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Pest severity, application timing key factors in pesticide treatment

Fungicide and insecticides are great tools to thwart soybean diseases and insects. However, payoff hinges upon infestation severity and plant growth stage.

That's what Golden Harvest researchers found when they evaluated fungicides and insecticides to control foliar disease and soybean aphids, respectively.

"We see the same trends that occur for other crop management inputs," says Scott Payne, Golden Harvest research agronomist, Ames, Iowa. "Corn borer control only pays when corn borer is present. Herbicides only pay when weeds are present."

CULTURAL FACTORS FIRST

Before spending a dime on pesticides, make sure you establish good cultural practices. Pesticides are important, whether or not you follow good cultural practices. Still, the following steps can help soybeans surge early in the growing season and withstand future stressors:

- Plant disease-free seed
- Plant resistant and/or tolerant varieties
- Manage residue
- Plant into a favorable seedbed
- Plant at a recommended planting date
- Rotate crops

Seed treatments also can help ensure a good start for plants so they'll be in top shape to face growing season maladies.

PEST TRIALS

In 2005 and 2006, Golden Harvest researchers conducted trials at 10 locations aimed at addressing any type of soybean pest pressure. To test variety interaction, they planted six to 12 varieties at each site.
In these site-years, the only pest pressure incurred was from soybean aphids in two cases.

Researchers applied Warrior at New Richland, Minnesota, in 2005, and also at Sherman, South Dakota, in 2006.
Heavy aphid pressure occurred at the Minnesota site, with over 100 aphids present in the uppermost expanded trifoliate of each plant. Researchers planted eight varieties at this site, ranging in maturity from Group 0.5 to 2.2.

Aphid pressure was lighter and began later in the season at the South Dakota site, with 21 aphids present on the uppermost expanded trifoliate of each plant. The same variety mix was planted.
Soybean treatment occurred at a mid-reproductive stage. The shorter-season soybeans were physiologically more mature than were the fuller-season soybeans at application.

INSECTICIDE PAYS UNDER PRESSURE

Among most varieties, Warrior applications paid hands-down in Minnesota.

"We saw a consistent increase in yield response as maturity increased," says Payne. "The biggest response occurred with the fullest-season varieties (Group 2.2), which were the least mature at the time of application."

In that case, the Warrior application returned over 30 bushels per acre above an untreated control. Meanwhile, just a 3-bushel per acre yield increase occurred with a Group 0.5 bean.

The difference for the gap? "Compared to the shorter-maturing beans, the 2.2's had longer time to recover from the aphid infestation," says Payne.

Results differed under lighter aphid pressure in South Dakota. There, insecticide applications showed no statistical yield benefit, even with long-maturing varieties.

TIMING IS KEY

"The difference in severity and the timing of infestations made a difference," say Payne. "That's typical through the Corn Belt. Where there was a severe aphid infestation, we saw an increased yield with Warrior for all but the shortest season variety. The longer maturing varieties had a longer time to recover yield than the shorter-season varieties."

This makes application timing particularly critical. If aphids reach thresholds early on in the reproductive phase, treatments should also pay on shorter-season varieties.

For example, an earlier application may have had a greater impact for the shorter season varieties in the Minnesota plot. Similarly, treatment costs may outweigh benefits if insecticides are sprayed later, regardless of variety maturity.

"We need to think about what stage that crop is in before spraying," Payne says.

IT ALL COMES BACK TO SCOUTING

The same principle of treating for a problem applies to fungicides. Golden Harvest researchers also evaluated four fungicides and fungicide combinations for foliar disease control in 2006. Test sites included:

- Pekin, Illinois
- Washington, Iowa
- Ames, Iowa
- Elk City, Nebraska

This followed a 2005 study where seven fungicide and fungicide/insecticide mixes were evaluated at the same Nebraska, Illinois, and Iowa sites.

Results for both trials were mixed. In some cases, statistically significant yield increases occurred. However, they were slight compared to the 2005 New Richland, Minnesota, soybean aphid site. And in most cases, the yield of check plots vs. treated plots did not statistically differ.

The key? All treatments were made under low or no insect or disease pressure.

"When we do have pest problems like we did in Minnesota in 2005, we had a good response with Warrior," says Payne. "All this goes back to scouting as a basis for making these types of decisions. Insecticides and fungicides can be great tools, but they have to be used in situations where they fit."

Fungicide and insecticides are great tools to thwart soybean diseases and insects. However, payoff hinges upon infestation severity and plant growth stage.

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