Insect thresholds for corn and soybean: what extension says, and when it said it
A threshold is the count where treating pays. I list the ones I have read in university extension sources, word for word where it matters, with the date of each source beside it. Where sources disagree I show both. Four rows come from a UW sheet dated February 2000; they are in grey, marked old, and sit next to the newer Wisconsin numbers.
Corn
Seedling and leaf feeders
| What and when | Threshold, as the source prints it | Source and date |
|---|---|---|
| Black cutworm seedling corn | Consider spot or border row treatment when 5% or more of the plants show damage and the larvae are 6th instar or smaller.The older sheet. The next two rows are newer. | UW Extension, Corn IPM Quick Guide: Corn Insect Pests of Wisconsin (L023) Feb 2000old (printed on the sheet as 2/00) |
| Black cutworm seedling corn | Treat when 2-5% of plants show cutting activity. | UW Extension A3646, Pest Management in Wisconsin Field Crops (Dewerff, Bick, Liesch, Nice, Renz, Smith, Werle) 2023 edition (the cover says 2023; the web file is dated Dec 2022) |
| Black cutworm seedling corn | Treatment threshold is met if 2-5% of plants have been cut: 5 to 12 of 250 scouted plants (five sets of 50 consecutive plants). Do not use moth trap counts to decide. | UW Crops and Soils, Managing black cutworm in Wisconsin corn fields Jul 11, 2025 (printed as last updated) |
| Armyworm June and July | Consider spot treatment when at least 1 armyworm per plant on 75% of plants, or 2 per plant on 25% of plants.The older sheet. The next row is newer. | UW Extension, Corn IPM Quick Guide: Corn Insect Pests of Wisconsin (L023) Feb 2000old (printed on the sheet as 2/00) |
| True armyworm corn | Treatment is suggested if larvae are 0.75-1.0 inch long or less and two or more larvae per plant can be found on 25% of corn plants, or if one larva per plant is found on 75% of corn plants. | UW Extension A3646, Pest Management in Wisconsin Field Crops (Dewerff, Bick, Liesch, Nice, Renz, Smith, Werle) 2023 edition (the cover says 2023; the web file is dated Dec 2022) |
| Western bean cutworm July | For field corn, treat when 5% of plants scouted have egg masses or small larvae. For processing sweet corn, treat when 4%. | UW Crops and Soils, scout for western bean cutworm (alert) Jul 2026 (the alert says emergence passed 25% on 7/20/2026) |
| Corn leaf aphid late whorl to early tassel | Treat if 50% of corn plants have more than 50 aphids per plant and the plants are in the late whorl to early tassel stage. | UW Extension A3646, Pest Management in Wisconsin Field Crops (Dewerff, Bick, Liesch, Nice, Renz, Smith, Werle) 2023 edition (the cover says 2023; the web file is dated Dec 2022) |
Silk feeders at pollination
Silk clipping only costs yield while the silks are fresh and pollen is shedding. Once the silks are brown it no longer matters. The sources below count beetles per plant or per ear, so read the unit before you compare.
| What and when | Threshold, as the source prints it | Source and date |
|---|---|---|
| Rootworm and Japanese beetle adults, silk clipping silking | Consider treating the adults when silks on several plants in each spot are clipped to within one-half inch of the tip of the husk before the silks brown. | UW Extension, Corn IPM Quick Guide: Corn Insect Pests of Wisconsin (L023) Feb 2000old (printed on the sheet as 2/00) |
| Beetles on silks pollination | Threshold guidelines: about 3 Japanese beetles per plant to cause economic damage (usually along field edges). On average about 5 to 6 rootworm beetles per plant to cause economic loss. Check that silks are being clipped to within ½ of the tip of the husk (unit as the post prints it).Only a problem during pollination. | UW Integrated Pest and Crop Management, Corn silk clipping insects and economic thresholds (Bryan Jensen) Jul 2020 (from the post's address) |
| Japanese beetle, adults tasseling and silking | Consider a foliar insecticide treatment during tasseling and silking if there are three or more beetles per ear, silks have been clipped to less than 0.5 inch, and pollination is less than 50% complete. Sample the field to see whether a border treatment is enough.The product table in the same guide gives the first two conditions only. | UW Extension A3646, Pest Management in Wisconsin Field Crops (Dewerff, Bick, Liesch, Nice, Renz, Smith, Werle) 2023 edition (the cover says 2023; the web file is dated Dec 2022) |
| Japanese beetle, adults tasseling and silking | Consider a foliar insecticide treatment during tasseling and silking if all three are true: 3 or more beetles per ear, silks clipped to ½-inch, and pollination less than 50 percent complete. It can still be applied to later plantings and to sweet corn at sensitive stages. Sample the interior of the field, since beetles clump along edges. | UW Integrated Pest and Crop Management, Japanese beetle scouting and thresholds for corn and soybean Jul 2012 (from the post's address) |
| Corn rootworm beetles pollination protection | Treat before pollination is 50% complete, when silk clipping is observed and five or more beetles are seen per plant. | UW Extension A3646, Pest Management in Wisconsin Field Crops (Dewerff, Bick, Liesch, Nice, Renz, Smith, Werle) 2023 edition (the cover says 2023; the web file is dated Dec 2022) |
| Japanese beetle and rootworm beetles on silks Iowa | Treat when all three are true. Japanese beetle: 3 or more beetles per ear, silks clipped to less than one inch, and pollination not yet complete. Rootworm beetles: 5 or more per ear, the same silk and pollination tests. In fields with adequate moisture the rootworm figure can go up to 15 beetles per ear.Counted per ear. The Wisconsin rootworm figures above count per plant. | Iowa State ICM News, Watch out for these silk feeding insects (Dean and Hodgson) Jul 9, 2024 |
| Rootworm beetles first week of pollen shed, South Dakota | An average of 5 or more beetles per plant in the first week of pollen shed. Or silks clipped to within one-half inch of the ear tip on 25 to 50% of scouted plants. Scout five plants in each of 10 places. Yellow sticky cards: two adult rootworms caught in one week means the plan for that field needs to change.No yield loss from silk clipping after the silks turn brown. | SDSU Extension, Monitor your corn for corn rootworm adults (Varenhorst and others) no date printed (copy read Oct 15, 2026) |
Rootworm, for planning next year
| What and when | Threshold, as the source prints it | Source and date |
|---|---|---|
| Rootworm beetles, to plan next year not a spray trigger for this crop | An average of 0.75 or more beetles per plant, and corn planted in the same field the following year, is the point to consider treatment for the following year.This is a count for deciding about next year's corn. It is not a reason to spray this year's silks. | UW Extension, Corn IPM Quick Guide: Corn Insect Pests of Wisconsin (L023) Feb 2000old (printed on the sheet as 2/00) |
Soybean
| What and when | Threshold, as the source prints it | Source and date |
|---|---|---|
| Soybean aphid through R5 | Treat when all three are true: an average of 250 aphids per plant, more than 80 percent of plants with aphids, and numbers increasing. R6: no economic threshold has been developed. Scout from mid to late June until R6.5. | UMN Extension, Scouting for soybean aphid reviewed 2023 (printed as reviewed in 2023) |
| Soybean aphid the same threshold, defended | 250 aphids per plant with more than 80% of plants infested and the population increasing, published in 2007. Treating below 250 per plant gave no yield gain in her trials, and a sliding scale that lowers it is not recommended. | Iowa State ICM News, Why use the economic threshold for soybean aphid (Hodgson) Aug 28, 2017 |
| Soybean aphid Wisconsin | Treat when approximately 80% of the field has reached an average of 250 aphids per plant and the population is actively increasing. Best response from treating between R1 and R5. Once pods reach full seed (R6) it is too late to protect yield and treatment is not recommended.Worded by field, not by plant; UMN above says more than 80 percent of plants. | UW Extension A3646, Pest Management in Wisconsin Field Crops (Dewerff, Bick, Liesch, Nice, Renz, Smith, Werle) 2023 edition (the cover says 2023; the web file is dated Dec 2022) |
| Japanese beetle soybean, by leaf area lost | Treatment is based on percent leaf defoliation, not beetles per plant: consider it at 30% defoliation pre-bloom and 15-20% from bloom to pod fill. | UW Extension A3646, Pest Management in Wisconsin Field Crops (Dewerff, Bick, Liesch, Nice, Renz, Smith, Werle) 2023 edition (the cover says 2023; the web file is dated Dec 2022) |
| Japanese beetle soybean, by leaf area lost | Consider treatment at 20% leaf defoliation for reproductive stage soybean, 30% for pre-bloom. Do not treat early below 20%, especially with twospotted spider mites present, because some pyrethroids can flare mites. | UW Integrated Pest and Crop Management, Japanese beetle scouting and thresholds for corn and soybean Jul 2012 (from the post's address) |
Soybean aphid check
Enter the average aphids per plant and the percent of plants with aphids.
Count adults and nymphs, including the small pale ones, from plants picked across the field and not the border. Do not count potato leafhoppers, whiteflies, cast skins or dead aphids (UMN). Small hot spots often collapse on their own.
About the old sheet
The UW corn insect sheet (L023) prints 2/00, February 2000, at the bottom. I kept its rows, in grey with that date, because they show how the numbers moved: black cutworm went from 5% damaged to 2-5% cut, and the silk-clipping count is now stated per ear for Japanese beetle. It also covers European corn borer and stalk borer, but those are worksheets where the answer depends on your yield, your price and what control costs, so I do not copy a single number from them. UW's current guide, A3646, has a stalk borer table (Table 2-8) built the same way; read it there.
Questions
What is the economic threshold for soybean aphid?
Iowa State and Minnesota give the same one: through R5, an average of 250 aphids per plant, more than 80 percent of plants with aphids, and numbers still increasing. All three must be true. UW A3646 words it as about 80% of the field at an average of 250 per plant and actively increasing, with R1 to R5 the best time to treat. Minnesota says no threshold has been developed for R6; A3646 says treating at R6 is not recommended. Sources: Iowa State ICM News, Aug 28, 2017; UMN Extension, reviewed 2023; UW A3646, 2023 edition.
How many rootworm beetles does it take to treat for silk clipping?
The sources do not agree on the unit, so I print each as written. UW Extension (Jul 2020) says about 5 to 6 beetles per plant. Iowa State (Jul 9, 2024) says 5 or more per ear plus silks clipped under an inch and pollination not done. SDSU says an average of 5 or more per plant in the first week of pollen shed. Silk clipping does not matter once the silks are brown.
Is the 0.75 beetles per plant number a spray threshold?
No. The old UW sheet (Feb 2000) uses 0.75 or more beetles per plant to decide whether to treat for the following year, when corn will follow corn in the same field. It is not a trigger to spray this year's corn.
Why is the UW corn insect sheet marked old?
Its own print date is 2/00, February 2000. UW has since published A3646, Pest Management in Wisconsin Field Crops (2023 edition), and Crops and Soils articles. For black cutworm, armyworm and silk feeders they give numbers that differ from the old sheet, so the old rows stay for comparison and the newer ones sit beside them. Where the old sheet is the only source I have, as for the next-year rootworm count, I say so.
What is the black cutworm threshold in Wisconsin?
A3646 (2023 edition) says treat when 2-5% of plants show cutting activity. UW Crops and Soils says the same, and works it out for you: 5 to 12 cut plants out of 250 scouted. The February 2000 UW sheet said 5% or more damaged with larvae 6th instar or smaller. Moth trap counts are not a reason to treat.
Why not print the European corn borer and stalk borer thresholds?
On the UW sheet they are worksheets, not single numbers: the answer depends on your yield, the price you expect, the cost of control and how many borers you count. I will not print a number that only holds for one set of inputs.
Related
Thresholds change with the pest, the crop stage and the year. Your county extension office knows what is local. This page lists counts, not products or rates. Something here that looks wrong: tell me.