For the 2015 season, perhaps the most troublesome pest for many soybean growers to this point has been soybean aphids. Although not dispersed throughout the entire Midwest, many fields from Wisconsin to South Dakota, northern Iowa, and Kansas have been sprayed for this pest. The question for the rest of the Midwest is: ‘if I haven’t had them, will I get them?’.
Soybean aphids do not overwinter in row crop fields but lay their eggs only on their alternate host: buckthorn, which grows as a large bush or small tree. Although there are many buckthorn species, those which are host to soybean aphids are found across the northern portion of the Cornbelt. With a limited distribution of buckthorn in the Midwest, every seasonal infestation must originate from areas where it grows. This typically means that infestations will begin in southern Minnesota or Wisconsin or in northern Illinois, Iowa, or Indiana. While infestations have occasionally originated from locations further to the south, it is possible that lady bugs and other beneficial insects make it difficult for those overwintering populations to survive or rapidly expand as spring turns into summer.
When aphids eggs hatch in the spring they begin feeding on buckthorn and eventually migrate to soybeans. Once a population is established in soybeans, whether it moves, and how far it will move depends on environmental conditions. Under optimal growing conditions of moderate temperatures and adequate but not excessive rainfall, populations can explode, doubling in size in 4 to 6 days. When aphids become ‘crowded’, individuals develop wings and are primed for flight, and will move on wind currents to adjacent fields, or, if surface winds are strong enough, to adjacent states. Since most infestations this season have occurred from Kansas and South Dakota across to Wisconsin, we would do well to keep an eye on fields to the south of that line, with the strong northwesterly winds being likely to move the unwelcome aphids greater distances. Although we have only a couple of weeks remaining for the soybeans to finish out, we should never discount the potential damage the aphids can do to our soybean crop. When scouting a soybean crop look for aphids in low spots and similar areas where these pests may have may have settled out as the winds died down.