Seasonal Timeline for Assessing Risk of Soybean Aphid

March-May

Risk to soybeans: None
Several states collect information regarding soybean aphid occurrence and abundance on buckthornâ"€their overwintering host. It is not clear yet 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. Several aphid generations occur on buckthorn with winged aphids produced throughout the spring. These winged aphids then migrated to soybeans in the spring.

June

Risk to soybeans: Small
Soybean aphids will migrate to soybeans shortly after the plants emerge. Once on the plant, these females reproduce asexually (no male required). As many as 18 generations may occur on soybeans.

Growers are encouraged to begin preliminary scouting during the end of June, although it is unlikely that economically damaging populations will develop during this time. Insect predators (such as lady beetles) within the soybean field have been shown to suppress early season population growth. Insecticides should not be sprayed prophylactically, below-threshold insecticide treatments would remove these predators and possibly exacerbate later aphid populations.

July-August

Risk to soybeans: High
Soybean aphid populations have traditionally increased substantially in size during mid to late July and early August. Weekly scouting of soybeans is recommended. Growers should scout five locations for each 20 acres in a field. Also, look 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 dead aphid mummies), can occur and do not require an immediate insecticide.

September-October

Risk to soybeans: Small to None
As soybeans reach maturity, the aphid population will start to decline. 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 ISU websites for information regarding this soybean aphid trapping network.