Posted 26 August 2011. PMN Crop News.
Soybean Aphids Start to Appear in Northern Indiana
Source: Purdue University Press Release. www.agriculture.purdue.edu
West Lafayette, Indiana (August 9, 2011)--Soybean aphids have begun establishing in new growth soybean foliage in northern Indiana, but according to Purdue Extension entomologist Christian Krupke, farmers shouldn't panic.
While there have been reports of a few fields over threshold being treated, aphid populations for the most part are low and are expected to stay that way.
"Aphids have been relatively scarce this year," Krupke said. "Since numbers in the upper Midwest haven't flared, we don't suspect that winged aphids have migrated to Indiana. Those finding aphids are not seeing many with wings, which is a good sign."
The weather in Indiana this summer also has kept aphid populations low. Krupke said the extreme heat isn't helpful for aphid reproduction or survival. While they don't die as a result of heat, he said they don't reproduce very quickly at temperatures above 90 degrees.
But recent rains in some areas and more tolerable temperatures have encouraged new growth in soybeans, Krupke said.
"That new growth is rich in nutrients that favor aphid development," he said. "Soybean fields that are in the early R growth stages, especially in the northern counties, should be scouted soon. Northern counties have historically had the highest aphid pressure."
Farmers scouting soybean fields should examine about 20 plants per field, concentrating on the backsides of new growth in the upper canopy and on the newly developing pods. If there are less than 250 aphids per plant, farmers should scout again a week later.
For fields where soybeans are in growth stages R1 (bloom) through R4 (pod growth), Krupke advises treatment when aphid populations reach 250 per plant. At R5, or seed fill, farmers should treat fields if fields are at 250 aphids per plant and aphid numbers are increasing. By stage R6, or full seed, treatment is necessary only when plants are under drought stress. Treating at below-threshold infestations rarely pays off, since heavy rains, predators and disease can knock down sub-threshold aphid numbers very quickly. And at stages R7-R8, or maturity, no treatment is necessary.
"The vast majority of Indiana soybean fields have virtually no aphids in them," Krupke said. "However, they are out there, and some populations will be increasing and marching toward the magic number of 250 aphids per plant. Farmers don't want to let their fields be among those that are not scouted until they are over threshold. They need to take the time for a quick survey."
A graphic treatment threshold guide by growth stage can be found in the July 29 edition of Purdue Extension's Pest and Crop Newsletter at http://extension.entm.purdue.edu/pestcrop/2011/issue17/index.html#aphid