Before soybean aphid was identified as a pest of soybean in the U.S. in 2000, insecticide applications to northern soybean crops were rare, targeting sporadic insect and mite outbreaks. Although large infestations have been relatively uncommon since the early to mid-2000’s, the soybean aphid is unquestionably still the key insect pest of soybeans in many North Central states. A tremendous amount of research and observational data has been obtained for this pest since its introduction and we have the tools and the knowledge to manage this pest effectively.
As farmers and their advisors find soybean aphids in their fields, the timing is right for a fact-based review of what is known about soybean aphid, their effect on yield, and cost-effective management of this pest.
How can soybean aphids reduce soybean yield?
The soybean aphid feeds on the phloem fluids (sometimes referred to as “sap”) by inserting piercing-sucking mouthparts directly into the phloem vessels that carry products of photosynthesis from the leaves to other parts of the plant. Prior to feeding, aphids “taste” the sap to determine if the plant is a suitable host species and if the quality is acceptable. Once they settle and begin feeding, the injury from soybean aphid infestations can reduce plant growth, pod number, seed number, seed weight and seed oil concentration (2, 24). Early and prolonged aphid infestations can affect all yield components, while later infestations tend to only reduce seed size (2). In addition, soybean aphids decrease photosynthesis rates of soybean plants (11).
Direct yield loss from soybean aphid feeding does not occur when the first (or five or ten) aphids begin feeding. Today’s soybean varieties are equipped to handle minor challenges, including a few aphids. Yield loss from soybean aphid is related to how many soybean aphids are present and for how long the aphids are present and feeding. The amount of aphid population pressure over time is calculated as aphid-days. Simply put, this is the average number of aphids on a plant multiplied by the number of days they are present. A single soybean aphid on a plant for 10 days is equal to 10 aphid-days, 200 aphids on a plant for 20 days is equal to 4,000 aphid-days, and so on. This aphid-day concept proved to be a good indicator of how soybean yield responded to aphid populations (23).
Feeding by aphids does not cause the plant to ‘leak sap’. Soybean aphids require specific nitrogen-rich amino acids that are present in plant fluids at low concentrations (16, 25). Therefore, aphids must consume large volumes of sap to acquire enough nutrition. Excess water and sugars from the sap are excreted by the aphids as waste. This is called honeydew. It is the sticky, shiny substance that accumulates on leaves of aphid-infested plants. The sugary honeydew is sometimes fed on and used by other insects as an energy source. A fungus called sooty mold also utilizes aphid honeydew and results in a dark coating on soybean leaves which, may block sunlight and interfere with photosynthesis (12, 10, 8).
Soybean aphids are not known to transmit fungal or bacterial diseases to soybean. Because soybean aphids and soybean pathogens are associated with certain environments, some people may wrongly assume that the presence of a fungal disease (such as charcoal rot) means that the disease was transmitted by aphids or that the disease entered through the wounds caused by aphids.
However, aphid feeding can transmit disease-causing viruses from one plant to another. The soybean aphid has been associated with the transmission of several viral diseases of soybean, such as soybean mosaic virus (6), alfalfa mosaic virus (17), and others (26). Because these viral diseases are not currently considered significant threats to soybean yield in the North Central region, they are not directly accounted for in general aphid management recommendations. The “tasting” or probing of plants by soybean aphids can also transmit viruses in plants that are not soybean aphid hosts, such as dry beans (18) and potato (3, 4). This effect in other crops is particularly pronounced when soybean aphid populations are high.
Economics of soybean aphid infestations: Math and biology matter
Figure 1. Relationship of insect population and crop yield (Modified from Pedigo et al. 1986).
The lowest level of aphid infestation that has been shown to cause yield loss in soybean is several thousand aphid-days. This value, referred to as the damage boundary, is a biological relationship between the insect, crop, and environment, and is independent of crop and input costs. Below the damage boundary, no damage can be measured. Therefore, management efforts directed at treating aphid levels well below the damage boundary cannot provide a return on investment.
The economic injury level (EIL) is the point at which the yield loss from insect damage is equal to the cost of a management action, such as an insecticide application. Insecticide applications made to pest populations that have not reached this point, and are unlikely to reach it, would not provide any return. To more readily apply this yield-loss relationship to field scouting and aphid management, a value in terms of aphids per plant was calculated as the threshold to apply an insecticide to threatening populations.
That brings us to the economic threshold (ET). This is the key number for management of the pest and is the insect population at which management action should be applied to prevent a growing population from causing economic injury. The ET can also be referred to as an action threshold or treatment threshold. In most thresholds for insect pests, including soybean aphid, the ET occurs well before the EIL to minimize the chance of the grower incurring economic loss. In fact, the ET for soybean aphid occurs before the damage boundary. In addition to costs, the ET may take into account factors like insecticide effectiveness, rate of insect reproduction, crop development, and lead-times for insecticide application.
All of the values and statements above are based on data. To determine the values for damage boundary, EIL and ET for soybean aphid, thousands of whole-plant aphid counts were taken at frequent intervals through the growing season. Furthermore, the large multi-state data set that went into these calculations included a wide range of soybean growing environments, including moisture variation and other stresses.
The economic threshold of 250 soybean aphids per plant, more than 80% of plants infested and aphid populations increasing (23) was established to prevent populations from reaching the EIL. These values were determined by closely monitoring aphid populations in research plots. It has been nearly a decade since the current threshold was developed and published, and University research continues to support these values. However, some people have questioned the continued validity of original soybean aphid ET of 250 aphids per plant, which was calculated including economic conditions from the mid-2000s. Calculations performed using current economic conditions (which are different now), but without regard to biology (which has not changed), may suggest that a lower ET should be used for aphid management. This is based on faulty logic. Aphid biology and how the plant reacts to aphids make the exercise meaningless. No significant gain can be found at those lower aphid numbers (remember the ET of 250 aphids per plant is already well below the damage boundary), AND low numbers of aphids often don’t reach the EIL.
The soybean aphid ET is best viewed as a fixed action or treatment threshold, unlike some, more flexible thresholds for other pests. In the case of soybean aphid, raising the threshold reduces lead-time for applications and increases risk of economic loss from rapidly increasing aphid populations. Lowering the threshold may provide a bit more lead-time for insecticide application, but it also reduces the chances for natural enemies and the sometimes-harsh environment of a soybean field to solve the problem for you. Lowering the threshold reduces your ability to treat only those fields facing a reasonably high risk for yield loss. Therefore, a sliding scale that lowers the ET is not recommended.
In university research across the North Central Region, treating below 250 aphids/plant resulted in no observable yield increase, supporting the conclusion that very low thresholds or zero tolerance of aphids is not necessary (https://www.ent.iastate.edu/soybeanresearch/content/extension).
Again, there are no published, peer-reviewed data that show that soybean aphid damage is likely below the ET. Therefore, for a positive return on investment, treat only the fields that have a reasonable chance of reaching economically damaging levels. Lowering the ET below 250 aphids/plant will not save yield. Instead, it will result in spending money unnecessarily on many more fields that would not have had economic loss from aphid injury. This threshold is conservative in that it allows plenty of time for action on the producer’s part before yield loss could begin. This is particularly true during late soybean growth stages.
Costs of treating soybean aphids too early
While some newer insecticides target a narrower range of insects, most insecticide applications are not specific. They will kill beneficial insects (lady beetles, parasitic wasps, etc.) as well as pests, later allowing soybean aphid populations to rebound in fields without those beneficial insects to slow them down. By using the ET, natural enemies will have a chance to suppress the aphid population and possibly prevent it from reaching economically damaging levels. After application, insecticide residues will kill insects for a short time, but insecticide activity invariably declines over time (generally, this is considered a good thing). With most insecticides registered for soybean aphid control (such as pyrethroids), soybean foliage emerging after treatment is not protected. Insecticides that are absorbed and translocated within soybean plants typically move upward only a leaf or two and eventually leave unprotected foliage, especially when applied early in the season.
Applying treatments early can result in a false sense of security and a reduced reliance on scouting. If a re-infestation is not detected before reaching the EIL, yield may be reduced. If detected, the cost of additional insecticide applications are incurred. Early treatment can reduce or eliminate the cost efficiencies of a single, well timed threshold-based treatment. Finally, unnecessary insecticide applications do nothing positive for a short-term return on investment. Importantly, long-term returns can be reduced if insecticide resistance becomes fixed in the soybean aphid population. This has happened many, many times in the history of pest management. We know that managing pesticide resistant pests is seldom “cheap and easy” (for example, consider the problems with herbicide resistant weed control).
How do you know if you have a soybean aphid problem?
All soybean fields are not equally likely to have a soybean aphid problem. Geographic, landscape, biological and agronomic factors all influence soybean aphid populations. The research results generated on commercial and University farms across the North Central Region can help identify when and where to target early- and late-season aphid scouting efforts. Early aphid infestations are often found in smaller fields near buckthorn (1) and are often more abundant near field edges. Soybean aphids prefer moderately dry soil moisture conditions (20). Soybeans grown in soils testing low in potassium contain higher levels of amino acids favorable for soybean aphid development (25) and aphid feeding can intensify potassium deficiency symptoms on these soils. Aphids are often most abundant in late maturing fields.
During vegetative growth stages, soybean aphids are often on the upper, newly expanding leaves. During reproductive growth stages, soybean aphids tend to move to leaves, stems and pods lower in the canopy (15). Soybean aphids should be scouted by examining individual plants throughout the field (7). Because soybean aphids remain attached to the plant while feeding and can occur throughout the canopy, use of a sweep net is not recommended for assessing populations of this pest.
It takes time for aphid populations to grow. It’s important to note that sometimes soybean aphid populations never grow at all – a single aphid does not invariably lead to hundreds! Soybean aphids can initially colonize a field and be rapidly wiped out by a combination of inhospitable environmental conditions and predatory insects. Once established, populations can grow at a rate where numbers double every 1 ½ days to as many as 6 days depending on environment and natural controls (14, 23). Most often, research documented that in-field populations doubled every 3 days (23).
Soybean fields should be scouted on a regular basis. Soybean aphid populations can increase rapidly, particularly with winged aphids migrating into fields. Early-season scouting can focus on fields with high risk for colonization or a history of early colonization by aphids. As aphid populations develop, more fields should be scouted. There may not be a need to visit every field every week, but fields with a history of high populations may need to be scouted weekly or more. The following resources may help to develop an efficient scouting strategy (Scouting guide for North Central Region).
Biology helps determine the profitability of crop production on your farm – Ignoring biology is expensive
None of what we have presented here is new, or groundbreaking information. However, all of what we have presented here is based on science that has been vetted and implemented over thousands of acres for more than a decade. Economic injury levels take commodity prices, labor and control costs into account.Fortunately, the biological components of an EIL are not sensitive to commodity or input prices.The insects on your farm do not eat faster or more when crop prices are high or insecticide costs are low; nor is your crop more sensitive to insect damage (remember the damage boundary). Yield loss occurs at the same level of pest population, regardless of market prices of commodities. It makes no sense to treat if there is no reasonable likelihood of damage.
Science is best when it does not sit still. New research on pest and crop biology and on new management tools may change EIL’s and associated ET’s over time. However, since the adoption of the 250 aphid/plant economic threshold, additional research has only confirmed the results of the original multi-state biological and economic research.
While some may view an insecticide costing “only a couple of dollars” as inexpensive when compared to other production inputs, it is still an added cost for no added benefit. These inputs add up with each acre applied. Farmers often find the “free” application costs when insecticides are tank-mixed with herbicides or other pesticides have unintended negative consequences – poor control from poor timing or application techniques of one or more products. Using an ET, based on sound, peer-reviewed research will help you apply your crop input dollars where they are most likely to produce a positive return on your investment and minimize the chances of creating other problems for yourself.
Using fear or faulty economic logic is tried and true as a very effective sales tool. It’s always prudent to be a skeptical consumer and consider the messenger when you evaluate information – a conflict of interest can arise if a profit motive underpins recommendations made without facts behind them. Be very wary of ETs that are based on “feel”, eyewitness accounts, or other anecdotes that are not supported by hard scientific data. ETs that are radically different from those recommended by agricultural research universities are another red flag.
Hopefully, this article has provided you with information that will help you sort through the information clutter on TV, radio, print media and especially the internet. A list of research references is included for those who would like to read and learn of some of the research supporting our soybean aphid management guidelines and economic threshold recommendations. Contact Extension for the most up-to-date and state-specific recommendations.
University of Minnesota: Bruce Potter, Robert Koch & Phil Glogoza
Iowa State University: Erin Hodgson
Purdue University: Christian Krupke
Penn State University: John Tooker
Michigan State University: Chris DiFonzo
Ohio State University: Andrew Michel & Kelley Tilmon
North Dakota State University: Travis Prochaska & Janet Knodel
University of Nebraska: Robert Wright & Thomas E. Hunt
University of Wisconsin: Bryan Jensen
University of Illinois: Kelley Estes & Joseph Spencer