Soybean Aphids More Unpredictable Than Usual

By Jane Fyksen Crops Editor
Friday, August 1, 2008 9:38 AM CDT

Growers are strongly advised to scout their soybeans for aphids. This pest is apt to be unpredictable this year, according to soybean experts.

UW-Madison entomologist Eileen Cullen reported late last week that while, overall, populations are still below economic threshold in most of Wisconsin, increasing populations are reminders “to remain alert and scout.”

“You may also be finding winged forms on plants as soybean aphids enter their summer migration phase,” she noted.

Cullen said that in some areas such as Dodge and Sauk counties, the increase was from near-zero activity to field averages of only 10 aphids per plant and 30 to 60 aphids per plant. “In other areas however, west central and east central Wisconsin for example, higher populations are starting to be reported closer to ‘economic threshold’ in a few fields,” she added.

Krista Hamilton, entomologist at the Wisconsin Department of Agriculture, Trade and Consumer Protection, reported late last week that a statewide survey of soybean fields was underway. Preliminary results from 122 fields in the southern and central crop-reporting districts indicated aphid densities were mostly low, with significant populations encountered in only four fields in Adams, Fond du Lac, Lafayette and Racine counties.

Hamilton suggested growers assess population densities and the rate of aphid build-up while most soybean fields are in the early reproductive stages of growth. “Chemical treatments are most effective in controlling aphids and minimizing their resurgence when applied during the R2 to R4 (full bloom to full pod) stages. Regular visits to fields may be required to determine if aphid colonies are actively increasing,” she said.

Densities per 20 plants examined by DATCP ranged from: 1 to 250 aphids on 25 to 90 percent of the plants in the southwest district; 1 to 166 aphids on 25 to 100 percent of plants in the south central district; 1 to 325 aphids on 20 to 85 percent of plants in the southeast district; 1 to 330 aphids on 100 percent of the plants in the central district; and 1 to 318 aphids on 30 to 100 percent of plants in the east central district. The highest population last week was 250 to 330 aphids per plant on 25 percent of the plants in Adams County.

“Several of the fields surveyed in Adams, Columbia, Dodge, Fond du Lac, Juneau, Kenosha, Ozaukee, Racine, Rock, Sheboygan, Walworth and Winnebago counties were 100 percent infested with moderate densities of aphids, and some of these may develop economic populations within the next two weeks,” warned Hamilton.

She added that "white dwarfs" producers may see are a distinct morphological form of soybean aphid thought to develop in response to physiological stress. These tiny, white aphids should be included in population estimates, she advised.

Lee Milligan, St. Croix County UW-Extension ag agent, reported that a number of soybean fields in the western portion of his county were already sprayed for soybean aphids in the last couple of weeks.

The soybean aphid “economic threshold” is 250 aphids per plant. This threshold is based on a field average from 20 to 30 plants collected from throughout a soybean field. The “economic threshold” of 250 aphids per plant incorporates a seven-day lead time between treatment action and preventable yield loss.

Milligan said research in six from 19 yield-loss experiments conducted over three years found no detectable yield differences attributed to treating soybean aphids at numbers below the “economic threshold” of 250 per plant. Treating soybean aphids below the “economic threshold” increases risk to producers by treating an aphid population that is growing too slowly to exceed the “economic injury level” in seven days, eliminates generalist predator insects, and exposes a larger portion of the soybean aphid population to selection by insecticides, which could lead to development of insecticide resistance

The best control occurs between the R1 (beginning bloom) and R5 (full pod) stage. At R6 and beyond it is too late to protect yield and treatment isn’t recommended, stated Milligan.

Kevin Steffey with the University of Illinois said, “Finding a significant number of aphids on a few plants in the field should qualify that field for more intense scrutiny.”

“The increase in numbers of soybean aphids in soybean fields this year is occurring later than usual. But then, what hasn't occurred later than usual this year?” he quipped.

Normally, this would not be a year for great concern about major outbreaks of soybean aphids, so fields should be carefully and continuously scouted, agreed Don Guy, field development and technical services manager for Makhteshim Agan of North America (MANA).

Extension entomologists noted low trap counts last fall, which usually point to lower soybean aphid pressure the following season, said Guy. "But… we are in uncharted territory regarding soybean aphid pressure. Anything can happen with the weather and crop conditions we've seen, the late planting or replanting, and degree days behind schedule in so many areas. Soybean aphid timing, duration, and intensity could be unpredictable."

"Once aphids reach threshold, there's no time to waste. Populations double every two to four days, so within a week they will hit levels that cause serious damage and yield loss," said Guy. "Usually aphid populations decline starting in mid-August, but this year could be different and growers should not let their guard down."

“Speed sampling” is the preferred scouting method these days. Extension entomologists recommend examining the upper trifoliate leaves on 11 plants about 30 paces apart. A plant is considered to be infested if it has 40 aphids or more, but you don't have to count beyond 40 on any one plant, which makes it easier to keep moving through the field. If 100 percent of the 11 plants are scored as infested, consider spraying. If there is uncertainly, scout additional sets of five plants up to 20 more plants. If 80 percent of the additional plants are infested, consider spraying, said Guy.

Weekly soybean aphid reports for Wisconsin are on this website, http://sba.ipmpipe.org.

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