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Corn
Volume 62 Number 17 Date 08/24/2017 CORN ROOTWORM - Beetle populations are the lowest since surveys for this pest began in Wisconsin in 1972. The annual survey completed last week found a pronounced decrease in adult rootworm counts in all nine crop districts as compared to 2016, with district averages ranging from 0.1 to 0.3 per plant. The very low 2017 state average of 0.2 beetle per plant is less than half of last year's average of 0.5 per plant. Only 23 of the 229 (10%) cornfields sampled this month had above-threshold averages of 0.8-2.9 beetles per plant, while 53 (23%) had below-threshold averages in the range of 0.1-0.7 per plant. No corn rootworm beetles were observed in 153 (67%) of the fields. Results of the survey indicate that adult rootworm pressure has been much lower than expected this season. The substantial decrease in beetle abundance may translate into fewer eggs being deposited into cornfield soils, and an overall lower risk of larval root damage next summer. EUROPEAN CORN BORER - Surveys show that larvae range in development from second- to fifth-instar in the central and northwest districts, as far north as Polk County. Larval infestations affecting 2-34% of corn plants were found in 8% of fields checked in the previous two weeks. Nearly all of the older, fourth and fifth-instar larvae present by late August will enter diapause and will not pupate until next spring. CORN EARWORM - The primary migration accelerated this week. Moderate to large flights of 40-111 moths per trap were reported from the Arlington, Cottage Grove and Ripon monitoring locations. A cumulative total of 288 moths were captured in 15 pheromone traps. Sweet corn growers should continue to follow corn earworm migration reports and maintain treatments as long as moth activity persists and green silks are available for oviposition. Counts for the week ending August 23 were: Arlington 111, Beaver Dam 16, Coon Valley 14, Cottage Grove 49, Hancock 2, Janesville 21, Madison airport 3, Manitowoc 0, Marshfield 17, Pardeeville 1, Ripon 40, Sun Prairie 3, Watertown 10, and Wausau 1. WESTERN BEAN CUTWORM - Cornfields surveyed in the central counties were 1-20% infested with 1-2 larvae per ear. The cutworms were mostly in the late instars and should enter the pre-pupal overwintering stage by September. This week's observations suggest that the increased pheromone trap counts documented in July and August were an accurate predictor of higher larval pressure in the field since infestations are relatively common in central Wisconsin, but also as far northwest as Barron County. Western bean cutworm larvae have been found at about 9% of the corn sites sampled this month. JAPANESE BEETLE - Adults are still numerous on corn silks in later-planted fields. Beetle emergence has peaked and much of the threat to the state's corn and soybean crops has passed, but scouting should continue in the eastern and northern areas, or in individual fields where pollination is incomplete and silk feeding remains a concern. Japanese beetle activity is expected to diminish by early September. -- Krista Hamilton, DATCP Entomologist 
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