Volume 63 Number 18 Date 11/15/2018
SOYBEAN APHID - Aphid populations reached the 250 aphid-per-plant treatment threshold in scattered fields during the first two weeks of August, but densities on a statewide scale were mostly low this season. The annual survey conducted from July 23-August 21 found a statewide average count of 14 aphids per plant. This was an increase from six aphids per plant last year and eight aphids per plant in 2016, still far below the threshold. One hundred and eighty-nine soybean fields in the R2-R6 growth stages were surveyed, with aphids counted on 40 plants per field. Only two sites, one each in Jackson and Trempealeau counties, contained above-threshold populations of 260 and 290 aphids per plant. Densities were below 100 aphids per plant in 96% of fields, and the majority of those sites (86%) had average counts of less than 25 per plant.
Results of the survey suggest that while aphid pressure was slightly higher in 2018 than in the previous two years, most sampled soybean fields did not meet treatment guidelines during the survey timeframe. In addition, no cases of pyrethroid insecticide failure were reported or confirmed in the state.
SOYBEAN GALL MIDGE - An emerging pest of Midwestern soybeans, the soybean gall midge (SGM) was not found in Wisconsin this year. Populations were confirmed in 12 western Iowa counties, as well as in Nebraska and South Dakota. Larvae of the SGM, a member of the Hessian fly family (Cecidomyiidae), feed internally at the base of soybean stems and cause stem discoloration. Infested plants snap off near the ground and the orange or white maggots can be found feeding inside. Much remains unknown about this insect, including the exact species and whether it is a direct or a secondary soybean pest. Consultants and soybean growers are encouraged to become familiar with SGM for 2019.
JAPANESE BEETLE - This insect was a leading pest of concern to Wisconsin soybeans again in 2018, second only to the soybean aphid. Surveys in July and August found defoliation in 72% of fields. In 2017, a banner year for Japanese beetle in Wisconsin, 87% of surveyed sites had some degree of feeding. Sweep net sampling during the August aphid survey yielded average counts ranging from 0-21 beetles per 100 sweeps in the state's nine crop districts. Areas with the highest numbers were the southeast (21 per 100 sweeps), south-central (17 per 100 sweeps) and west-central (13 per 100 sweeps) districts. The state average was 8.4 beetles per 100 sweeps. The prevalence of Japanese beetles documented by the survey signals that this invasive pest continues to pose a significant threat to the state's soybean crop.
-- Krista Hamilton, DATCP Entomologist
SEEDLING ROOT ROT - DATCP surveyed 54 soybean fields from June 11-July 6 for seedling root rot diseases. Twenty seedlings from each field were tested at the Plant Industry Bureau Laboratory (PIB Lab) for Phytophthora sojae, general Phytophthora species, and general Pythium species, using molecular methods. Testing confirmed 46% (25 of 54) of fields were positive for P. sojae and 96% (52 of 54) were infected with Pythium. The Phytophthora rate was a marked increase from the two previous years when the pathogen was found in 24% (in 2017) and 32% (in 2016) of fields. Surveys in the past decade have found P. sojae prevalence ranging from 13% in 2011 to 49% in 2014. The increase in Phytophthora root rot was likely due to excessively wet spring conditions.
In addition, another Phytophthora species, Phytophthora sansomeana, was found in three fields in Jefferson, Rock, and Winnebago counties. Since the first Wisconsin detection in 2012, P. sansomeana has been documented in twelve counties: Calumet, Dane, Dodge, Dunn, Eau Claire, Green, Jefferson, Outagamie, Marathon, Rock, Sheboygan, and Winnebago. This year both Rock and Winnebago were new additions to this list.
-- Sam Christianson, DATCP Plant Industry Lab