Concerns over soybean aphid continue

The soybean aphid continued to be a dominant insect concern in eastern Iowa during the second week of August. George Cummins, extension crops specialist, has found aphids in Blackhawk, Grundy, Howard, and Tama counties. Mark Carlton, extension crops specialist, confirmed soybean aphid in southeastern Iowa in Mahaska, Monroe, and Wapello counties. He notes that populations have not reached the "severe" level but many fields do have populations of the insect. Jim Fawcett, extension crops specialist, has found fields with fairly high populations (500-1000 aphids per plant) in Linn, Johnson, Iowa, and Benton counties in east central Iowa. Some fields have been sprayed in Linn County. Jim also reports some fields with high populations of beneficial insects. Virgil Schmitt, extension crop specialist, reports many fields in Jones County with aphid populations of 50 insects per leaflet (150 per trifoliate leaf). Jim Baumer, SOYGENETICS, LLC, has found "very light" infestations west of Webster City in Hamilton County, which is the westernmost infestation in Iowa that has been reported to me.

Soybean aphids on soybean stem.

Lady beetle adults and larvae are very abundant in some fields and they will help reduce the aphid population. They probably will be most beneficial in fields with populations that have not reached damaging levels. In fields with noticeable plant stunting and very large populations, the lady beetles probably cannot reduce the aphid population quickly enough to prevent economic damage.

Soybean aphids suck sap from the plant, which drops onto leaves as honeydew. Black sooty mold often grows on the honeydew and discolors the plant.

A major concern is estimating aphid population size and determining an economic threshold or treatment level. Because this is a new pest to the Midwest, no local research addresses this problem. The best that can be done is to develop a nominal threshold, which is a threshold based on the subjective determinations of a person's experience. Before applying an insecticide during August, I would suggest that three criteria be met:

  1. aphid populations are heavy and cover the upper trifoliate leaf on a majority of plants,
  2. lower leaves are covered with honeydew and turning black from sooty mold, and
  3. infested plants appear stunted. If plants also are under stress from dry soil conditions, feeding effects of the aphids could worsen.

Four insecticides are labeled for soybean aphid (or Chinese aphid on some labels):

  • Furadan 4F (1/2 pint, 21-day preharvest interval),
  • Lorsban 4E (1-2 pints, 28-day preharvest interval),
  • Penncap-M (1-3 pints, 20-day preharvest interval), and
  • Warrior (1.92-3.2 ounces, 45-day preharvest interval).

Be certain to consider the preharvest interval before using any of these insecticides. Read and follow all label directions.

This article originally appeared on pages 163-164 of the IC-486(21) -- August 20, 2001 issue.

Updated 08/19/2001 - 1:00pm