IPM STUFF 2024-3
Volume 27 number 3
5/29/2024
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 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.
We are a bit ahead on degree-days and slightly ahead on precipitations. The SWROC missed some of the heavier recent rains, but other areas have struggled more with crusted soils and standing water.
Alfalfa Weevil Issues in Southern MN. Cutting early or spraying a short residual insecticide before harvest are two effective management tools for high alfalfa weevil populations in 1st cutting alfalfa. Unfortunately, some alfalfa weevil infested alfalfa fields in southern Minnesota are seeing the results of doing neither. Rain and conflicting fieldwork priorities have delayed cutting and are leading to significant yield loss in heavily infested fields (Fig. 1). Fields take on a frosted appearance as weevil feeding in the upper canopy increases.
Small, black-headed, green larvae in sweep net samples or on the ground after shaking alfalfa stems (Fig. 2) confirms an ongoing alfalfa weevil infestation. Use the stem count method to
Is there a way to make the best of a difficult situation? When weevil populations are threatening but crop damage is not severe, decide whether to cut or to use an insecticide with a short pre-harvest interval (PHI). The rain in the weather forecast further complicates decisions. Do not let hay sit in windrows for an extended period. Whichever management tool is used, monitor the second cutting for survivors.
Fields that are heavily weevil infested and already damaged present more difficult decisions. As damage increases, relying on cutting involves more risk.
determine if there are still enough larvae present to justify management (Updated alfalfa weevil management).
Because significant yield loss has already been incurred, management increasingly shifts to salvaging any remaining 1st crop yield and minimizing impacts on the remaining cuttings. Cutting as a management tool is less effective in these fields. The feeding of large numbers of larvae concentrated under windrows can injure shoots and even crowns, delaying emergence. This injury will delay the 2nd cutting regrowth. Although the PHI will delay harvest, consider using insecticide first. The yield and quality implications for delaying cutting for the 3-7 days of short PHI insecticides increase with alfalfa development and calendar date.
Bean leaf beetles (Fig. 3) survived the winter well. Pay close attention to early emerging soybean fields and the distinctive round holes caused by overwintered bean leaf beetle feeding. Some reports of heavier populations have come in.
Remember soybeans are very resilient. The economic threshold for vegetative soybeans is 30% leaf area removed.
I have received one report of infestations on neonicotinoid insecticide treated seed. The issues were in saturated soils and likely related to reduced insecticide concentration rather than resistance.
Bean leaf beetle eggs are laid in the soil and the larvae feed on soybean root nodules. This feeding is not believed to cause economic injury.
While most fields will not require a foliar insecticide at this time, watch fields with higher beetle populations for 1st generation beetle feeding on foliage and pods
Happy trails,
Bruce