High Plains Journal
09-03-06
Soybean aphids not reducing 2006 Iowa yields
By Jennifer Bremer
The 2006 Iowa soybean crop appears to be avoiding the soybean aphid problems that have been a yield reducing problem the past few years.
On Brian Weber's Tama County farm in east central Iowa, only a few soybean aphids can be found in a field close to his building site.
"There has only been one incident of soybean aphids that I am aware of in this area and after spraying the field, it appears to be taken care of," he says.
Iowa State University assistant professor of entomology, Matt O'Neal says that so far the reports of soybean aphid outbreaks this year have been limited to
Minnesota and parts of the Dakotas.
"There have been a few fields that have needed insecticide applications in Iowa, but mostly in the northern third of the state," he says. "Perhaps the higher temperatures this summer have helped prevent a spread since aphids do not do well in high temperatures."
O'Neal's research is focused on developing ecologically and economically sustainable insect pest management programs for soybeans, with current research being focused on soybean aphids. "Understanding the biology of this pest at landscape scales is critical for effective management within Iowa, as aphid migration can generate infestations hundreds of miles from source populations. Managing soybean aphid may constrain current pest management practices for other soybean pests," he explains.
Weber, of Dysart, farms about 2,000 acres of corn and soybeans with his father and also sells seed corn and beans.
Identify soybean aphid problem early
"Even though a field may not seem to have a problem with soybean aphids today, it can have a problem tomorrow," Weber says.
Soybean aphids get their name by being pests of soybeans, however they survive through the winter on the buckthorn plant. Buckthorn is the common name for a group of woody shrubs found in forest edges, woodlots, roadsides, fencerows, field borders and ornamental plantings. The aphids actually use the

buckthorn to produce two generations prior to the third generation migrating to soybean fields.
"The thing to remember about soybean aphids is that they have a three day life cycle and they are born pregnant, so a plant may have 10 one day and 1,000 the next," he says.
In Iowa aphid outbreaks generally starts in late July and early August. Weber uses the 250 aphid per plant threshold when scouting to determine if a problem is present.
The soybean aphid is a small, greenish-yellow insect with black "tailpipes" or cornicles near the tip of its abdomen. Only the size of a pin head, it feeds on the under side of the soybean plant's leaves.
Treatments need to be applied before lower leaves are covered with honeydew or turn black from sooty mold, and before plants are noticeably stunted.
O'Neal says his research shows that aphid populations of less than 250 per plant on average doesn't seem to cause a significant decrease in soybean yields.
The direct feeding of the aphid on the plant can reduce yields. He says that aphids can also spread viral diseases, but little is known about how much of a yield reduction this causes.
Weber says that 2005 yields in his area were reduced due to soybean aphids, but the worst problem was in 2003 when yields were reduced by as much as 15 bushels per acre.
Treatment is necessary when problems are found
He recently purchased equipment to do preventative seed insecticide treatments for his customers, which he hopes has helped with the soybean aphid problem this year.
Treating affected fields with labeled organophosphates and pyrethroids works well according to O'Neal, but he says timing is the most important step in treating the problem. When a problem is scouted--treatment must be taken.
An analysis of Iowa acres treated for soybean aphids in the past few years shows that in 2003 approximately 4 million acres were treated. That number dropped to 250,000 acres in 2004, but increased to 2 million acres in 2005.
"Although we did not reach insecticide usage levels in 2005 that we saw in 2003, it is remarkable to note that 10 years ago it was estimated that very little if any insecticide was used in U.S. soybean production," says O'Neal.

Other preventative measures include the use of natural predator insects that feed on the aphids, but do not pose a threat to the soybean plant. Lady beetles, green lacewing, and other beneficial insects eat aphids in soybean fields.
Weber says that seed companies are working on identifying varieties that are more tolerable to soybean aphids. In the future he hopes to see soybean aphid resistant varieties as well.
O'Neal adds that right now there is very little the grower can do to prevent an aphid problem. "Insecticides applied to the soybean seed do not provide significant protection late in the season," he says. "There are a number of seed companies investigating the potential for resistant varieties to limit if not completely prevent outbreaks, but it will likely be three to five years before those are available."
Since major outbreaks have been seen in odd years in Iowa, Weber and O'Neal both stress the importance of planning ahead for the coming year's crop.
"Whether it's using more tolerant varieties or treating soybean seed prior to planting, being prepared for an outbreak and preventing the problem is important to the bottom line," stresses Weber.