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Soybeans
Volume 63 Number 13 Date 07/26/2018


SOYBEAN APHID - Observations from the annual aphid survey currently underway suggest populations are increasing but remain low for late July. Only two of 53 fields sampled this week had an average count greater than 20 aphids per plant on 100% of the plants. The highest count documented as of July 25 was 72 aphids per plant in a Waupaca County field. All other sites had averages below 10 aphids per plant.

Although surveys indicate populations are low, soybean aphids can reproduce rapidly under the mild, dry weather pattern forecast for early August, with the greatest population growth occurring at temperatures of 70-80F. Soybean producers are reminded that insecticide treatment is not advised until the threshold of 250 aphids per plant on 80% of the plants throughout the field has been exceeded. Insecticide treatment, if required, is most effective when applied during the R2-R4 (full bloom to full pod) stages.

AMERICAN PAINTED LADY - Larvae of the American painted lady butterfly, also known as thistle caterpillars, have been observed at low levels in soybean fields this month. Large populations develop in some years, but treatment is discouraged since the mortality rates generally are high and the solitary caterpillars, which construct a messy silken web between the leaves, seldom cause significant defoliation. The larvae observed in the past week were full-grown and likely to pupate soon. Adult butterflies should begin emerging by early August.

JAPANESE BEETLE - Light to moderate defoliation is widespread in soybeans, though surveys have found no fields with above threshold injury during the three-week period from July 9-26. The economic threshold for Japanese beetle and other leaf feeding soybean pests is 20% defoliation between bloom and pod fill. Spot treatment is an acceptable form of control for fields with the heaviest injury occurring in the margins.

-- Krista Hamilton, DATCP Entomologist