Volume 64 Number 19 Date 11/07/2019
SOYBEAN APHID - Populations recorded during the annual survey were very low, aside from a few western Wisconsin fields with moderate pressure. The state average count in 160 fields sampled from July 26-August 26 was only five aphids per plant. For comparison, the 2018 survey found an average of 14 aphids per plant, the 2017 average was six aphids per plant, and surveys from 2010-2016 documented counts of 7-55 aphids per plant. This season's state average was the lowest in the 18-year history of Wisconsin soybean aphid surveys. In addition, no cases of pyrethroid insecticide failure were reported in the state in 2019.
JAPANESE BEETLE - Defoliation was observed in 75% of the soybean fields examined in August. Counts taken during the soybean aphid survey ranged from 1-184 beetles per 100 sweeps, with a state average of 14 per 100 sweeps (the 2018 average was 8 per sweep). The highest counts of 50 or more beetles per 100 sweeps were noted in the southern and west-central districts for the second year in a row (see table 3 page 159). The prevalence of Japanese beetles documented by the survey signals that this invasive pest is becoming an increasingly significant defoliator threat to the state's soybean crop.
WHITE MOLD GALL MIDGE - This new gall midge was found for the first time in Wisconsin this year. The white mold gall midge (WGM) Karshomyia caulicola (Diptera: Cecidomyiidae) was collected from a Pierce County soybean field in August and identified by a USDA ARS entomologist on October 2. The species is similar morphologically to the soybean gall midge (SGM) Resseliella maxima, but there are important differences between the two. First, the WGM is a fungus feeder and, unlike the SGM, is not considered a significant crop pest. Second, the larvae appear later in the season (after flowering) and can be found throughout white mold-infected fields. By contrast, SCM infestations can develop by the V3 stage and show an edge effect. Although the WGM is associated with soybean white mold, it does not spread or promote infection.
Soybean producers are advised to be alert for both this species and the soybean gall midge next season. The SGM has not yet been found in Wisconsin.
-- Krista Hamilton, DATCP Entomologist
SEEDLING ROOT ROT: - DATCP surveyed 52 soybean fields from June 21-July 19 for seedling root rot disease caused by Phytophthora sojae, general Phytophthora species, and general Pythium species. Testing at the Plant Industry Bureau Laboratory confirmed 38% (20 of 52) of fields were positive for P. sojae and 100% (52 of 52) were infected with Pythium. The Phytophthora rate decreased from the previous year when the pathogen was found in 46% of fields. Surveys in the past decade have found P. sojae prevalence ranging from 13% in 2011 to 49% in 2014.
-- Sam Fieweger, DATCP Plant Industry Lab