August 28, 2003

 

Field Trial at Prosper Research Farm on Aphid Economic Threshold

A field trial at a North Dakota State University research farm near Prosper will help determine, for the region’s farmers, the economic threshold for aphid spraying in soybeans, according to Phillip Glogoza, North Dakota State University entomologist. Similar trials are being conducted at several locations in Minnesota and Wisconsin.

"We are still gaining insight into an economic threshold for the region that addresses possible interactions with aphid population and soybean growth stage," Glogoza says. "Hopefully these field trials will provide insight into thresholds for next year. We are looking at various aphid populations and their impact on different growth stages."

The widely accepted minimum economic threshold for soybean aphid is 250 per plant with 80 percent of the plants infected at beginning bloom (R1) to full bloom (R2) stages. Some field trials in Minnesota last year demonstrated yield losses when aphid infestations occurred during the full bloom to full pod stages (R4), even with population counts below 250 aphids per plant.

Field trials conducted in Wisconsin during 2001 and 2002 suggest a threshold of 250 aphids per plant at full bloom, 1,000 or more aphids per plant at the beginning pod stage and 1,500 plus per plant at full pod.

"So the question is, which threshold to use?" Glogoza says. "We need more research to verify any numbers under a range of growing conditions, environments, soybean varieties and control tactics."

For now, Glogoza suggests producers stick with the 250 aphids or more with 80 percent of the plants infested and growth stages are R1 to R4, but be concerned about later-planted fields that still may be in mid-stages of growth.

"Last year the populations started increasing at the beginning pod stage, much like this year," Glogoza says. "Because of the dramatic increase, the response was to widen the risk window to include the beginning and full pod stages. But the jury is still out on the threshold and growth stage interactions, particularly when we reach growth stages where seed filling is completed."

Treating fields that are in the advanced growth stages may not produce a significant economic return. "This situation may be very similar to the cereal aphids in heading wheat that we experience," Glogoza says. "Yes, there may be a slight yield advantage in treating a field but is the potential yield increase enough to justify the cost?

Aphid populations have been increasing, especially in eastern North Dakota and Minnesota. However, Glogoza feels there is a high likelihood that aphids will soon be abandoning fields, which would suggest minimal yield impact. "Many of the aphid nymphs are developing wing buds which means they will be taking flight soon," Glogoza says. "Many aphids you see today may be gone in a few days. It may be that a good rain would do more good than an insecticide treatment in the vast majority of these fields."

At the Prosper research farm, Glogoza treated aphids on a weekly basis for four weeks. He will compare the data from that farm with the other field trials in Minnesota and Wisconsin.

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