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Locally severe thunderstorms swept across much of Wisconsin on May 21, with several confirmed tornadoes and reports of widespread wind damage and power outages. The five tornadoes touched down at locations in Buffalo, Marathon, and Trempealeau counties, according to the National Weather Service (NWS). Throughout the state, winds led to structure damage and trees falling on power lines as wind gusts reached more than 80 mph. Utility providers reported a combined total of about 20,000 customers without power across the state after the storm passed.
Tuesday evening's storm also brought heavy rainfall of 1-3 inches to parts of central and southern Wisconsin. The Stevens Point and Wisconsin Rapids areas received about 2.6 inches of rain, the NWS reported. The showers and wet soils interrupted fieldwork in some areas, but the rain was welcomed overall as summer crop emergence begins to ramp up.
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Damage by alfalfa weevil larvae has increased in the past week and will become even more pronounced in the week ahead, especially if predicted rain delays harvest of first-crop alfalfa. Estimates of leaf tip feeding recorded during this week’s alfalfa surveys mostly varied from 10-20%, though a few scattered fields had higher larval pressure (3.9-8.6 per sweep) and showed above-threshold levels of tip defoliation ranging from 40-70%. |
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 Alfalfa weevil feeding damage | K. Hamilton DATCP
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The peak alfalfa weevil damage period occurs in Wisconsin between 600 and 800 degree days (sine base 48°F) and is now beginning in Grant, Iowa, and Rock counties, as shown in the DATCP degree-day table. With the first alfalfa harvest less than 10% complete and several days of rain in the forecast for late May, routine scouting is strongly recommended for unharvested first-crop alfalfa fields next week and until new growth of the second crop is established. Alfalfa stands with 40% or more of the leaf tips throughout the field showing weevil feeding would benefit from an early cutting (for fields more than seven days from the scheduled harvest date). |
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The primary black cutworm damage window for seedling corn has opened in southern and western Wisconsin. Leaf feeding and possible stem cutting from larvae produced by the mid-April and early May significant moth flights should be underway. Localized infestations could develop in the next two to three weeks and scouting emerging corn and soybean fields is strongly recommended.
While black cutworm outbreaks are sporadic and unpredictable, the key factors that influence outbreak potential are late tillage and planting, which allow unworked fields to develop weedy pre-plant conditions that provide preferred egg laying habitat. Fields with excess crop residue (i.e., reduced-till and no-till), low spots in a field, and corn planted into soybean residue are also attractive egg laying sites.
Heavy moth flights in April and May can further contribute to the risk of spring cutworm outbreaks. DATCP’s black cutworm counts have generally been lower this season than last year’s levels and the long-term average. The 76 survey traps have collected a cumulative total of 1,682 moths since March 31, for an average of 22 moths per trap. This season’s average is well below the 10-year average of 36 moths per trap and the 72-per-trap average recorded in 2017, the last time serious black cutworm infestations were documented in the state. It is also far less than the 2023 count of 2,307 moths in 50 traps, or 46 moths per trap.
All corn acreage should be closely monitored for cutworm feeding or below-ground tunneling injury from emergence until the five-leaf stage. It is important to be aware that Bt hybrids and corn treated with seed-applied insecticides are at reduced risk of damage, but the larvae are still capable of cutting young plants. The economic threshold for black cutworm is reached if 3% of plants have been cut and larvae are still present in the field. |
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A sharp increase in codling moth emergence has been reported from Wisconsin apple orchards. Twelve of 27 cooperator sites captured above-threshold counts of 5-34 moths this week. Most southern and central monitoring locations (those that do not use mating disruption) have recorded a sustained flight and biofix as of May 23.
Apple orchards with a history of high codling moth pressure that register a large first flight of greater than 10 moths per trap per week are candidates for a first larvicide application at 250 degree days (base 50°F) from the biofix. At this time of year, 250 degree days are equal to about 14-21 calendar days, depending on location.
For orchards where the spring codling moth flight has been inconsistent, growers may delay applications until 350 degree days after the biofix date, when approximately 60% of egg hatch will be targeted by one application. With the consistent sustained flights observed this spring, many orchards will likely opt to apply a larvicide at the earlier 250 degree days target point.
As a reminder, density of one trap per 2.5 acres, or one per five acres where blocks are uniform in size, shape and topography, is suggested for accurate codling moth monitoring. |
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Home gardeners and potato growers can expect to see overwintered Colorado potato beetles on plants by the first week of June. Managing these early adults will lower egg laying potential and help reduce crop damage. For small to moderate-scale farms, control methods such as planting a trap crop two weeks in advance of the main crop and destroying the trap crop with a mechanical method, such as a soil chopper, can be effective in combination with other tactics. Crop rotation and relocating this year’s crop at least ½ mile from last year’s field (if practical) can also help reduce early-season pressure. This insect has developed resistance to several insecticides and effective control requires a varied approach that uses non-chemical methods.
Another familiar vegetable pest now appearing in gardens is the imported cabbageworm. Adult butterflies have been active since late April, and small larvae have become noticeable on cabbage transplants. Growers are encouraged to inspect gardens and larger cabbage plantings regularly for the yellow eggs laid on plants and velvety green caterpillars with a faint yellow longitudinal stripe. The economic threshold for this pest in cabbage is 30% infestation at the transplant-to-cupping stages.
Routine inspection of seedling and recently transplanted vegetables for evidence of black cutworm feeding is also encouraged since larvae are reaching the most damaging plant-cutting stages. Cutworms feed on the stems of young plants at the soil line and can be destructive where transplants are planted through black plastic or a similar weed barrier. These barriers provide a protective covering for cutworms, making them more difficult to control. Beans, cabbage, carrots, celery, corn, lettuce, peas, peppers, potatoes, and tomatoes are all at risk of injury. |
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