Western IPM scouting map updates for 2023

Anthony Hanson & Angie Peltier, UMN Extension Regional Educators

During the 2023 growing season, U of MN Extension IPM workers  have been scouting wheat and soybean fields for pests in northwest and west-central Minnesota. As we move into August, scouts are wrapping up wheat sampling for wheat head diseases, and will continue to scout through mid-August for soybean insects, especially soybean aphid.

This Western IPM Pest Survey continued this year with support by the Minnesota Soybean Research and Promotion Council (MSRPC) and Minnesota Wheat Research and Promotion Council. Weekly maps are produced jointly with NDSU to show growth staging and pest pressure and to help alert growers when they should be scouting their fields for pests. Usually, three scouts are hired based out of Crookston, Moorhead, and Morris to cover the majority of the western half of the state.  Unfortunately, we could not find someone to fill the Morris position this year, so while the maps in this article do not show fields for southwestern Minnesota, do not assume there are not pest issues in that part of the state. There are more details related to that below.

Growth stages
During July 24-Aug 4,  wheat fields were mostly at the milk or dough stage (70-89 Zadok’s scale) in northwest counties with some fields ripening near Moorhead and along Interstate 94 (Fig. 2). These represent roughly half of the wheat fields scouts have been visiting in previous months, and maps for remaining fields will be available next week here.

Figure 1. Wheat growth stages (Zadok’s growth scale) from fields in ND and MN July 24- Aug. 4, 2023.

For the most part, soybeans in northwest counties were at the beginning seed (R5) or full seed (R6) growth stages, maturing more quickly than soybeans further south (Fig. 2). Further south, including in areas that have recently received rain, soybeans had all reached the full pod (R4) to beginning seed growth stages (R3).

Figure 2. Soybean growth stages from fields in ND and MN July 24-Aug. 4, 2023.

Small grains diseases
As expected in a dry year, small grains diseases have not been very prevalent in fields scouts have visited this year. A few cases of tan spot and bacterial leaf streak were found in June, though only a small percentage of plants were infected in these fields. During this most recent week of head disease scouting before harvest, a few fields have been found with ergot, but these fields also had a low percentage (2-6%) of infested plants (Fig. 3).

Figure 3. Percent of wheat plants infected with ergot. July 24- Aug. 4, 2023

Soybean diseases

SCN. In dry areas of the state, people may see soybean plants maturing before they would in a year with ‘normal’ rainfall. Soybean cyst nematode (SCN) parasitizes nutrients and water from roots of the soybean plant, causing stunted plants, poorly closing rows, yellow leaves and early maturity (Fig. 4). If you have an interest in determining whether your field is infested with SCN or in getting a better understanding about whether SCN population densities are rising or falling in your field, contact Angie Peltier (apeltier@umn.edu) for sample bags and sampling instructions for an SCN testing program sponsored by the MSRPC. 

Figure 4. Soybean roots with cysts (female SCN filled with eggs) of the soybean cyst nematode. As digging up plants to look for cysts is neither possible for much of the growing season nor will it provide an indication of a field’s population density, collecting soil samples for analysis is the preferred method to detect and monitor SCN population densities.

Recent field visits in northwest Minnesota found large patches of soybeans or individual plants among perfectly healthy soybeans suffering from Rhizoctonia root rot, caused by the fungus Rhizoctonia solani, or Phytophthora root and stem rot, caused by the oomycete Phytophthora sojae. While there is no ‘rescue’ treatment that can be used to manage either disease at this point during the 2023 growing season, it is important that the next time that soybeans are planted in the field with Phytopthora root and stem rot, a variety with both R-gene-mediated and partial resistance be selected. Seed treatment active ingredients effective against P. sojae do not provide protection against R. solani, and vice versa, and so careful selection of seed treatment active ingredients based on a field’s disease history and perceived risk is urged.

Soybean insects
Soybean aphids are the primary insect concern across the state right now. Most fields in NW MN and following the western border south still have relatively few aphids per plant (Fig. 5). However, there are multiple fields with increasing populations that are now over 200 aphids per plant over the past few weeks if you roughly follow Interstate 94 southeast from Clay County. The highest population found in the last week of July was 805 aphids per plant in a Douglas County field. Some fields in Becker and Mahnomen counties were also between 380 to 480 aphids per plant. Fields can vary significantly, even on the same farmstead,  so this is only an indication that scouting should definitely be occurring in these areas rather than assuming a field has an aphid problem based on nearby reports. For more information on soybean aphid treatment decisions, and precautions to take when also managing  two-spotted spider mite, read this recent article from Soybean Entomologist Dr. Robert Koch and IPM Specialist Bruce Potter.

Figure 5.  Average soybean aphids per plant in sampled soybean fields July 24- Aug. 4, 2023.

While not on these maps due to our lack of scout based out of Morris, southwest MN has also seen its fair share of soybean aphid issues with many fields being sprayed in the last couple weeks. That said, be sure to scout your own fields first rather than making decisions based on what neighbors are doing.
Sometimes soybean aphid populations don’t continue increasing. One field in Stearns county (A. Hanson’s family field) was at about 80-100 aphids per plant in late July that appeared prime for fast population growth. Last week, only about 20 aphids per plant were found. What happened in that week’s time? Aphids were heavily parasitized by parasitoid wasps (i.e, aphid mummies), hot temperatures in the 90s may have slowed down development, and heavy rains (a rarity this year) in the area all could have contributed to a population drop. The take home message here is to not get too excited to apply insecticide early and instead give the 250 aphid threshold time to work. Once that threshold is exceeded, plant damage still isn’t occurring, but it does give you roughly 5-7 days to apply an insecticide before populations reach about 670 aphids per plant, which is the economic injury level. There’s still time for populations to reach treatable levels to justify continued scouting, but many growers could also be able to avoid needing any insecticide applications this year.

When sampling soybean fields, scouts also use sweep nets near grassy field edges, typically along road ditches, to monitor the density of grasshoppers. Soybean fields sampled in NW and WC MN so far had relatively few grasshoppers (Fig.6), though you may see higher numbers in pastures and hay fields. If populations increase in August, it is possible we may see increased feeding in soybeans. Treatment thresholds for grasshoppers are defoliation above 30% pre-bloom and 20% between bloom and pod-fill, though be sure to determine the average defoliation across the whole plant rather than only looking at the top leaves.

Figure 6. Grasshoppers captured by sweep net in soybean field margins (20 sweeps per field).

 

To view the soybean and wheat pest maps, visit this link to view previous maps and future ones from U of MN and NDSU  as they are released. We aren’t hiring for 2024 yet, but if you know of anyone that would be interested in working as an IPM summer scout (e.g., college students, science teachers looking for summer work, etc.) at the Crookston, Moorhead, or Morris locations, have them get in touch with Dr. Anthony Hanson at hans4022@umn.edu.