Farm Horizons, Feb. 2002

What is eating my soybeans?

By Myron Oftedahl, CCA

A very small pest called the soybean aphid is rapidly invading the United States, the Midwest in particular.

The soybean aphid was first identified in Wisconsin, Illinois, and Michigan in 2000. Fields in southeastern Minnesota were identified in July 2001 with sever infestations.

By September, soybean aphids had spread and been identified in almost every county in Minnesota.

The soybean aphid is very small, about 1/16 of an inch long or less, a pale yellow color, and is the only aphid that will colonize on soybeans. The aphid is native to China, where the soybean is also native.

Crop damage is due to the aphids piercing the soybean tissue and sucking the sap from the plant. Another concern is that the aphids are capable of transmitting several viruses, which can cause additional losses.

The soybean aphid life cycle is rather complicated, although fascinating. It overwinters as an egg on a woody shrub called buckthorn. (I am told the easiest way to identify buckthorn is that it is the last tree/shrub to lose its green leaves in the fall and that it is very common.)

This egg hatches, and the aphid will produce three generations on buckthorn before moving to the soybean plant. All generations of the aphid are females until the fall, when the aphid moves back to buckthorn to lay eggs for the winter.

The summer females grow quickly and are capable of bearing offspring within seven days. They can bear approximately 15 to 18 generations per season. Populations can double in two to three days.

When the plant gets crowded from too many aphids, or the plant becomes stressed for moisture or disease, or possibly Soybean Cyst Nematode, the aphids will produce the next generations with wings so that future generations will transfer to other plants/fields.

Very limited research has been done to establish economic thresholds for soybean aphid. Sever crop damage is capable, with yield losses often ranging up to 50 percent loss.

Insecticide treatments are an option. Current suggestions are to scout fields often and to consider treatment when populations approach 200 or more aphids per plant. Speculation is that this should be about mid-July, so it may be possible to mix the insecticide treatment with the second spraying of Roundup on Roundup-ready soybeans.

If you have questions this summer, contact your chemical dealer, crop consultant, or extension agent.

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