Risk is often seen as the probability of some future event that is viewed as undesirable and how harmful that event would be if it occurs. It follows that soybean aphids in soybean fields are viewed by soybean producers as an undesirable event. The level of harm incurred by the aphids, however, will depend on their population density and the soybean plant stage.
We have developed the following risk assessment timeline based upon our observations of soybean aphids in Iowa during the past five years. It broadly summarizes when we would expect yield loss to occur during a year when aphid populations develop into economically damaging populations.
Risk to soybeans: None
Several states collect information regarding soybean aphid occurrence and abundance on buckthorn--their overwintering host. It is not yet clear how strongly correlated aphid numbers on buckthorn relate to risk of injury to soybeans. Eliminating buckthorn will not reduce the risk of aphid infestations in soybeans because winged aphids have the potential to migrate long distances on wind currents. Three or four aphid generations occur on buckthorn with winged aphids produced during the spring. These winged aphids then migrate from the buckthorn in search of soybeans.
Risk to soybeans: Small
Soybean aphids may migrate to soybeans shortly after the plants emerge, depending on the region of the state. Once on the plant, these females reproduce asexually (no male required) and as many as 18 generations may occur on soybeans.
Growers, particularly those in extreme northeastern Iowa, are encouraged to begin preliminary scouting during the end of June, although it is unlikely that economically damaging populations will develop during this time. Winged soybean aphids have been found in this area as early as June 5-18. In the rest of Iowa, soybean aphids have been found later with the first occurrence during July 7-19. Insect predators (such as lady beetles) within the soybean field have been shown to suppress early season population growth. Insecticides should not be sprayed as a preventative measure when soybean aphids are not present or below the economic threshold. Insecticides will kill off insect predators, which are of value in helping to suppress small populations of soybean aphids.
Risk to soybeans: High
Aphid populations increase substantially in size during mid- to late July and early August. Weekly scouting of soybeans is recommended beginning in mid-July (or late June in northeastern Iowa). Growers should scout five locations for each 20 acres in a field. Begin by looking for ants or lady beetles on the soybean plant--they are good indicators of the presence of aphids. When aphids are found, estimate the population size per plant. An insecticide application is recommended if the average number of aphids is 250 per plant and the population appears to be increasing in size. Stagnant or declining populations, such as those with parasitic wasps (as indicated by swollen and brown aphid mummies), can occur. These fields should not require an insecticide because the aphid population is declining.
September--OctoberRisk to soybeans: Small to none
Soybeans are at very low risk of economic damage once they reach stage 5.5 (halfway between beginning seed and full seed). Aphid populations will start to decline at this plant stage.
Both male and female winged aphids will fly back to buckthorn where they will mate and lay eggs. The eggs will remain on the stems of buckthorn, hatching the next spring. Fall estimates of these winged aphids will be made using a network of suction traps. Information collected from these traps will be used to estimate the aphid potential for the following year. Watch Iowa State University Web sites for information regarding this soybean aphid trapping network.
This article originally appeared on pages 18-19 of the IC-496 (1) -- January 23, 2006 issue.