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Pest severity, application timing key factors in pesticide treatment

Agriculture.com Staff 02/19/2007 @ 8:25am

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.

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