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Corn
Volume 64 Number 17 Date 08/22/2019


CORN ROOTWORM - Surveys in the last three weeks (August 1-21) have found variable but generally low beetle populations, with district averages in the central and northern areas nearly equal to last season's very low counts (<0.2 per plant). Averages in south-central and southwestern Wisconsin have shown a noteworthy increase, from 0.3 in 2018 to 0.5 per plant this year. The preliminary state average of 0.3 beetle per plant is slightly higher than the 0.2 survey averages in 2018 and 2017. An average of 0.75 or more adult corn rootworms per plant in continuous corn indicates control should be considered to prevent root damage in 2020, either in the form of crop rotation, using a Bt-rootworm hybrid, or applying a soil insecticide at planting. Beetle populations exceeding this threshold have been recorded in 24 (12%) of the 201 fields surveyed this month, represented by orange circles on the map below. The annual survey will be finalized next week.

CORN LEAF APHID - Colonies of this aphid are appearing on corn ears and leaves in southern and western Wisconsin. Corn leaf aphids usually do not interfere with pollination unless they develop early and populations grow rapidly, and a large percentage of corn tassels become saturated with aphids and their honeydew secretions.

WESTERN BEAN CUTWORM - Moth counts have declined at most monitoring locations. The traps in Door and Kewaunee counties continued to register moderate captures of 17-55 moths per trap this week, while counts at nearly all other sites fell to near zero for the reporting period. As of August 21, the state cumulative total is 3,582 moths in 56 traps (60 per trap average). The highest individual count for the 10-week monitoring period is 405 moths near Princeton in Green Lake County.

EUROPEAN CORN BORER - The treatment window for second-generation larvae has closed near Beloit, Eau Claire, La Crosse, Lancaster and Madison, but will remain open for 1-2 more weeks in parts of central and northern Wisconsin. Final inspections should be performed before degree day accumulations surpass 2,100 (modified base 50F) and larvae begin boring into corn stalks.

CORN EARWORM - A locally significant flight of 55 moths per trap was reported from the Beaver Dam (Dodge County) monitoring location, while the 13 other sites collected no more than 16 moths per trap during the week. A cumulative total of 634 moths have been captured in 15 pheromone traps to date. Sweet corn growers should continue to follow CEW migration reports and maintain treatments as long as moth activity persists and green silks are available for oviposition. Counts for the week ending August 22 were: Arlington 1, Beaver Dam 57, Bristol 5, Coon Valley 7, Cottage Grove 4, Janesville 0, Madison North 5, Marshfield 0, Mayville 16, Pardeeville 0, Ripon 11, Sun Prairie 0, Watertown 4, and Wausau 0.

-- Krista Hamilton, DATCP Entomologist