Brian Lang, extension field specialists in crops, reported on July 23 that soybean aphid populations had dramatically increased in some Winneshiek County fields. He noted that some plants hosted 5,000-10,000 aphids and these densities could be found throughout the field. Insecticides were being sprayed to control soybean aphids and several chemicals were giving 95 percent control. Herb Eichenseer, research scientist at Pioneer Hi-Bred International, reported small populations of soybean aphids near Johnston on July 18. Mummies of parasitized aphids were found along with eggs of lady beetles and green lacewings on the plants, suggesting that some natural control would help to hold these smaller populations down.
The big question is what to do with fields that have very large populations of aphids. There is no clear answer because of the lack of research on this pest in the United States. Last year, University of Wisconsin entomologists, found that 80 to 100 aphids per leaflet reduced soybean yields by approximately 8 bushels per acre. There are no mid- or late-season economic thresholds. Heavy rains and beneficial insects may reduce large populations slightly, but insecticides will be the only option in achieving a substantial population reduction.
If an insecticide is sprayed, an unsprayed test strip should be left in the field to compare and evaluate against the sprayed sections. The unsprayed test strip is needed to effectively compare the real value of the insecticide treatment and determine its performance. Also, not all insecticides provide equal levels of control. Ted Radcliffe, University of Minnesota entomologist, reports that the soybean aphid appears to rebound from some insecticides. University of Minnesota plots showed rapid increases in soybean aphid numbers after being sprayed with Warrior at St. Paul and with Sevin at Rosemount. Presumably these increased aphid numbers resulted from the suppression of beneficial predators.
Insecticides labeled for soybean aphids include Furdan 4F at the rate at 1/2 pint per acre and Lorsban 4E (stated as Chinese aphid on the label) at 1-2 pints per acre. For Furadan, do not apply the last treatment within 21 days of harvest or Lorsban with 28 days of harvest. University of Illinois entomologist Kevin Steffey reports that Warrior has been approved for a supplemental label for "Chinese aphid" but I have not received a copy of this label.
This article originally appeared on page 156 of the IC-486(20) -- July 30, 2001 issue.