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Crop weather
Rainfall, air and soil temperatures, degree-days, soil moisture, 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 https://swroc.cfans.umn.edu/weather. Some darn welcome rain around here recently.
Keep up to date on crop and pest development.
Degree-day development models and blacklight insect captures are available at https://vegedge.umn.edu/degree-day-models-select-insect-pests-midwest-region and https://vegedge.umn.edu/weekly-moth-flights.
Corn growing degree day models along with temperature and precipitation maps can be found at the Midwest Regional Climate Center https://mrcc.purdue.edu/
Tired yet?
Many of you have experienced a long pest management season and depending on your area of operation, you’ve been struggling with drown outs, replants, drought, and less-than-optimal herbicide performance. This has been a heck of a year for arthropod pests starting with big migrations of black cutworm and armyworm. These and alfalfa weevil problems have subsided for this year but grasshoppers, green cloverworms, soybean aphids, and twospotted spider mites still required attention. Will cooler weather and rain slow these pests or create crop disease issues? Some of you have seen outbreaks of these pests before but for others, they are an uncomfortable, new experience. Other than resistance issues, the pests are behaving the same as they always have.
When will it be over?
Late-season pests and diseases can impact corn yields into R5 (dent) stage and soybeans into the R7 (beginning maturity) stage. Fortunately, pests and diseases need time to develop to yield-reducing levels and yield impacts lessen as the crop approaches maturity. Because pest management is best applied ahead of any economic yield loss, scouting can wind down ahead of those growth stages. On the other hand, the wide range of crop development stages in some areas, even within some fields, could stretch the scouting window later this year.
Corn
Corn stages range from blister to dough sometimes in the same field.
Increasing northern corn rootworm populations require increased attention in Northwest Minnesota and in rotated corn throughout the state. You don’t have to hang sticky traps in every field, but over the next couple of weeks try to take a quick peak in fields, especially those later in development. Areas where you observe large numbers of northern corn rootworm beetles in late season pollinating weeds and flowers are at higher risk for northern corn rootworms and future extended diapause. Unless you plan on retiring soon, planting BT-RW hybrids on most acres is probably not the wisest rootworm management strategy. I don’t suppose it’s wise even if you plan on retiring soon.
Cereal aphids are present in some corn. Despite their name, corn leaf aphids are typically less common in MN fields than bird cherry-oat and English grain aphids. Based on the infestations I have seen; these species tend to infest different parts of the plant. Infestations arrive in corn as small grains and grasses mature. They tend to build, sometimes to very high levels, during the milk stages and leave corn during the dough stage. Earlier insecticide applications may favor aphids by removing beneficial insects. Fields with heavy aphid infestations often see stalk rot later and it may be that the aphids are responding to host quality. I have not seen convincing data on yield benefits from insecticide control of aphids in post-pollination corn. There could be several reasons for this.
Areas seeing recent rains and dews may see corn leaf disease development but remember yield risk lessens as epidemics are delayed. Please let Dean Malvick or me know if you observe corn tar spot in your fields this season.
Soybean
Most soybeans range from late R4 to R5 with early planted and/or short-season soybeans at R6. Some replants and beans planted after peas may still be R3. Some nasty multiple pest problems in a few fields.
2023 is shaping up to be a bad year for soybean aphids. Perhaps as bad, or worse, than 2015. Most reports have been from areas in the western half of the state. However, even there, not all fields have an aphid problem.
As soybeans reach the R5 stage, many of the remaining aphids are smaller and paler than those you saw in the upper canopy earlier. These are not the cast skins left as aphids molt to later nymph and adult stages. I would count these “white dwarves” the same as other soybean aphids. They may consume less sap than larger aphids but reproduce just fine. As host quality improves in the late R5 and R6 stages, typical light green aphids will be produced.
Migrating winged aphids are abundant now. Fields with poorly managed aphids produce large numbers of winged aphids to infest new fields, sometimes quite a distance away. These aphids will tend to infest plants still producing new leaf growth. That includes previously sprayed fields that have produced new leaves. Those sprayed with broad-spectrum insecticides are likely short on generalist predators and parasitoids. Change the insecticide mode of action if you need to re-treat a field.
Portions of the soybean aphid population are still likely resistant to pyrethroid insecticides. That includes bifenthrin. Unfortunately, you don’t know what insecticides the aphids arriving in your field were previously exposed to.
Very large aphid populations can still impact yield in R6 (full seed) stage soybeans. Using the 250/plant threshold until R6 can prevent late-season aphid surprises. Early-planted and short-maturity group soybeans will be at or near R6 now.
Make sure aphids are still increasing if you decide to use insecticide. Aphids may begin moving to buckthorn as soybeans mature and temperatures drop. Cool, wet weather may trigger fungal disease outbreaks.
Twospotted spider mite problems continue in areas of Minnesota with longer-term moisture stress. As soybeans enter the R5 stage, field edge symptoms have started to appear in more areas of SW Minnesota during the past week.
Spider mite feeding causes a loss of photosynthetic area. I would be nervous about allowing much over 25% upper canopy loss as late as R6 stage soybeans.
Populations can collapse rapidly. The change to cooler weather will slow mite reproduction and wetter conditions might help trigger an outbreak of mite-killing fungi. Check for live mites before you spray. Agent 86 has been conducting intensive surveillance for diseased spider mites in an area of SC Minnesota with significant recent moisture. His initial reports have been negative.
Green cloverworms continue to cause concern in some fields. This time of year, you might see overlapping generations with moths, eggs, and several larval stages present in the field at the same time. We are still using a 20% whole canopy defoliation action threshold through R6 soybeans. In most years, green cloverworms are controlled by predators, parasites, and diseases. Green cloverworms and grasshoppers have negatively impacted many aphid and mite management plans this year. Don’t overreact to what you see. Even a highly trained professional like me tends to overestimate defoliation.
We are seeing grasshopper adults moving into soybeans. Winged adults will move further into the field reducing border treatment options. Large grasshoppers are also harder to kill with insecticides. I tend to lean more toward % defoliation thresholds as grasshoppers disperse further into the field.
Natural enemies can still help moderate this summer’s grasshopper populations while a warm, dry fall will favor grasshopper egg-laying. Keep track of soybeans, alfalfa, and grasslands with unusually high fall hopper populations for scouting next spring.
Downy mildew and bacterial blight are the two soybean diseases I am seeing most often now. A new generation of SCN females is visible on southern MN soybean roots now.
Happy trails,
Bruce
Bruce Potter
University of Minnesota Extension IPM Specialist
University of Minnesota SW Research and Outreach Center
23669 130th Street
Lamberton, MN 56152
Cell: (507) 276-1184
[email protected]
https://swroc.cfans.umn.edu/ag/pest-management
Products are mentioned for illustrative purposes only. Their inclusion does not mean endorsement and their absence does not imply disapproval.
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For University of Minnesota Extension crop production information: http://www.extension.umn.edu/agriculture/crops/
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