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Farmers should remain vigilant for crop pests

Colleen Kottke
Wisconsin State Farmer
Corn earworm larva snack on kernels of corn in a Wisconsin farm field.

Cooler weather with lower humidity levels maintained favorable yield prospects for summer crops in Wisconsin.

Wisconsin Department of Agriculture, Trade and Consumer Protection Entomologist Krista Hamilton says highs temperatures were below normal for early August and ranged from the 60s to lower 80s. Overnight lows on August 4 dropped to the upper 30s across the north-central region with 40s and 50s elsewhere.

Aside from a few stray showers and a record rainfall of 4.8 inches set at Milwaukee on August 3, much of the state remained dry.

With many crops developing ahead of schedule, dry conditions and lack of extreme heat helped farmers and producers get out in the fields to harvest alfalfa, potatoes, small grains and sweet corn.

According to Hamilton's latest Wisconsin Pest Bulletin, soybean aphid densities have remained generally low.

"Although a few soybean fields have developed populations of 50-150 aphids per plant since late July, most fields have average counts below 20 per plant, Hamilton reported, adding that final aphid treatments, if required, must be applied before the R5.5 (mid-seed) growth stage to provide any economic benefit.

Adult Japanese beetles continue to cause variable damage to soybeans throughout much of the state.

Soybean growers have also seen increasing defoliation by grasshoppers, green cloverworms, Japanese beetles, leafrollers, redheaded flea beetles and stink bugs. Hamilton said the damage is especially noticeable around the perimeters of fields.

"A combined defoliation rate exceeding 20% for soybeans in the seed-filling stages may justify treatment if the insects are actively feeding and damage is expected to intensify," she noted. 

Although the emergence of Japanese beetles has peaked for the season, Hamilton says that adults continue to cause variable damage to soybeans throughout much of the state.

Defoliation levels in nearly all fields surveyed since late July have been below the 20% economic threshold for soybeans in the seed-filling stages, but economic defoliation ranging from 20-60% has been observed at a few southwestern sites (Crawford, Grant, Lafayette, and Richland counties), as well as in one Clark County field.

Pressure from Japanese beetles has also been reported in cornfields in Crawford, Grant, Green, Lafayette, Dunn and Pepin counties in the southwest and west-central districts where the insect is still common on silks in cornfield margins.

"As a reminder, a fieldwide average of three or more beetles per ear is considered high and may be a concern for fields not yet pollinated," Hamilton said.

A significant flights of 78 moths was registered at Arlington in Columbia County, while 12 other monitoring sites for corn earworm captured fewer than 10 moths for the week ending August 6.

Although the corn earworm flights recorded over the last four weeks have not been especially large (with the exception of the Arlington flights), fresh-market sweet corn growers should continue to monitor silking sweet corn fields and follow CEW flight reports through early September.

Jamilton reported that summer moth emergence for the European corn borer has peaked across the southern half of the state. While larval development is variable at this time, with 1st to 4th-instar caterpillars observed in the past week she recommended that controls directed against second-generation larvae must be applied during the period after egg hatch and before the caterpillars bore into corn stalks and ears, prior to the accumulation of 2,100 degree days (modified base 50°F).

"The treatment window for second-generation larvae will close by August 20 in southern Wisconsin," she reported.