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Intermittent rain and heat persisted in Wisconsin, favoring summer crop growth. Afternoon high temperatures in the 80s to around 90°F prevailed, and weekly average readings were within 4°F above normal for mid-July. Rainfall soaked parts of the state for several days as the season’s active heavy precipitation pattern continued. The highest amounts (2.0-5.0 inches) fell in the southwest and northwest areas. Platteville in Grant County collected 3.27 inches July 11, while Solon Springs in Bayfield County tallied 5.11 inches July 15. The precipitation maintained adequate soil moisture reserves for crops, but surplus rain in parts of north-central and northeastern Wisconsin in recent weeks has left many fields with standing water.
Meanwhile, crop development continued on pace with the five-year average. At the start of the week, 13% of the state’s corn acreage was in the silking stage, while 44% of this season’s soybean crop was flowering and 5% were setting pods. Crop conditions held steady with good to excellent ratings ranging between 72% and 81%. The latest USDA NASS crop progress report shows 78% of the corn crop, 81% of oats, 74% of soybeans, and 80% of hay as good to excellent.
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Mid-season soybean surveys across southern Wisconsin indicate that advanced fields have reached the R2-R4 (full bloom to full pod) growth stages and insect pressure is increasing. Soybean aphid counts are still very low overall, but a small percentage of fields have moderate counts of 25-50 aphids per plant. The typical average is less than 10 aphids per plant, based on observations in 125 soybean fields sampled July 3-16. Aphids have not yet dispersed throughout entire fields and are generally colonizing fewer than 50% of the plants fieldwide.
As populations begin to increase more rapidly toward the end of the month, growers and crop advisors are reminded that the treatment threshold for soybean aphid remains at 250 aphids per plant on 80% of the plants. Insecticide application is not economical until this threshold is met. Aphid counts were still far below this level in the soybean fields surveyed by DATCP in the past two weeks. |
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In addition to aphids, soybean fields are also showing 1-10% of plants with light to moderate leaf injury caused by Japanese beetles, bean leaf beetles, green cloverworms, silver-spotted skipper larvae, and other defoliators. The sampling method for defoliators is to select 10 plants throughout the field, choosing a trifoliate from the upper, middle, and lower canopy on each plant, for a 30-leaf sample. Compare the 30 leaflets with an online defoliation estimating guide to determine the average percent defoliation. Defoliation that meets the 20% threshold between the bloom and pod-fill stages and 30% in the pre-bloom soybean may warrant control. Scouting several areas in the field interior, in addition to field edges where Japanese beetles are most numerous, is required for accurate assessment. Routine scouting will be most critical in the next three weeks as soybeans advance to the later reproductive growth stages.
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Soybean field scouting in late July and August should include looking for two newer soybean pests: soybean gall midge and soybean tentiform leafminer. Neither insect has been observed in Wisconsin to date, but both occur in neighboring Minnesota and soybean gall midge (SGM) was also recently found in eastern Iowa.
Earlier this week, SGM was confirmed in Dakota County, Minnesota, just across the river from St. Croix and Pierce counties in Wisconsin. According to the Soybean Gall Midge Alert Network, the current known distribution of SGM includes 180 counties in seven Midwestern states (Iowa, Kansas, Minnesota, Missouri, Nebraska, North Dakota, and South Dakota).
In eastern Iowa, the recent SGM finds in Tama and Poweshiek counties were identified based on wilted and dead soybean plants in the field margins. Adult gall midges are weak fliers that emerge and fly to the nearest soybean plants to lay eggs—usually infesting plants along field edges next to last year’s soybean fields. Symptoms to scout for are stems with blackened or discolored bases and deformed, stunted, or wilted plants. Infested plants become brittle and break easily along the stem. The larvae can be found by peeling back the epidermis tissue, as shown in the photo below. |
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 Soybean gall midge larvae | Justin McMechan UNL
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A second new soybean pest to be alert for in late July and August is the soybean tentiform leafminer (STLM). Similar to SGM, infestations of this insect also develop along field edges, especially near wooded areas. Signs of STLM on soybean leaves are raised, blister-like mines that form in a speckled oval pattern on the upper surface (refer to leaf at the left below). On the reverse side of the leaf, the larvae feed internally between the tissues and form distinctive hollowed-out leaf blotches or mines (refer to leaf on the right). |
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 Soybean tentiform leafminer | UMN Extension
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Several mines can develop per leaf and heavy mining may impact soybean productivity by reducing photosynthesis. Any suspected SGM or STLM infestations can be reported to the DATCP Pest Survey Program by emailing a photo to Krista Hamilton at Krista.Hamilton@wisconsin.gov. |
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Early, heavy corn earworm moth flights this season are prompting Wisconsin sweet corn growers to begin management programs about a month ahead of the usual schedule. Moths started arriving in the state during the first week of July and monitoring sites in Dane and Rock counties have registered incoming flights of 100-276 moths per trap per week. The highest weekly count to date is 276 moths near Clinton in Rock County, and the current cumulative total is 1,402 moths in 13 traps.
Corn earworm is a perennial migratory pest in Wisconsin, though the main influx of moths typically does not occur until mid-August. This season’s early arrival means that protective treatments will be needed earlier than normal to keep the larvae from infesting corn ears. Sweet corn fields are susceptible to corn earworm egg laying as long as the silks are green and exposed. Delaying control by even one day during heavy flights can lead to infestations affecting 10%-15% of corn ears (UW Vegetable Entomology). Treatment is warranted when pheromone traps begin registering 10 or more moths per night for three consecutive nights, with reapplication every three to four days until the silks turn brown. Corn earworm trapping network participants are reminded to replace lures on a weekly basis.
Moth counts will be posted each Thursday for the next nine weeks on DATCP's Corn Earworm Network page and on the Great Lakes and Maritimes Pest Monitoring Network site. |
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In contrast to the corn earworm, emergence of western bean cutworm moths is off to a slow start this season. Counts have been low since the annual flight began a month ago. DATCP’s monitoring network captured a total of 139 moths in 59 traps (2.4 per trap average) July 11-17, which is lower than the 82 moths in 31 traps (2.6 per trap average) collected the week before and far below the 2,805 moths (41.3 per trap average) captured during the same week last season. The highest individual trap count recorded as of July 17 is just 32 moths near Spring Green in Sauk County.
Although counts are still low for mid-July, the annual flight should peak next week across much of southern Wisconsin and in areas near La Crosse. Based on the western bean cutworm degree-day model, peak moth activity, or 50% emergence, coincides with the accumulation of 2,704 degree days (refer to the modified base 38°F column in DATCP's degree day table). |
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The window of peak moth activity also marks the primary egg laying period for western bean cutworm. The opportunity to scout for egg masses is quickly passing in the south but will remain an option through the first week of August in central and northern Wisconsin. For these areas, the bulk of this year’s western bean cutworm eggs will be laid in pre-tassel and pollinating corn in the next two weeks. Scouting requires checking 20 consecutive plants in five separate areas of the field for egg masses laid on the upper surface of the top leaves. The treatment threshold has been met if egg masses or small larvae are found on 5% or more of the plants. |
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