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Nursery & Forest

Degree Days

Volume 65 Number 17 Date 08/27/2020

CORN ROOTWORM - The annual survey conducted from July 30-August 26 documented an increase in adult rootworm populations across much of Wisconsin, and the highest state average in five years. Average counts rose from 2019 levels in seven of the nine crop reporting districts, and remained unchanged in the east-central and northeast areas. The largest increase was recorded in the south-central district where the count escalated considerably from 0.5 beetle per plant last year to 1.3 per plant this season. Beetle pressure was also relatively high in the southwest and central regions (0.6-0.7 per plant).

In addition, above-threshold counts of 0.75 or more beetles per plant were found in 62 of the 229 (27%) fields surveyed, compared to last year's 27 fields (12%). The 2020 total count of 1,341 beetles was 47% higher than the 711 beetles counted in 2019. Seventy percent of this season's beetles were the northern corn rootworm, which has been the predominant species in the state for seven consecutive years.

Based on the overall increase in beetle pressure this summer, evaluating adult rootworm populations by early September is strongly advised. An average count of 0.75 or more beetles per plant in continuous corn indicates control should be considered to prevent root damage in 2021, either in the form of crop rotation, using a Bt-rootworm trait package, or applying a soil insecticide at planting. Planting continuous corn with the same trait should be avoided.

EUROPEAN CORN BORER - The second flight of moths continued at very low levels (≤ 11 moths) at a few black light trap locations. Meanwhile, surveys in corn found light infestations and larvae ranging from third- to fifth-instar in Brown, Clark, Door and Oconto counties. The European corn borer treatment window has closed for the season across the southern and central counties.

WESTERN BEAN CUTWORM - The June-August trapping program collected a cumulative total of 3,783 moths in 58 traps, or 65 per trap. This average ties 2019 for the second highest count since surveys began in 2005 and is well above the 16-year average of 28 moths per trap. The survey record of 79 moths per trap was set in 2010. Localized damaging populations resulting from this summer's relative large flight have been observed in scattered cornfields in the west-central and central counties this month. In all instances, the o.75 to 1.5-inch caterpillars were located in the ear tips where control is ineffective. Most larvae are now in the late instars and should enter the pre-pupal overwintering stage by mid-September.

CORN EARWORM - Late-season moth migrations escalated this week. The DATCP pheromone trapping network captured 645 moths in 18 traps between August 20 and 26, for a cumulative total of 1,650 moths since mid-July. The high count for the reporting period was 184 moths at Arlington in Columbia County. The latest activity signals that fresh market and processing sweet corn remains at risk of infestation and should be monitored until harvest. Counts for the week ending August 26 were: Arlington 40, Arlington North 184, Beaver Dam 153, Bristol 27, Coon Valley 6, Cottage Grove 3, Hancock 3, Madison airport 11, Mayville 115, Ripon 81, Sun Prairie 15, Sun Prairie North 6, and Wausau 1.

-- Krista Hamilton, DATCP Entomologist