Recently, soybean aphids have been confirmed at higher populations than typically expected in August. While many folks are scouting for worms and stinkbugs, we also recommend looking for aphids in your soybean fields as well. The key variables to consider regarding managing soybean aphids are the number of aphids per plant and the soybean growth stage. Information from our Pest Management Guide on Soybean Aphids in summary including info on thresholds and sampling are as follows (VCE Field Crops PMG, Publication 456-016):
Number of aphids per soybean plant
The current economic threshold for aphids is an average of 250 aphids per plant, on two consecutive field visits spaced about 5-7 days apart. This is because aphid populations can “crash” quickly due to heavy pressure by natural enemies like lady beetles, parasitic wasps, and fungal diseases. When scouting, choose a “Z” or “W” shaped pattern to cover the entire field and sample at least 20 to 30 plants per field by examining the entire plant, including stems and upper and lower leaf surfaces. Use the aphid/plant average for determining the need for treatment.
The soybean growth stage
The threshold of 250 aphids per plant applies to soybeans through the R5 growth stage (3 mm long seed in the pod at one of the four uppermost nodes on the main stem), after which time plants can tolerate 1,000+ aphids with no threat to yield. If an insecticide is applied for aphids, pyrethroids (e.g. bifenthrin, Warrior II, Mustang Max, etc.) can be effective for management, but choosing a more selective insecticide can preserve natural enemy populations and limit future flare ups of aphids or other pests.
Carrie Ortel, Extension Soybean Agronomist, Virginia Tech Tidewater AREC
Mark Reiter, Soils and Nutrient Management Extension Specialist, Virginia Tech Eastern Shore AREC
Joseph Haymaker, Postdoctoral Associate, Virginia Tech Eastern Shore AREC
Many of the full-season soybean fields in Virginia are beginning to flower, which is a great time to start tissue testing if there is a concern for crop nutrition. Tissue testing is an effective way to monitor crop nutrition when done correctly. To get reliable results, choose the correct, uppermost fully expanded soybean leaf during optimal conditions and carefully interpret the results. Tissue testing may begin as early as V4 but is most reliable during flowering and can continue through pod filling.
Step 1: Plan Your Sampling During Good Field Conditions
Time It Right – Field Conditions Matter
Field conditions at sampling time greatly affect the accuracy of tissue tests. Aim to collect samples when plants are actively transpiring, which usually means:
Adequate soil moisture (not drought or waterlogged).
Moderate temperatures and healthy plant function.
Avoid sampling during:
Drought, which can limit nutrient uptake even when nutrients are present in the soil.
Saturated soils, which can temporarily inhibit root function.
Shortly after foliar nutrient applications—wait at least a week and ensure a rain event has occurred to allow nutrients to be absorbed and leaf surfaces to clear.
Address In-Field Variability
Soybean nutrient levels can vary within a field due to differences in soil texture, drainage, or previous management practices. For meaningful results:
Divide fields into management zones based on known variability.
Take one composite sample per zone, collecting at least 18 trifoliolate leaves randomly throughout that area (Ortel et al., 2023).
This helps identify localized deficiencies and supports more precise nutrient management.
Step 2: Collect Your Leaf Sample
Choose the Right Plant Part
To get consistent and accurate results, it’s critical to sample the correct, uppermost fully expanded trifoliate leaf, as nutrient concentrations differ between leaves. The uppermost fully expanded trifoliate leaf is typically located on the second, third, or fourth node from the top of the plant (Figure 1, shown below).
Figure 1. Soybean plant at the full flower (R2) growth stage. The uppermost fully developed leaf is shown as the leaf on the third node from the top of this plant.
Look for the highest leaf that is dark green, full-sized, and has a coarse texture.
Avoid leaves with a velvety feel or are lighter in color than others—these are still developing and can falsely indicate nutrient levels.
Check with your testing lab for specific guidelines. Some labs also recommend sampling the whole plant during vegetative stages or including/excluding the petiole (the stalk attaching the leaf to the stem). Be sure your sampling method agrees with the lab’s interpretation standards.
Collect the Sample Properly
Once you identify the correct leaf, follow these best practices:
Collect 18–25 leaves per sample for a good composite.
Take samples across a consistent management zone (based on yield history, soil type, or other field characteristics).
Place leaves in a paper bag (not plastic) to allow drying and prevent mold.
If your lab uses critical nutrient thresholds without the petiole (e.g., Virginia Tech), remove the petiole before bagging.
Step 3: Interpret Results with Context
Getting accurate lab results is only half the process—understanding them correctly is essential.
Use growth-stage-specific critical concentrations when available, such as potassium in soybean (Slaton et al., 2021). These values indicate the threshold below which yield may be affected.
If no critical values are available, use sufficiency ranges cautiously—they are less precise and based on broader surveys instead of replicated research.
Also, consider nutrient mobility:
Mobile nutrients like nitrogen (N), phosphorus (P), potassium (K), and magnesium (Mg) can move within the plant and are often relocated from leaves to developing seeds and pods (Bender et al., 2015). Lower concentrations in leaves during reproductive stages may not signal a deficiency.
Immobile nutrients like calcium (Ca), sulfur (S), and most micronutrients remain in their original locations and offer more stable indicators.
Although some nutrient deficiencies may not be visually apparent (hidden hunger), only take action with a corrective application of fertilizer if a deficiency occurs. Prophylactic foliar fertilizer applications have not been shown to increase yields (Matcham et al., 2021). When a nutrient deficiency does occur, a corrective application of granular fertilizer should be used to correct macronutrients, while a foliar fertilizer may be used to correct micronutrients.
Take Home Points
Collect a composite sample of at least 18 of the uppermost fully expanded soybean trifoliate leaves from each management zone.
Only collect tissue samples during favorable field conditions and adequate soil moisture.
Consider the plant part collected (petiole included or excluded), growth stage, and nutrient mobility with interpreting results.
Bender, R. R., Haegele, J. W., & Below, F. E. (2015). Nutrient uptake, partitioning, and remobilization in modern soybean varieties. Agronomy Journal, 107(2), 563–573. https://doi.org/10.2134/agronj14.0435
Matcham, E. G., Vann, R. A., Lindsey, L. E., Gaska, J. M., Lilley, D. T., Ross, W. J., Wright, D. L., Knott, C., Lee, C. D., Moseley, D., Singh, M., Naeve, S., Irby, J. T., Wiebold, W., Kandel, H., Lofton, J., Inman, M., Kleinjan, J., Holshouser, D. L., & Conley, S. P. (2021). Foliar fertilizers rarely increase yield in United States soybean. Agronomy Journal, 113(6), 5246–5253. https://doi.org/10.1002/agj2.20889
Ortel, C. C., Roberts, T. L., Hoegenauer, K. A., Poncet, A. M., Slaton, N. A., & Ross, W. J. (2023). Mapping variability of soybean leaf potassium concentrations to develop a sampling protocol. Agrosystems, Geosciences and Environment, 6(4). https://doi.org/10.1002/agg2.20439
Slaton, N. A., Drescher, G. L., Parvej, R., & Roberts, T. L. (2021). Dynamic critical potassium concentrations in soybean leaves and petioles for monitoring potassium nutrition. Agronomy Journal, 113(6), 5472–5482. https://doi.org/10.1002/agj2.20819
First, I would like to introduce myself. I am Tim Bryant, the new assistant professor of entomology and extension specialist at the Tidewater AREC, in Suffolk, VA. I did my master’s degree here at the TAREC under Dr. Sally Taylor, and I am excited and grateful for the opportunity to come back and work with the farming community in Virginia. I am also looking forward to continuing to provide updates on this platform as regularly as we have updates to provide. Now, on to a quick update from my first few weeks on the job…
Soybean aphids have been reported in Mecklenburg and Prince George counties (First reports 8/28). Aphids can reproduce asexually, and are capable of rapid population growth under the right conditions, so close monitoring is important. Our threshold for managing soybean aphids is 250 per plant at R4 (full pod) or prior, and 1000+ at R5 (beginning seed) and beyond. The below image shows 300 aphids on a single leaflet as a point of reference (Photo credit: Roger Schmidt, University of Wisconsin-Madison, Bugwood.org). There is likely to be multiple developmental stages of aphids present at one time, and early stages are very small.
There are a couple key considerations for managing aphids; 1) they are highly susceptible to population crashes from natural enemies or weather events, and 2) broad-spectrum insecticide applications (i.e. pyrethroids) may kill aphids, but also the afore-mentioned beneficial insects. Eliminating beneficials can potentially flare up spider mites or other pests later on. If you think you may have an aphid problem, scout several areas of the field to estimate the number of aphids per plant and pay close attention to the number of beneficial insects present. Where we scouted for aphids, there were also large numbers of lady beetle adults and larvae, which are excellent aphid predators. If you reach the aphid threshold, sample again in 5-7 days to ensure the population has not crashed. Narrower spectrum insecticides (e.g. sivanto) may preserve some beneficial insects if an application for aphids is made. See our pest management guide (beginning on page “4-62”) for more details on labeled products, application rates, etc.
The below image shows a lady beetle larvae, which were found in abundance along with the aphids we are reporting on here, feeding on an aphid (Photo credit: Winston Beck, Iowa State University, Bugwood.org).
Corn earworm (= bollworm) moth catches were exceptionally high this week, continuing to climb in Dinwiddie, Prince George, and Suffolk, VA black light traps. The average number captured per night was 38 in Greensville, 79 in North Dinwiddie, 72 in Prince George/Disputanta, and 124 in Suffolk. Thanks to Sara Rutherford, Scott Reiter, and the Tidewater AREC entomology crew for their reports. Here is the Table.
Corn earworm (= bollworm) moth catches increased greatly this week in southeastern Virginia black light traps. The average number captured per night was 50 in Greensville, 42 in North Dinwiddie, 39 in Prince George/Disputanta, and 67 in Suffolk. Thanks to Sara Rutherford, Scott Reiter, and the Tidewater AREC entomology crew for their reports. Here is the Table.
We have evaluated over 250 moths in our 2024 vial tests, with 57% surviving the 24-hour exposure to the pyrethroid insecticide, cypermethrin, at 5 micrograms per vial.
Corn earworm (= bollworm) moth catches really started to increase this week in southeastern Virginia black light traps. The average number captured per night this week was 4 in Greensville, 11 in North Dinwiddie/Petersburg, 18 in Prince George/Disputanta, and 30 in Suffolk. Thanks to Sara Rutherford, Scott Reiter, and the Tidewater AREC entomology crew for their reports. Here is the Table
We have evaluated over 150 moths in our 2024 vial tests, with 61% surviving a 24-hour exposure to the pyrethroid insecticide, cypermethrin, at 5 micrograms per vial. Even though cypermethrin is no longer widely used, survival rates this high suggest we need to watch for some pyrethroid control failures, and to consider alternative chemistries when an insecticide is needed.
I want to remind everyone that the Virginia Ag Expo, Virginia’s largest field day, is next Thursday, Aug 4. The Expo is an educational, marketing, and social event that farmers and agribusiness look forward to each year. It moves from one location to another each year and strives to showcase the diversity of Virginia Agriculture. Our host this year is Mill Creek Farms and the event will be located at Camden Farm, which is located near Port Royal, right off of Highway 17 in Caroline County.
Precision for Profits is the theme for the 2022 Virginia Agricultural Expo. There will be a wide variety of agribusinesses present (currently over 85 exhibitors) showcasing the latest equipment, technology, goods, and services. Virginia Cooperative Extension will be working with NRCS to highlight the diversity of soils on this farm and the high potential for precision agriculture practices. Field plots are once again a walking, go-at-your-own-pace tour designed to fit your interest and schedule. Breakfast and Lunch will be catered by the vendors shown below.
The Virginia Ag Expo is jointly sponsored by the Virginia Grain Growers and Virginia Soybean Associations, in Cooperation with Virginia Cooperative Extension.
This event is FREE to the public. Click the link below to let us know you are coming!
The upcoming weekend rainfall (our meteorologists seem assured that it’ll be widespread) should greatly help parts of Virginia that are short on or getting short of topsoil moisture and position us perfectly for a good start to the soybean growing season.
However, with the rainfall comes cooler soil temperatures. Below are predictions for 4-inch soil moisture and soil temperature over the next 10 days from Orange (one of the coolest parts of Virginia) and Suffolk (one of the warmest parts). As you see, soil temperatures will plummet from relatively warm (>60o) to nearly 50o or less over the weekend. While soybean seed will germinate and emerge in a reasonable amount of time at 60O, germination and emergence will be very slow when temperatures dip below this.
Therefore, heed my suggestion and make sure that you have a good fungicide seed treatment on anything that you are planting now or early next week.
Below is the fungicide efficacy chart from our Pest Management Guide. The main diseases that we need to concern ourselves with are Rhizoctonia and Fusarium sp. If in wet soils, Pythium and Phytophtora sp. could also be a problem with slow-emerging soybean. Be sure to protect a slow-emerging crop with a fungicide containing the active ingredients that provide good to excellent control of the appropriate disease.
Many may have already planted. Some have stopped planting other crops such and peanut and cotton due to cool soils and started planting soybean instead. Regardless, an advantage of soybean is that it tolerates a wide range of planting dates. I’ve found no advantage to planting soybean before May and have seen little yield penalty from planting as late as the first week of June, under most conditions. There are advantages to planting early and planting late, but that is not the subject of this post. Contact me for more detailed discussion on this matter-it will take more that a few comments.
With that said, I’ve always preferred May planting due to less risk of frost damage and slow emergence that is common with earlier planting. Still there are big differences in management when planting in early- versus late-May. Here I’ll review a few of these.
Plant earliest maturing varieties first, then move to later maturing varieties. This will spread risk and harvest dates. We have however observed that early-planting and/or early-maturing varieties work better under productive soils/fields. If planting on poor soils/unproductive fields, later-maturing varieties will extend the growing season and push the critical pod- and seed-filling stages to less stressful times of the year.
Always plant into moisture. But don’t plant more than 1 inch deep when soils are cool or emergence will be delayed substantially. One-half to 3/4 of an inch is adequate. For later planting dates when soils are warm, one can plant as deep as 1.5 inches and get rapid emergence.
If planting into cool soils (usually before mid-May), use a good fungicide seed treatment. It could take 10 days or more for the soybean to emerge; plenty of time for soil-born seedling diseases to attack.
A final plant population of 70 to 100 thousand plants per acre is usually adequate for full-season plantings. This means that 95 to 130 thousand seed per acre is enough, depending on expected emergence. Our research and in other states have observed that less seed is needed with low-yielding fields and greater seeding rates are needed with high-yielding fields. The problem is knowing how much the field will yield.
As always, contact me for more details regarding these tips or discuss other early-season soybean issues.
