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Soybean Aphid Story: 2003

September 5, 2003

After a notable absence in 2002, the soybean aphid made itself known in soybean fields across Illinois in 2003. In fact, aphid densities were at their highest since the insect was first observed in 2000. Let's recap how things transpired this past summer.

The first winged soybean aphid adults in Illinois this year were found by Dr. David Voegtlin, research entomologist with the Illinois Natural History Survey. He and his coworkers had been monitoring Rhamnus cathartica (a buckthorn species), the overwintering host for soybean aphids. On April 23, hundreds of soybean aphids were found on some of the small plants. At that point in time, these aphids had not begun their movement to soybeans, as there were none in the vicinity.

However, almost a month later, aphids were spotted on soybeans for the first time by Jeff Wessel, farm manager of the J.F. Richards Demonstration and Research Farm at Joliet Junior College. On May 29, he found varying numbers of soybean aphids and as many as 40 aphids on one V2-V3 stage soybean plant. A natural area on the college campus has a significant number of buckthorn plants. This discovery indicated that soybean aphids had begun their move to the early-planted soybeans in this area. Reports similar to this occurred in other midwestern states around the same time.

  • Marlin Rice, extension entomologist at Iowa State University, reported that soybean aphids were found on V1-stage soybeans in a field in northeastern Iowa on June 5.
  • In Michigan, extension entomologist Chris DiFonzo, Michigan State University, reported soybean aphids on V0-stage soybeans on June 3 on the MSU campus.
  • On June 2, University of Minnesota entomologists found a few soybean aphids on soybean plants near the campus.

During June, reports of spotty aphid infestations made their way to our desks by e-mail or phone. One very important recurring theme in these messages was the noted absence of natural enemies such as lady beetles. During this time, Drs. David Voegtlin and David Onstad (entomologist with the Department of Natural Resources and Environmental Sciences) and crew began their township surveys. Kendall Township in Kendall County and St. Joseph Township in Champaign County are the two townships sampled in this ongoing survey. Sampling in Kendall Township began on June 24 this year, compared to early July the previous two years. Even though sampling began earlier than the two years before, 100% of the fields examined were infested with soybean aphids. Approximately 29% of the soybean plants were infested with soybean aphids in Kendall Township, compared to 3% in early July 2002.

For the next few weeks the numbers of soybean aphids in northern Illinois continued on the increase, with no sign of slowing down. Reports that filtered in had similar findings--low densities across many fields, with "hot spots" embedded throughout the area. Our neighbors to the north (Wisconsin, Minnesota, and Michigan) all echoed similar findings. As the densities of aphids increased and infestations became severe, aphids began moving from the top of the plants to lower within the canopy. The aphids distributed themselves on stems and lower leaves deeper in the canopy. While aphid populations were widespread across northern Illinois, the presence, or should we say absence, of natural enemies was still a concern.

The presence of winged aphids (alates) was common among soybean plants in mid-July. It is alates developing on soybean plants that will leave those plants and fly to other soybeans to feed and reproduce. It was also common to find all stages of soybean aphids--alates, alatoid nymphs, and nymphs--on plants, indicating that winged individuals were landing on plants and producing offspring regardless of the size of the population on the plant.

Another indication that the aphid flight was greater and earlier than previous years was the capture of winged aphids in suction traps. Winged aphids were found for the first time this year in the Freeport, Joliet, and DeKalb traps the week ending July 11. As soybean aphid densities continued to increase, the movement of winged adults from field to field also increased, as indicated by the enormous numbers found in the soybean aphid suction traps. As we progressed from July to August, aphids made their way downstate, aided by northerly winds.

Soybean aphids took central and southern Illinois by storm. The first week in August saw many reports of heavy infestations south of I-80, primarily in Champaign, Ford, Kankakee, Iroquois, and Livingston counties, with many other counties on the western side of the state also reporting the presence of aphids. Several reports indicated aphid densities in hundreds per leaflet and thousands per plant in central Illinois on R3-R6 stage soybeans. By the end of the week, numbers of aphids in southern counties had begun to increase, as well. Over the course of the next two weeks, aphid populations in-creased dramatically. Suction traps operating in the southern half of the state began capturing exceedingly high numbers of alates just as the northern Illinois traps had earlier in the season. Traps in Brownstown, Monmouth, and Perry all had around 1,500 aphids the week ending August 15. Soybean aphid alates were also found in the Dixon Springs suction trap.

Soybean aphids were still a major concern for much of Illinois a little over a week ago. While most activity in northern Illinois had died down, soybean aphids in other parts of the state showed little signs of slowing. Captures of winged aphids were still at record highs in many of the suction traps across the state. Very little of the population (less than 1% to 2%) on soybean plants in central Illinois was composed of alatoid nymphs (aphids that are forming wings), indicating that these populations were going to be around for a while.

The torrential downpours and continuous rain over Labor Day weekend will undoubtedly affect many aphid populations. That, combined with an increase in natural enemies in many areas, will play a part in how the populations rebound in the next several days.

This past summer was very frustrating at many different levels. There were several questions running through the minds of producers: When is the best time to spray? How long should I wait before I treat? At what stage do I get the most benefit from treating? What level of aphids can a soybean plant tolerate and still be unaffected? What benefit do insecticides have on soybeans stage R5 and beyond? The decision to spray or not to spray probably caused many a sleepless night this summer. It was frustrating from our end too; many of these questions plagued the minds of researchers as well.

The soybean aphid is a relatively new pest to North America. Though reported in Illinois in 2000, it was seen at economic levels for the first time in many areas in this state. Many unanswered questions still remain. Research is being conducted in many areas of the Midwest, and hopefully in 2004 we'll know more about this insect. Ron Estes, coordinator of the Insect Management and Insecticide Trials, has completed an insecticide efficacy trial for soybean aphids.

As a part of the efficacy trials, a soybean aphid trial was conducted at the Kellogg farm outside of Yorkville, Illinois. The trial consisted of eight insecticide treatments, each replicated four times. The treatments (plots) were four rows by 30 feet in size. The spray solution was applied at a volume of 30 GPA, via a two-row hand boom. Aphid counts were taken at five plants from every plot. Three trifoliates, from the upper, middle, and bottom third of each plant, were counted. Average aphids per trifoliate were calculated from these counts. Prior to insecticide application, the average number of aphids per trifoliate was 48 across all plots. Table 2 gives the average number of aphids per trifoliate at 0, 7, 14, and 21 days after insecticide treatment (DAT).

Initially, all products performed equally well, with Nufos showing slightly better control after one week. By 14 days after treatment, all products, with the exception of Nufos, had aphid numbers increasing to 20 per trifoliate. While these densities may not be at economic levels, the differences are still noteworthy. At 21 days after treatment, the Asana and Warrior treatments were statistically the same, as were Lannate and the untreated check. Overall, Nufos showed the greatest residual control of the products tested. Although there were statistical differences among products, only Lannate showed an increase in aphid numbers that reached a level of 25 aphids per leaflet.

Additionally, we realize that the trial shows a very limited comparison of the array of products that are available. However, in the spring when our insecticide studies were planned, industry representatives and growers alike showed little interest in the soybean aphid. This was likely because of the low aphid populations in 2002, as previously mentioned.

Further research on the soybean aphid is ongoing both here and at other University of Illinois research farms throughout the state. Information will be available later this year. As the soybean aphid story of 2003 comes to an end, we look forward to hearing from our readers about their own experiences with this insect.--Kelly Cook and Ron Estes

Author: Kelly Cook Ron Estes


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