What the research measured
The cited six-state study of 19 yield-loss experiments (2003–05) calculated an economic threshold of 273 aphids per plant (95% CI 111–567) giving a 7-day lead before the economic injury level of 674 (95% CI 275–1,399); Minnesota, Iowa and South Dakota extension round this to 250 aphids on 80% of plants. The same study found populations doubled every 6.8 days and peaked at R3–R5 in 18 of 19 location-years. Iowa State reports yield losses up to 40% from aphid feeding; Minnesota and Iowa bioassays found pyrethroid mortality cut up to 39-fold in the most resistant field population.
A peer-reviewed 19-location-year threshold study in six states, three state extension guides (Minnesota, Iowa, South Dakota) that apply it, and a peer-reviewed bioassay series of 27 population-years testing pyrethroid resistance in Minnesota and northern Iowa.5 independent sources.What it is. A foliar insecticide, most often a pyrethroid such as lambda-cyhalothrin or bifenthrin, sprayed when whole-plant counts average 250 soybean aphids per plant on 80% of plants with numbers rising, from late vegetative stages through R5. The threshold trials used lambda-cyhalothrin (Warrior) at 16.8–28.0 g a.i./ha by ground once a target aphid density was reached.
Study conditions and limitations
- The threshold is supported through R5: aphid peaks fell at R3–R5 in 18 of 19 location-years, Iowa State says yield loss usually does not occur after R5, and Minnesota advises scouting until R6.5 because larger numbers are needed to cause loss in early R6.
- Pyrethroid resistance is documented: the most resistant Minnesota field population showed 39-fold lower mortality than a susceptible colony, and Iowa State expects every aphid in Iowa to carry at least one resistance mutation to pyrethroids such as bifenthrin and lambda-cyhalothrin.
- Spraying below threshold carries costs the study authors spell out: it treats a population growing too slowly to reach the injury level in 7 days, removes generalist predators and exposes more aphids to insecticide selection.
- South Dakota State notes the damage boundary, where yield loss becomes detectable, is about 485 aphids per plant, so injury levels below that have no practical value; the threshold range of 111–567 reflects differing control costs, prices and yields.
- The threshold needs real counts: 20 to 30 plants per field every 7 to 10 days, or speed scouting in which a plant counts as infested at 40 or more aphids and 11 plants are assessed first.
- Populations can double in 2–3 days under favorable conditions (Iowa State), against the 6.8-day field average behind the 7-day lead time.
Read the sources · 5
- Economic Threshold for Soybean Aphid (Hemiptera: Aphididae) ↗
Nineteen yield-loss experiments in six states over 2003–05 under a common protocol: field doubling time averaged 6.8 ± 0.8 days; the economic threshold averaged 273 aphids per plant (95% CI ±38, range 111–567), giving a 7-day lead before the economic injury level of 674 (95% CI ±95, range 275–1,399). Peak density fell at R3–R5 in 18 of 19 location-years.
- Why the 250 Threshold is Still Appropriate for Soybean Aphids ↗
SDSU recommends 250 aphids per plant on 80% of plants, rounding the 273 threshold from six-state 2003–05 data; it places the economic injury level at 674 and the damage boundary at about 485 aphids per plant, so injury levels below 485 have no practical value. Speed scouting starts with 11 plants and a 40-aphid infested cutoff.
- Soybean aphid ↗
From vegetative stages through R5, treat when average counts exceed 250 aphids per plant and more than 80 percent of plants are infested; sample at least 20 to 30 plants, scout infested fields every seven to 10 days and continue until R6.5.
- Soybean Aphid ↗
Aphid feeding can cut yield up to 40%; applications are warranted until R6 at 250 aphids per plant with 80% of plants infested. Populations can double in 2–3 days, and pyrethroid resistance (bifenthrin, lambda-cyhalothrin) has been reported in Iowa, where the authors expect every aphid to carry at least one resistance mutation.
- Evidence for Soybean Aphid (Hemiptera: Aphididae) Resistance to Pyrethroid Insecticides in the Upper Midwestern United States ↗
Aphids from 27 population-years in Minnesota and northern Iowa were tested in leaf-dip bioassays with lambda-cyhalothrin (2013–15) and glass-vial bioassays with lambda-cyhalothrin and bifenthrin (2015–16); the most resistant field population showed 39-fold lower mortality than the susceptible laboratory colony.
Before you buy, ask
- What disease pressure did the trials with a yield response have, and do my fields see it?
- What application timing and rate did the cited studies use?
- Is there a scouting threshold that would tell me whether to apply this season?
Record
- Sources checked
- October 12, 2026. Every figure shown is printed on the linked page; unit conversions are noted on the chart.
- Company response
- None yet. Makers and suppliers of this product can send corrections, additional trials, or a statement; it is published here, labelled, and never edits the research summary. Send a correction →