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Nursery & Forest

Degree Days

Volume 62 Number 15 Date 08/10/2017

SOYBEAN APHID - Surveys continue to indicate that populations remain far below the established 250 aphid-per-plant treatment threshold in Wisconsin soybean fields. Densities have increased slightly since late July, but not to the point where control is required. The average count in 63 fields surveyed from since August 1 was only three aphids per plant. Only two fields, both in La Crosse County, had averages above 20 aphids per plant, and 13 fields had no detectable population.

GREEN CLOVERWORM - Larvae ranging from mid- to late-instar were found at low levels in the southern and western counties. Defoliation levels in surveyed fields were minor at less than 5-10%. This sporadic soybean pest is highly susceptible to parasitism and disease, and is normally controlled biologically without insecticide use.

SOYBEAN DEFOLIATORS - Defoliation by grasshoppers, green cloverworms, Japanese beetles, leafrollers, stink bugs and thistle caterpillars was prevalent in sampled fields, particularly around the perimeters. A combined defoliation rate exceeding 20% for soybeans in the seed-filling stages may justify treatment if the insects are actively feeding and damage is expected to increase. Defoliation estimates should be based on all parts of the soybean canopy (not just the injured portion) to avoid overestimating leaf injury and thus making unnecessary insecticide applications.

-- Krista Hamilton, DATCP Entomologist