Foliar application best with aphids

Will an insecticide seed treatment control soybean aphid, and is it economically justifiable when a foliar insecticide application may also be needed? Extension entomologists have been conducting research to answer these questions.

In 2008, North Dakota State University Extension entomology evaluated the following treatments at Johnson, Minn., for control of soybean aphid in a large strip trial:

• untreated check

• insecticide seed treatment alone (Cruiser MAXX at 3 fluid ounces per cwt.)

• foliar insecticide alone (Warrior II at 1.28 fluid ounces per acre) applied at the economic threshold, or ET, of 250 aphids per plant in 80% of the field

• combination of seed treatment plus foliar application at ET

Soybean aphids reached ET during the first week of August, and Warrior II insecticide was applied on Aug. 7 by a commercial applicator. Soybean was at R3 to R4 growth stage.

In 2008, aphid invasion occurred in late July, and the population built up very rapidly. For pre-spray aphid counts on Aug. 7, there were no significant differences in aphid densities among treatments.

Key Points

• Foliar tops seed treatment in controlling soybean aphids.

• Scouting and applying insecticide at ET proves to be most economical.

• On 550 acres, savings would total more than $8,000 per year.


For aphid densities at seven days after application (Aug. 14) and at 14 days after application (Aug. 20), foliar sprayed treatments had significantly lower densities than the seed treatment alone or the untreated check. Treatment means on these dates indicated that aphids were well above ET for the untreated check and for the seed treatment alone, while mean aphid counts on the foliar treatment and seed treatment plus foliar were considerably below ET. For yield, the foliar treatment and the seed treatment plus foliar had significantly higher yields than the untreated check and the seed treatment alone.

These data indicate that there was no advantage in using an insecticide seed treatment to control soybean aphids when aphids invade late in the season.

The North Dakota Soybean Council funded this study.

U-M trials

In 2008, Ian MacRae, University of Minnesota Extension entomologist, looked at the economics of an intensive management approach costing $30 to $35 per acre (insecticide seed treatment at $10 to $12 per acre, foliar fungicide at $14 per acre, and foliar insecticide at $10 per acre) versus an IPM approach (treating at ET) costing $16 per acre (scouting costs at $6 per acre, foliar insecticide at $10 per acre).

The difference between the intensive and IPM treatments was at least $15 per acre.

With 550 acres of soybeans, a farmer would save approximately $8,250 if using the IPM approach over the intensive management approach!

These data indicate that a well-timed foliar insecticide applied at ET was the best pest management strategy to optimize control of soybean aphid and prevent unnecessary repeated insecticide applications. The foliar insecticide applied at ET generally had higher yield and was the most economical treatment.

Seed treatments may have some early-season control of aphid infestation only when infestation occurs early in the season (June). However, peak soybean aphid infestations typically occur later in the year (late July or early August) in North Dakota and northwestern Minnesota.

Insecticide seed treatments are effective in controlling other early-season pests of soybeans such as wireworms, seed corn maggots and overwintering bean leaf beetles.

Knodel is an NDSU Extension entomologist. Contact her at janet.knodel@ndsu.edu.

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APHID ATTACK: Soybean aphids feed on the underside of a soybean leaf. NDSU and U-M trials indicate properly timed folair insecticide applications are the most economical way to reduce their population and take the pressure off soybean plants. Photo: P. Beauzay, NDSU, Department of Entomology.

This article published in the February, 2010 edition of DAKOTA FARMER.

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