Soybean aphid populations remain at low densities in monitored fields across the state, but the known distribution for 2006 now covers most of Iowa. On June 19, Extension field crops specialists reported soybean aphids from northeastern (Allamakee, Clayton, Winneshiek counties), southeastern (Lee County), central (Story County), and northwestern Iowa (Cherokee County). Aphid densities were less than one aphid per plant in all counties. In the June 5 newsletter, we reported an aphid colony containing 40 insects, but two weeks later when 80 plants were inspected in the same field, only a single plant was found with only four aphids on it.
Cool spring temperatures and dry conditions are both considered factors that would benefit soybean aphid survival and reproduction. In order for these conditions to produce large soybean aphid populations, two events must occur: aphids must first successfully arrive, and those aphids must escape predation. The movement of aphids from buckthorn to soybeans has already begun; however, it is not yet clear the intensity of this immigration. The soybean aphid suction trap network has been activated for the 2006 growing season. These traps will give us a relative measure of the abundance of winged aphids across the North Central Region, including Iowa. Beginning in July, visit the Regional Soybean Aphid Suction Trap Network for weekly updates from the four traps deployed in Iowa as well as those in neighboring states.
As important as estimating aphid immigration rates for estimating the potential risk from soybean aphids, we must still consider the potential of aphid survival once it reaches a soybean field. Although soybean aphids have yet to colonize most (i.e., 99.9%) of the plants within a soybean field, their longevity on those plants is not guaranteed. There are several predators that will feed on them including spiders, daddy-longlegs, and insidious flower bugs. During the early part of the growing season, this community of predators can keep up with the aphids' reproduction and limit their population growth. However, if the aphid immigration rate increases, and the number of plants colonized and the number per plant increases, then these predators are unlikely to maintain soybean aphids below the economic threshold.
The soybean aphid has arrived in many parts of Iowa, but the current populations are very low and do not require insecticide treatments. The extent to which the very low populations will result in economic outbreaks later in the season is not yet clear. We will continue to update you on the soybean aphid population expansion throughout the summer.
Soybean aphids will colonize the upper stem and newly developing leaves on soybeans. (Marlin E. Rice)
This article originally appeared on page 179 of the IC-496(16) -- June 19, 2006 issue.