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Each growing season, DATCP Pest Survey field specialists survey alfalfa, corn, soybeans, and small grains across the state to collect data on the leading economic pests of concern to Wisconsin crop producers. Our crop surveys also target invasive national priority pests that are currently not known to occur here or have limited distributions in the state. This work is funded in part by the USDA Cooperative Agricultural Pest Survey (CAPS) Program and is conducted from April through October.
To accomplish our annual survey and detection priorities, we enlist the help of volunteer cooperators to set insect traps and report their counts on a weekly basis. Insect trapping data supplied by these networks indicate current risk levels and help growers prepare for pest threats. Cooperators represent a variety of agricultural backgrounds, including agronomists, crop consultants, farmers, fruit growers, gardeners, and UW-Extension staff. In 2024, our eight pest monitoring networks were made possible by the support of 87 cooperators in 48 of Wisconsin’s 72 counties.
This end-of-season issue of Field Notes summarizes our corn, soybean, and wheat pest survey results and provides analysis of our trapping network data. Additional reports featuring our forest, fruit, and vegetable pest survey results will be published later this month. We wish to offer our gratitude to all the cooperators who contributed data to our agricultural pest surveys in 2024.
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DATCP’s historic, 82-year-old annual survey to assess the fall overwintering larval population of European corn borer (ECB) in Wisconsin was completed two weeks ago. Several important updates were made to the survey methods this season. Dating back to 1942, this long-term effort has traditionally included 200 or more randomly selected grain corn fields sampled in September for second-generation ECB larvae and damage. However, with recent confirmation of Bt-resistant corn borer populations in Connecticut and parts of Canada, determining the Bt status of fields in which ECB larvae are found has become more important for detecting resistance development. The updates made this season include adding known non-Bt sites to the sampling plan, conducting in-field Bt testing at sites with ECB larvae (using Agdia ImmunoStrip tests), reducing the number of randomly selected survey sites, and collecting larvae for lab analysis of Bt resistance.
For 2024, we asked agronomists, crop advisors, and farm managers to volunteer non-Bt grain corn fields for sampling. The survey included 145 randomly selected fields and 17 volunteered non-Bt fields. European corn borer larvae were found in 35% of the non-Bt fields and only 5% of the random fields. The state average count in the 145 random fields was 0.03 borer larva per plant, or three borers per 100 corn stalks, which is similar to the 2023 survey average of 0.02 borer per plant and only slightly above the all-time lowest average of 0.01 larva per plant recorded in 2018 and 2019. The average in the non-Bt fields was 0.12 larva per plant, or 12 larvae per 100 corn stalks. Of the combined 162 fields surveyed, only 8% had detectable ECB larval populations and a single field in Grant County had an economic average count above the 1.0 larva per plant threshold (indicated by the orange circle in the map below).
Results of the survey confirm that ECB populations remain very low in Wisconsin, where Bt corn accounts for about 82% of all corn acres in the state (USDA NASS, 2023) and continues to have a “halo effect”. As expected, ECB was more prevalent in the non-Bt fields than in the randomly selected fields, but the average ECB count in the volunteered non-Bt fields (0.12 larva per plant) is still far below the established economic threshold of 1.0 larva per plant. Larvae collected from infested fields have been sent to the University of Minnesota for Bt resistance testing and results are pending. A similar survey is planned for 2025, though our program hopes to increase the number of volunteered non-Bt fields sampled next season. |
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Beetle pressure in Wisconsin corn fields was generally moderate this season and similar to levels recorded in 2023. Surveys in 229 fields in August found a state average count of 0.5 beetle per plant, the same average as last season. Average beetle counts decreased in five of the state’s nine crop reporting districts and increased in the other four districts, as shown in the map and table below. The highest averages were recorded in the southwest (0.8 beetle per plant), east-central (0.7 beetle per plant), and northwest (0.6 beetle per plant) regions, though only the southwest district average beetle count exceeded the 0.75 beetle per plant economic threshold. Corn fields with high or above-threshold beetle populations comprised 15% of this year’s 229 sites, a decrease from 18% of last year’s sites.
In addition, the 2024 total count of 1,151 rootworm beetles on 2,290 corn plants sampled statewide was very comparable to the 1,174 beetles tallied in 2023. While the overall corn rootworm count was not surprising, the season’s most unexpected finding was the shift in beetle composition in favor of the western species. Fifty-two percent (600 out of 1,151) of this season's beetles were the western corn rootworm, 45% were the northern species (523 out of 1,151), and 3% were the southern species (28 out of 1,151). The western corn rootworm has not been the predominant species in the state since 2013.
Although the survey documented the continuation of mostly low and moderate beetle populations in Wisconsin corn fields in 2024, areas of high rootworm pressure were found in the southern, east-central, and northwestern districts. Effective corn rootworm management in these regions will continue to require a multiyear plan emphasizing crop rotation (out of a continuous corn cropping system), selection of a dual or stacked mode of action Bt-rootworm (RW) and RNAi corn seed product and, in some situations, the use of a soil-applied insecticide at planting—usually for corn products without Bt-RW protection. Crop rotation remains the most effective regulator of corn rootworm populations by breaking the lifecycle. |
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Moth counts registered during the June to mid-August flight period increased for the fourth consecutive year to set a 20-year survey record. The 2024 trapping program captured an average of 227 moths per trap (15,685 moths in 69 traps), surpassing the previous record of 222 moths per trap (9,351 moths in 42 traps) set in 2023, and nearly quadrupling the 20-year average of 57 moths per trap. The highest individual trap catch for the 11-week survey period was 1,834 moths at Crivitz in Marinette County, while one-third of the sites recorded cumulative counts above 250 moths per trap.
This season’s extensive network of 69 traps in 30 counties helped identify high moth count areas in the western and northeastern regions of the state, beyond the traditional WBCW high-pressure zone in central Wisconsin. Fall surveys found corresponding hot spots in Buffalo, Jackson, Sauk, and Trempealeau counties, where as many as 15-21 larvae were counted per 25 corn ears sampled in each field. Overall, western bean cutworm caterpillars were observed in ear tips in 8% of the 229 corn fields sampled in September and early October. |
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DATCP’s pheromone trapping network collected a cumulative total of 8,849 corn earworm moths in 14 traps this season (632 moths per trap average), with the largest flights recorded August 23 to September 5. Flights peaked at 126 moths per night near Beaver Dam in Dodge County, 95 moths per night at Ripon in Fond du Lac County, and 56 moths per night near Sun Prairie in Dane County during this timeframe.
While this year’s total count was 37% lower than the 13,996 moths collected in 12 traps in 2023 (1,166 moths per trap average), the migration still posed a late-summer threat to fresh market and processing sweet corn. Corn earworm larvae were observed in 9% of the grain corn fields sampled as part of the fall corn pest survey and were slightly more prevalent than western bean cutworm larvae. The trap location with the season’s highest cumulative catch was Beaver Dam in Dodge County, where 2,204 moths, or one-quarter of the 2024 total moth catch, were collected. |
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An exceptionally wet spring, late weed control, and substantial May moth flights pointed to a high potential for true armyworm problems at the start of 2024. Survey traps began collecting moths on March 28 and the first large flights occurred by May 9. Signs of first-generation armyworm activity became noticeable in corn and wheat by early June, but counts of larvae and damaged plants were well below economic thresholds. The 2024 trapping network captured a cumulative total of 5,597 moths in 45 traps (124 moths per trap average) March 28-August 8. Peak first-generation moth flights were recorded May 3-9, while the summer flight showed no discernable peak. True armyworm feeding was inconspicuous after late June and the second-generation of larvae in July was not destructive.
In addition to the true armyworm network, a new fall armyworm trapping network was established with assistance from UW-Extension to provide advanced warning of potential late-season larval infestations in alfalfa, field corn, sweet corn, and pasture grasses. From July 26-September 5, cooperators set 31 pheromone traps and captured 4,743 moths, for an average of 153 moths per trap. Moth activity peaked August 30-September 12 at most monitoring locations. Despite high weekly counts above 250 moths per trap at a few sites, damaging fall armyworm populations were not observed or reported. |
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Soybean aphids | K. Hamilton DATCP
 
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Soybean aphid populations remained low throughout July and peaked in mid-August, after the optimal control window had closed. Surveys for economic soybean pests during July found aphid infestations in the range of 1-20 aphids per plant on less than 50% of the plants throughout the field. Only two of the 168 fields sampled July 5-31 had an average count exceeding 25 aphids per plant (see map below). The highest count recorded was 76 aphids per plant on 84% of the plants, in Waushara County.
By mid-August, scattered fields across southern and central Wisconsin had developed moderate to heavy aphid pressure approaching the 250 aphid-per-plant threshold, but most soybeans had already reached the R6 (full seed) growth stage when insecticide treatment does not produce a yield benefit. Some replanted and late-planted soybean fields that were still at the R3-R5 stages in August qualified for treatment. |
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Surveys in wheat fields across the southern and east-central areas of the state in June found a marked increase in foliar disease symptoms, due in part to prevailing wet spring conditions. DATCP specialists sampled 61 fields in Brown, Calumet, Columbia, Dane, Dodge, Door, Fond du Lac, Kewaunee, Manitowoc, Sheboygan and Winnebago counties June 6-21. In each field, a sample consisting of 20 wheat heads and leaves was collected for disease testing at the Plant Industry Bureau Laboratory.
The map below shows the results of disease testing. Fungal diseases were prevalent in the surveyed fields, particularly Fusarium head blight, which was found in nearly half of the fields checked (27 fields). These findings align with UW reports indicating the reappearance of Fusarium head blight this season for the first time in three years: Wheat Diseases Resurge in Wisconsin (midwestfarmreport.com). Other diseases detected on the wheat samples included Alternaria head mold (seven fields) and rust (10 fields). |
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All samples tested negative for the wheat blast pathogen Magnaporthe oryzae Triticum pathotype, a national priority pest predicted to spread through wheat-growing regions of the world under climate change conditions. Wheat blast originated in Brazil and is currently found in several countries in South America, as well as in parts of Africa and Asia. Where this disease occurs, infection can be severe with up to 100% yield loss. DATCP targeted wheat blast as part of a USDA APHIS early detection survey cooperative agreement. |
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