SW MN IPM Stuff 2022 Issue 16

Volume 25 Issue 16| August 19, 2022

This newsletter and the advice herein are free. You usually get what you pay for.

Crop Weather

Rainfall, air and soil temperatures, degree-days, soil moistures, and other current and historical weather data for the University of Minnesota Southwest Research and Outreach Center (SWROC), a little spot about two miles west of Lamberton, MN, can be found at http://swroc.cfans.umn.edu/weather.  

As of 8/15, the SWROC accumulated 1955 GDD and 8.74 inches of precipitation since May 1. This compares to historical averages of 1853 GDD and 12.67 inches of precipitation. Soil moisture is still a concern. As of 8/15, the SWROC had 8.77 inches in the top 60 inches of the soil profile. There was no moisture remaining in the top 24inches ( https://swroc.cfans.umn.edu/weather/graphs). This is still ahead of soil moisture levels (1.26 inches) at this date in 2021, but well below the long-term average.

Yesterday’s rain will help but more will be needed. Rain came too late for some fields on light soils.  A consultant sent a picture of an ear of corn. The ear showed when the plant got a shot of water – the top of the ear pollinated and the bottom did not.

Corn is approaching or at R4 (dough) stage and most soybeans range from early to late R5.

We are seeing developing bird cherry-oat and English grain aphid populations in late-planted SWROC corn fields that are planted near small grain studies. Post-pollination aphid populations in corn can be messy but not necessarily yield limiting. Populations in corn tend to peak during the dough stage and decline at dent.

Corn rootworm beetle populations continue to cause concern in some fields. Sticky trap captures in some fields are very high.  The EXCEL spreadsheet I sent to cooperators limits data entry to 0-500 rootworm beetles/species/trap/week. Agent 86 managed to find a trap with 609 westerns on it somewhere in or around Watonwan County.

  • If you sprayed adults to reduce egg laying, remember to re-evaluate beetle populations after 7-10 days. Gravid females and more than one beetle/plant may require a second application and possibly a third.
  • Rootworm beetle feeding on corn ear tips is another concern. There is no threshold post-pollination for this type of feeding. Fortunately, the small tip kernels contribute little yield. The injury might open the ears to ear mold fungi and picnic beetles, but that horse is already out of the barn. Hybrids that expose more tip kernels can see more injury. It is not likely, but if rootworm beetle feeding on the ears is progressing further into the ear, an insecticide intervention may be needed. Grasshopper feeding can be more aggressive but is often limited to field border rows.
  • Very high beetle populations indicate changes in rootworm management are needed in the field, rotation out of corn being the most helpful.
  • Check corn roots in fields with very high beetle populations. If the beetles emerged from that field, they may have significant root injury with little lodging.

Watch for pod feeding by grasshoppers, crickets, and bean leaf beetles as pods fill. Dry conditions tend to increase the risk of pod feeding.

Soybean aphid populations continued to increase over the past weeks. Based on conversations with growers and consultants the area with threshold infestations appears to be expanding to the east and north. Late maturing fields are attracting the most migrating aphids. Focus most of your scouting on later planted and/or full-season beans now.

This is a tough time of year for soybean aphid decisions. A declining probability for an insecticide yield response on older soybeans, a chance that aphids will begin leaving soybean fields for buckthorn, and insecticide pre-harvest intervals complicate decisions now. Summer help heading back for more education does not help get fields scouted either.

  • The recommendation is to use the 250 aphid/plant stage through R5.  As soybeans progress through the R5 (Beginning seed) stage, it becomes less likely, but still possible, to receive a yield benefit from an insecticide treatment.
  • Soybeans can suffer yield loss into R6(full seed). If you decide not to treat a 250 aphid/plant or more field during R5, it would be prudent to revisit the field in early R6 to make sure aphids did not get out of hand.
  • A few emails, phone calls, and research plot results at Lamberton indicate that pyrethroid insecticide-resistant soybean aphid populations are still here. Do high crop prices pay for high-risk aphid management behavior?
  • Some fields have high levels of wasp parasitism. Large numbers of mummies can indicate the aphid population will stall. With some cooler, wet weather in the forecast fungal diseases may also start to collapse some aphid populations.
  • We will be watching buckthorn to see if we can detect when aphid movement starts. Aphids will not leave all fields at the same time.

Two pest management field day opportunities

Pest management field day. Wednesday, August 31. 8:30 am– 12:30 pm at the UM SWROC. Pre-registration by August 26 is encouraged. For details see:https://swroc.cfans.umn.edu/events/ipmday

Corn and soybean disease and insect field day. Thursday, September 1. 9:00 am – 12:30 pm at the Rosemount Research and Outreach Center.  For details see: Corn and soybean disease and insect field day - Rosemount

As always, I appreciate hearing what you are seeing out there.

Happy trails,