Window for pest damage to crops now open

Krista Hamilton

Crops across Wisconsin are emerging earlier compared to last year and the pest population that impacts those plants are showing up on schedule.

Early June heat accelerated crop emergence and growth across Wisconsin. Afternoon temperatures were the warmest of the year so far, with highs on June 2 reaching or exceeding 90°F at Appleton, Eau Claire, Racine, Wausau and many other locations. A daily-record high of 93°F was set in Milwaukee.

The very warm and humid atmosphere on Tuesday also provided a favorable environment for storms that became severe, producing damaging winds, large 2-inch hail, and downpours across central and portions of southern Wisconsin.

Rainfall was otherwise scattered and light during the week, and the weather was suitable for gardening, weed management and other fieldwork. Alfalfa producers continued harvesting the first crop and soybean planting reached 88% complete, with 53% of acreage emerged. Crop prospects generally continued to improve with the heat, and the latest USDA NASS report rates 82-83% of the state’s corn, oats and soybeans in good to excellent condition.

The spring flight of the European Corn Borer moth began by June 3 with the first reported moth capture in the Columbia County black light trap.

Corn pests on the move

The primary damage period for corn is now open. Much of the state's corn acreage is under a low threat of larval infestation of black cutworm this year based on early field preparation and the relatively late arrival of significant moth flights. However, localized infestations are still possible this month. Routine inspection of emerging corn (including Bt hybrids) for larvae and cut plants is advised until the five-leaf (V5) stage.

Degree-day accumulations across southern and central Wisconsin have surpassed the 374 heat units (modified base 50°F) required for spring moth emergence of the European corn borer (ECB). The first ECB moth of the season was collected in the Columbia County black light trap site in the past week. Egg laying is beginning, and the spring flight is expected to peak by June 11 in advanced southern areas.

Based on the European corn borer phenology model, the majority of moths should emerge by June 11 in advanced southern areas and June 19 in the central counties. Egg laying is starting in areas of the state where 450 degree days (modified base 50°F) have accumulated, such as Beloit, Madison, La Crosse and Lone Rock.

The primary damage period for seedling corn is now in progress and will extend through mid-June this year. No larval infestations were observed in corn surveyed May 28-June 3. A rescue treatment is justified if more than 3% of plants are damaged and larvae are still present in the field. Spot treatment is an option for sites with patchy damage.

Small true armyworm caterpillars ranging in length from ½-¾ inch are fairly common in alfalfa sweep net collections. Based on this observation and the moderate local flights of 40-60 moths reported during the previous two weeks, more concentrated scouting of corn and wheat should begin next week.

Longitudinal leaf streaks indicative of slug activity are also evident in damp cornfields. These mollusks become prevalent during periods of wet weather and are usually most damaging in no-till or reduced-till systems and very weedy corn, where surface residue and high moisture favor their development. Corn in the V4 stage or younger is particularly vulnerable to slug feeding.

Larvae of the stalk borer will begin migrating from grassy areas into corn in the next two weeks. The recommended scouting procedure is to spot-check the marginal 4-6 rows for plants with holes in leaves, wilted whorls and other signs of damage starting at 1,400 degree days (base 41°F). Control measures may be in order for corn fields with infestation rates in the range of 5-10%.

Alfalfa weevil

Alfalfa Weevils

Surveys indicate that leaf feeding damage inflicted by alfalfa weevils in first-crop alfalfa remains well below the 40% threshold, but leaf tip damage should intensify next week in uncut alfalfa fields. It will be particularly important to scout remaining first-crop alfalfa in the week ahead.

Larval counts for alfalfa weevils are low for early June. Surveys in western Wisconsin alfalfa, including Chippewa, Dunn, Eau Claire, Pierce and St. Croix counties, found 0-51 larvae per 100 sweeps, and an average of 17 per 100 sweeps. Alfalfa in Adams, Green Lake, Fond du Lac, Juneau and Monroe counties also had low counts, with a range of 6-41 per 100 sweeps.

Alfalfa weevil leaf injury is evident on these alfalfa plants.

In Calumet, Manitowoc and Winnebago counties in eastern Wisconsin, egg hatch had just begun as of May 29 and surveys found no more than 10 larvae per 100 sweeps. Leaf tip feeding was below 20% in all sampled fields, but harvesting the first crop in the week ahead will be important for avoiding damage by the larger late-stage weevil larvae. Scouting is recommended until new growth of the second crop is established.

Arrival of soybean aphids

The spring dispersal of winged aphids to soybeans is likely to begin in the week ahead. Currently over 53% of the state's soybean acreage has emerged, 18 days ahead of last year and six days ahead of the 5-year average.

A soybean aphid and nymph are busy at work on the underside of soybean leaves.

Persistent moist soils in eastern Wisconsin this spring are extremely favorable for slug activity. Spot-checking newly emerged soybeans for feeding scars on the hypocotyls and cotyledons, as well as for distorted or tattered unifoliate leaves, is recommended. Stand losses from slugs usually occur when soils are wet and seed furrows do not close fully during planting, creating a ?highway? for the slugs to feed on and destroy the growing point of consecutive seedlings. A sample size of 20 plants in each of five areas of the field is suggested. No specific thresholds have been developed for slugs in soybeans, and spot application of a molluscicide bait to problem areas should be considered only as a last resort for severe infestations. Growers must follow labeled use rates and distribute the product evenly over the infested area.

Hamilton is an entomologist with the WI Dept. of Ag, Trade and Consumer Protection