July 17, 2003

 

Monitor For Soybean Aphids

Soybean growers should start monitoring for soybean aphids, according to Phil Glogoza, North Dakota State University Extension entomologist. Counties where aphids have been found include: Richland, LaMoure, Dickey, Steele, Traill, and Grand Forks with more expected.

"Producers should survey field margins, particularly near shelterbelts and other structures," Glogoza says. "Field colonization patterns observed last year and already this year indicate those are areas where aphids settle after their migration. Eventually, the entire field will have aphids but last year it took about five weeks for fields to become generally infested after first detection."

Glogoza suggests growers evaluate individual leaflets on a 0 to 4 scale. Zero would mean no aphids while four would mean finding 100 or more aphids on the leaf. Treatment is advised if the aphid population reaches 3 on the scale, 26 to 99 aphids per leaf.

Glogoza suggests following these guidelines when making treatment decisions:

• Begin scouting soybean fields at the V3 to V4 stage to determine if soybean aphids are present in fields. No treatment is recommended at this time and is discouraged so insecticides do not reduce the presence of predators and parasites.
• The critical growth stage for making most soybean aphid treatment decisions is during the late vegetative to early reproductive stages. (Vn to R2). Assessing aphid populations at this time is critical.
• For best results, aphid treatment should occur between mid July to early August.

Treatment to manage soybean aphid is recommended at early flowering (R1 to R2) when aphids are abundant on most plants. A population of 200 aphids per plant during susceptible growth stages (R2 to R4) can result in a yield loss of 6 bushels per acre. That yield loss is near or above the break-even point for the cost of an insecticide application.

"When spraying for aphids, the objective is to achieve well over 85 percent control," Glogoza says. "Minimizing the number of aphids will give the soybean plant a chance to progress to a growth stage where aphid feeding will have minimal impact on yield.

Best results will be achieved with treatments in the upper range of the label rates, Glogoza notes. Using low rates will increase the risk of leaving enough aphid survivors to result in a rapid increase in the population, especially in the absence of predators and parasites that will be killed by the insecticide treatments. A rapid recovery of the population could result in the need to reapply treatments for adequate protection over the risk window of R1 to R4 growth.

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Source: Phil Glogoza, (701) 231-7581, pglogoza@ndsuext.nodak.edu
Editor: Rich Mattern, (701) 231-6136, richard.mattern@ndsu.nodak.edu