colonies increase. "We've picked out some sentinel plots in the area to do our spot checks, and growers and dealers are watching as well," he says. "Levels of infestation are generally still below what we would consider treating in many areas so far. If the weather continues to stay hot and dry, levels can blow up quickly. We plan on checking the sentinel plots every couple days."
What is the economic damage threshold you should use for deciding when it pays to treat for this pest? Treatment thresholds for spider mites do not exist, notes McGrath.
He points out that 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."
~~~PAGE_BREAK_HERE~~~Spider mites thrive in hot, dry weather, and they pierce the bean leaves and draw plant sap out, McGrath explains. The damage to the soybean plant leaves causes more water loss and plant stress; yield losses of 40% to 50% or more are possible. Shatter loss can increase significantly in fields that had spider mite pressure.
Deciding when to spray with an insecticide—guidelines to use
If you see spider mites present in your field, should you spray with an insecticide? "Farmers need to understand that yield potential is hard to predict and it is dropping right now due to the drought, and that crop insurance implications complicate this even more," says McGrath. "You need to work with your local ISU Extension field agronomist and your local retail agronomist or crop consultant if you want more input on decisions."
He adds, "I spent the summer of 1988 as a school kid/crop scout at Terra Industries in Lenox and I learned from the best in the business. We have fought spider mites several more times while I was with Agriland FS and now with ISU. I have learned a few lessons about controlling spider mites." The lessons include:
* You have limited insecticide choices for spider mite control -- chlorpyrifos (Lorsban Advanced, Yuma, NuFos, Pilot, etc.), dimethoate (Dimate), bifenthrin (FanFare,
Brigade, Sniper, etc.) and a zeta-cypermethrin/bifenthrin mixture (Hero). There are a few combo products (like Cobalt) and a few more trade names, but the bottom line is that these are it in soybeans.
All have some pros and cons, but with proper application rates, timing and good coverage these products should all knock the spider mite populations down for around a week or so in your field, depending on a number of other factors. Always keep an eye on treated fields starting about day five to six after your spray -- watch for another flush of spider mites to possibly develop.
Also, watch the preharvest intervals that are required. The interval is spelled out on the insecticide label. These label restrictions specify the number of days after spraying that you must wait before you can legally harvest the beans.
* Probably around $18 to $24 per acre is the cost to treat for spider mites, depending on application rates and whether you use a ground rig sprayer versus an airplane versus a helicopter.
~~~PAGE_BREAK_HERE~~~"Personally I have no preference if any of these application methods are applying an adequate carrier volume to get good coverage--which is critical," says McGrath. "Yes, aerial application costs more money (application with a ground rig is around $7 per acre, aerial is $10 to $13 per acre) but ground rigs run down the beans, which can cost a couple bushels per acre (or more depending on row spacing, number of tracks, weather, etc.). However, planes and helicopters can't always get the corners, next to tree lines, terraces, power lines. So you need to choose which application method fits your field and timing."
* What about applying fungicide/miticide tank mixes? McGrath has been hearing of some of those being applied. The idea is that since "we are making the trip across the field anyway, we might as well mix the insecticide and the fungicide together and apply both in the same trip."
The ISU Extension plant pathology specialists at Ames can address this tank mix practice, but McGrath is not so sure that tossing in a fungicide is a good idea "given our lack of disease or disease conducive conditions this summer."
* Can you scout field margins early? McGrath has tried to scout and treat field margins early and avoid treating whole fields for spider mites for his customers. He's tried that a few times—each time unsuccessfully—"and we ended up coming back and treating the entire fields a week or so later anyway," he says.
* Don't treat too early. "We made that mistake a few times and had to apply the insecticide treatment again," says McGrath.
* Do you get any residual control from these products? The residual control after you apply spider-mite insecticide products depends on the application coverage, the weather and a few other variables. "We tell farmers and crop consultants to start checking fields for spider mite population growth again at around five to six days after treating," says McGrath. "And keep watching until you treat again or when beans reach the later reproductive or "R" growth stages, potentially through R5 growth stage. In 1988 we had bean fields that needed two treatments of insecticide. If there is a second trip, you need to change to an insecticide product that has a different mode of action."
* Scout, scout and scout some more. "Some areas are still worth protecting if spider mites hit," says McGrath. "Don't give up." For more information on managing spider mites, go to U Minn TSM Info.