With their quick and explosive ability to reproduce in vast numbers, tiny soybean aphids qualify as a heavyweight in the pest world with the potential to severely knock down your yields.
“No question about it, these aphids can extremely limit yields in soybeans if left unchecked,” says Palle Pedersen, Iowa State University Extension agronomist. “Under the worst circumstances, soybean aphids have caused yield losses of 20 bu./acre and, in a few cases, even more.”
First reported in southeastWisconsin in 2000, Pedersen says the soybean aphids can be found just about anywhere — as far west as Nebraska, all over the upper Midwest and on the eastern seaboard, including three provinces in Canada.
Soybean aphids are small, soft-bodied insects, and may be winged or wingless, depending on the season and the plant's condition, according to Pedersen. The soybean aphid has markings that include a yellow-green with black cornicles (“tail pipes”) and a pale-colored tail projection. The aphid is small, about the size of a pinhead. Nymphs are smaller.
In the U.S., common buckthorn (Rhamnus cathartica.) — an invasive European woody shrub — is the most common overwintering host for the soybean aphids' eggs, which are extremely cold-hardy and can survive temperatures as low as -29°F.
“To some extent, the soybean aphid is an odd pest because you essentially have an Asian species using a European variety of buckthorn as its primary overwintering host in North America,” says David Ragsdale, University of Minnesota entomologist. “Yet during the growing season, the soybean aphid is almost exclusively found on soybeans.”
Ragsdale and other entomologists have observed that soybean aphid often exhibits a biennial life cycle — one year where populations may be high followed by low numbers the next.
“Further research will be needed, but it appears that major predators, particularly the Asian multicolored ladybird beetle, along with some native ladybug beetles, seek out buckthorn to feed on the wingless soybean aphids that are laying overwintering eggs,” says Ragsdale.
“In years of high populations, these predators will invest the time and energy to decimate the egg-laying soybean aphids in buckthorn, which then usually translates into lower survival and fewer aphids the following spring.”
But Ragsdale says trying to pinpoint the chief reasons for the reported biennial life cycle of the soybean aphid may not be all that cut and dried.
For example, the impact of spraying, weather conditions, aphid-infecting fungi, soybean variety selection, planting date and feeding habits of the predators will likely shed important light on the soybean aphids' life cycle swings.
“Basically, the soybean aphids' life cycle is complex,” says Ragsdale. “In the spring, the eggs will hatch and the soybean aphid will have two to three generations on buckthorn. The number of winged offspring increase each successive generation and will eventually move into soybeans in early June.”
The winged females are fertile without mating (known as “parthenogenetic” reproduction) and bear living young. Several generations (up to 18) of wingless females can be produced quickly due to parthenogenesis. The stem tips and young leaves of growing soybeans are colonized first.
Later the aphids appear on the underside of leaves of mature plants. Aphid development is favored in late June to early July and at optimal temperatures of around 82°F.
Most offspring on soybeans are wingless, but a variety of environmental (temperature and day-length) and plant quality factors combined with high aphid numbers can all contribute to the development of winged offspring.
Winged females will leave the field and migrate to new fields with better quality and lower aphid populations, according to Iowa State University.
In the summer, the population in soybeans is comprised of females that essentially clone themselves and give birth at rates of three to eight aphids per day for about a month. The generation time is 7-10 days, depending upon temperature.
The result is an exponential growth rate where populations can double in just two to three days under favorable conditions.
However, plant breeding programs are already underway in several Midwestern states that show promise in resistance to the soybean aphid.
“Germplasm in some Group 7 and Group 8 varieties from the University of Illinois and Michigan State University are being used in some field testing in Illinois, Iowa, Michigan, Minnesota and South Dakota,” says Ragsdale. “And the resistance to soybean aphids looks very promising.”
Here are a few scouting tips from the University of Wisconsin and University of Minnesota:
Begin scouting weekly in late June or early July and continue through pod set. Check 20-30 plants at random per field, covering 80% of the field, and pay particular attention to late-planted fields, or fields under moisture stress.
Examine the entire plant, particularly the new growth at the top and side branches. Count the number of aphids on each plant and then calculate the average number and use that figure to make your decision on whether or not treatment is needed.
Since this can be time-consuming, especially when aphid numbers are dense, Minnesota entomologist David Ragsdale says another scouting method — called “speed scouting” — can be used with fairly good accuracy.
Speed scouting basically involves getting your eye acquainted with what a group of 40 aphids looks like. Once you can pretty well judge what that amount looks like in size, then sample 11 or so plants in a field, using perhaps a “W” or “Z” pattern.
“We call it speed scouting because you don't have to meticulously count each aphid, and you can make a decision quicker whether or not to spray,” explains Ragsdale.
If you find that the first 11 plants have 40 or more aphids per plant, then treatment is recommended. If you find fewer than 11 plants have 40 or more aphids, Ragsdale recommends sampling another five plants.
“It is extremely important to note that 40 aphids are not — and I mean not — to be considered a new threshold level,” stresses Ragsdale. “Speed scouting is based on a mathematical model using detailed field data.”
So far, Ragsdale says that this new method corresponds pretty well mathematically with the traditional way of calculating the treatment threshold of 250.
In the traditional method, use a threshold of 250 aphids/plant, especially if populations are actively increasing, to make your decision about when to spray. This action threshold should be based on an “average” of 250 aphids/plant from 30 plants sampled throughout the field. Regular field visits are usually required to determine if soybean aphid populations are increasing.
Pyrethroids (i.e., Warrior, Mustang Max, Asana, Baythroid) and organophosphates (i.e., Lorsban) are two insecticide classes labeled for soybean aphids on soybeans and commonly used in chemical control programs.
Organophosphates exhibit a “fuming” action, which may work better in heavy canopies or at higher temperatures. Pyrethroids tend to provide longer residual than organophosphates or carbamates (Furadan) and are most effective at temperatures below 90°F.
For soybean aphids, good coverage is important. Higher spray volumes (15-20 gal./acre) and higher pressure help to move the insecticide down into the canopy, says Iowa State's Palle Pedersen.
Unless aphids are at threshold levels and actively increasing, adding insecticide to early season glyphosate applications as “insurance” is not recommended and may make the situation worse, he says.
The University of Minnesota has developed a Soybean Aphid Growth Estimator (SAGE) model that can estimate the maximum reproductive rate possible.
The inputs into the SAGE model include forecasted temperature and aphid density per plant. The model, which uses Microsoft Excel to perform the calculations, is available at: www.soybeans.umn.edu/crop/insects/aphid/aphid_sagemodel.htm.
For speed scouting, more details and downloadable worksheets are available at the following Web site:
Iowa State University offers the following Web site for more information about the soybean aphid: www.soybeanaphid.info.
Further information about the soybean aphid and Checkoff-funded research can also be located at www.planthealth.info
Palle Pedersen, Iowa State University Extension agronomist says, “Essentially, what is important to keep in mind is that aphids can multiply up to tenfold in a week. So, if you have 150 aphids/plant, the population, if left untreated and conditions are good, could quickly leap to 1,500-2,000/plant within a week — a very high and damaging level. This is why frequent scouting and monitoring of population and dynamics in the fields are extremely critical.”
Presently, treatment is recommended when a threshold of 250 aphids/plant is found. This threshold provides growers with a seven-day lead time before damaging aphid densities are reached.
If you are unsure if aphids are growing at a fast enough rate to reach damaging levels, you can scout every three days once the threshold is reached and then be ready to spray at a moment's notice.
In 2005 some aphid populations reached 250/plant, but never reached the damaging densities of more than 1,500 aphids/plant, according to Minnesota entomologist David Ragsdale. Consequently, treatment was unnecessary.
“Despite its prolific reproduction ability, the good news is that soybean aphids are still easy to manage because they are very sensitive to insecticides,” says Pedersen. “There are enough available products labeled for the soybean aphid to do the job.”