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Corn
Volume 62 Number 16 Date 08/17/2017 CORN ROOTWORM - The 2017 rootworm survey found the lowest beetle population in the 46-year history of the annual count, based upon observations in 229 cornfields from July 31-August 16. The state average of 0.2 beetle per plant is less than half that of last season and the lowest since Wisconsin rootworm beetle surveys began in 1972. District averages were uniformly low at 0.1 to 0.3 beetle per plant. The most substantial reductions in beetle abundance occurred in the north-central and northeast areas, where average counts declined from 0.7 beetles per plant last year to 0.2 per plant in 2017. A significant decrease from 0.7 to 0.3 beetles per plant was also recorded in the southwestern counties where beetle populations have historically been the highest. Economic populations of 0.75 or more beetle per plant were found in only 11% of the 229 fields sampled, and no beetles were found in 67% of the fields, which is an unusually high percentage. The map below summarizes this year's unexpected survey findings. WESTERN BEAN CUTWORM - Infestations have been observed in the last two weeks at sites in Adams, Barron, Dunn, Jackson, Marquette and Sauk counties, where 2-10% of ear tips were infested with one or two larvae. An exceptional field in Columbia County was estimated to have 75% of the ears infested with small caterpillars. Most of the larvae have reached the intermediate instars by now and should enter the pre-pupal stage before the end of the month. A few late moths are still being captured in eastern and northern pheromone traps, but the flight has effectively ended. EUROPEAN CORN BORER - Second-generation larvae range from first- to third-instar in the southern and central counties. Larval infestations affecting 32-56% of the plants have been observed in a few cornfields, but most sites have lower populations involving less than 16% of plants. The treatment window for summer corn borers is expected to close statewide in another week. Final management decisions for sweet corn must be made before the caterpillars have started boring into corn stalks and ears. CORN LEAF APHID - Small colonies of 20-40 aphids per plant are appearing on corn ears and leaves in southern and western Wisconsin. Corn leaf aphids usually do not interfere with pollination unless aphids appear early and populations grow rapidly, and a large percentage of corn tassels become saturated with aphids and their honeydew secretions. JAPANESE BEETLE - An unusually heavy infestation was encountered this week in the Sparta area of Monroe County. As many as 25 beetles per ear were feeding on the silks of edge row plants, though most ears were infested with 5 or 6 beetles. The infestations extended 10-12 rows into the field. This insect has caused economic damage to field, fruit and vegetable crops across the state this season, with populations being the highest in many years. -- Krista Hamilton, DATCP Entomologist 
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