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Corn
Volume 64 Number 15 Date 08/08/2019


WESTERN BEAN CUTWORM - Moth counts have peaked across southern and central Wisconsin and should begin to decline soon. The annual flight is expected to peak next week in the northern counties. Pheromone traps captured a total of 1,619 moths from August 1-7, compared to 1,121 the week before. Preliminary results of the 15th annual trapping survey show that the 2019 state count is 3,054 moths in 57 traps (52 per trap average). The highest individual trap total to date is 258 moths registered near Princeton in Green Lake County.

CORN ROOTWORM - Beetle counts at most sites are low for early August. Surveys in 97 cornfields in the southern, central and northwest districts found no detectable beetle population at 69 (71%) of the sites. Twenty fields (21%) contained low adult rootworm populations ranging from 0.1-0.4 beetles per plant, while only eight (8%) of the sites had economic averages of 0.75 or more beetles per plant. An exceptional field in Lafayette County had an extremely high average of 7.2 beetles per plant. The official 2019 corn rootworm survey is now underway.

JAPANESE BEETLE - Silk pruning has become evident along field edges, although at most sites the heaviest feeding is limited to the outer rows and the infestations do not extend more than 5-6 rows into the field interior. Control of this pest in corn is warranted if fieldwide populations exceed three beetles per ear and pollination is less than 50% complete. Chemical treatment of entire fields is rarely necessary. Border area spot treatments are usually sufficient for reducing beetles during the critical pollination period.

EUROPEAN CORN BORER - The peak in summer moth activity has occurred in the southern counties and should be reached by August 12 in the central areas. The treatment window for second-generation corn borers will remain open for approximately two more weeks. Controls directed against the summer larvae must be applied during the period after egg hatch and before larvae bore into the stalks, prior to the accumulation of 2,100 degree days (modified base 50F). The larvae observed during this week's surveys were in the early to intermediate (3rd instar) development stages. Degree day totals as of August 7 were: Beloit 1,936, La Crosse 1,863, Madison 1,915, and Eau Claire 1,770.

CORN EARWORM - Fewer moths arrived this week compared to the week before. Nine pheromone traps captured a total of 67 migrants from August 1-7, a decrease from 179 moths collected in 11 traps during the previous week. However, the arrival of even a few corn earworm moths in traps (5-10 moths for three consecutive nights) signals that sweet corn producers should increase monitoring of fields with green silks. Corn earworm caterpillars ranging from -1 inch long were found this week in corn in Green, Rock and Sauk counties.

-- Krista Hamilton, DATCP Entomologist