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Field Notes session talks alfalfa weevil and nutrient management

Angie Peltier, UMN Extension crops educator, Anthony Hanson, Extension IPM educator, and Brad Carlson, Extension nutrient management and water quality educator

Figure 1. Alfalfa weevil larva. Alfalfa weevil can be distinguished
from other weevil species by the dark brown or black head capsule.
 Clover leaf weevil larvae can be confused for alfalfa weevil, but i
instead have a brown or tan head capsule. Photo: Anthony Hanson.
The following information was provided during a 2024 Strategic Farming: Field Notes session. Use your preferred podcasting platform or listen online to a podcast of this Field Notes session hosted by UMN Extension crops educator Claire LaCanne.

Alfalfa weevil

IPM Extension educator Anthony Hanson provided information about how best to manage alfalfa weevil during this week’s program. Alfalfa weevil larvae feeding injury can cause significant yield losses both before and after first cutting. Current best management practices for managing alfalfa weevils are routine scouting, timely cutting and/or insecticide applications when thresholds are met, promptly picking up hay and implementing insecticide-resistance mitigation strategies.

Both alfalfa and alfalfa weevil growth and development are temperature-driven. Alfalfa growth and development in 2024 has been as varied as field conditions throughout the length and breadth of Minnesota, with first cuttings in the rear-view mirror for some and others nearing first cutting. Larvae of alfalfa weevil hatch from eggs and begin feeding, growing larger and molting over time as they reach developmental milestones. As larval size increases, so does feeding risk to the crop.

Scouting tips and treatment thresholds

While sweep nets can be used to capture adult weevils and some larvae, there is a much easier and more accurate way to determine the number of larvae per plant throughout the field. To better determine when thresholds have been met, it is recommended that folks take the following with them into the field:
  • a ruler or tape measure
  • a pocket knife or pruners
  • a 5 gallon bucket
  • something in which to record the information gathered
Stem height is measured before the stems are cut near the base of the plant and cut plants are beat against the inside of the bucket to dislodge larvae. Larvae are then counted and the average number of weevil larvae per stem determined.

One way to approach treatment thresholds is that thresholds have been met when 35-40% of leaf tips show feeding injury and there are two or more live larvae per stem. Another threshold strategy varies based on the average number of larvae per stem takes into account crop growth and development, the length of the pre-harvest interval of the insecticide you plan to use, the economic value of the hay crop and the per-acre treatment cost.

Click here for more detailed information about alfalfa weevil larvae ID and scouting.

When treatment decisions become a bit more complicated…..

Pre-harvest intervals will come into play when determining whether to make an insecticide treatment before cutting. While delays in making a first cutting could risk crop quality, there are also risks to not treating before cutting. Alfalfa weevil larvae can continue to grow and develop under windrows, risking losses to the regrowing crop.

After the first cutting has taken place, it is important to continue scouting to determine the level of infestation in the stubble. Scouting should continue until weevils develop from larvae into pupae and feeding has ended for 2024. Focus on scouting those field areas where windrows lay as this is where larvae will tend to be concentrated. Consider treating the crop if there are 6-8 larvae per square foot (6 larvae in sandy soil and 8 in other soils). Measure and cut stems and beat them in a bucket if enough regrowth has occurred to make square-foot counts difficult.

When 2024 scouting can end

While adults will hatch from pupae that develop this year, there is no cause for concern as adults will overwinter in the field and lay their eggs in 2025.

Based on current growing degree modeling of eastern strain alfalfa weevil growth and development, larvae are entering the pupal stage in northern Iowa. Southern Minnesota alfalfa fields will soon have pupating larvae and as degree days continue to accumulate, with weevils in more northerly fields pupating a bit later. However, if one also has the western strain of alfalfa weevil, scouting will need to go further into the growing season because this strain’s development can be 1-2 weeks behind that of the eastern strain.

Insecticide resistance concerns

While pyrethroid-resistant alfalfa weevil populations have been confirmed in western states, in-field failures of pyrethroids in Minnesota have not been confirmed as caused by resistance. However, populations in Minnesota have not yet been formally screened for resistance either. There is always the risk that an alfalfa weevil larvae will hatch that carries one or more genes that allow it survive an insecticide application to reproduce. The genetic recombination that takes place during sexual reproduction and randomly occurring genetic mutations are both naturally occurring processes. It is what we do that can make it so that those individuals able to survive an insecticide application mate to produce offspring that over time can shift a field’s weevil population from insecticide-sensitive to insecticide-resistant for any given insecticide group.

We only need to look back to the pyrethroid-resistant soybean aphid to best understand how to minimize the speed at which we select these populations. Unnecessary, prophylactic insecticide applications and repeated and exclusive use of single insecticide group can speed the shift toward resistance.

Only treating when thresholds have been reached and rotating insecticide groups are two ways to preserve the effectiveness of our insecticides over time. As weevil parasites are also killed by insecticides, only treating when thresholds have been reached has the added benefit of allowing other insects that can parasitize alfalfa weevils to provide some ‘free’ weevil control.

Current insecticide group options and example trade names include pyrethroids such as Warrior and bifenthrin, organophosphate chlorpyrifos formulations approved for use in 2024 in Minnesota include Pilot 4E and Warhawk and an indoxacarb formulation called Steward EC are the extent of the insecticide ‘tools’ that are in our ‘tool box’. To reduce the chance of resistance occurring in your alfalfa weevil population, it is recommended that in any 3-year period of alfalfa growth, one only treats once per year and uses an a.i. from a different insecticide group each time.

Nutrient management

Recent rainy weather has people wondering about the fate of nitrogen applied to supply the 2024 crop. Nutrient management and water quality educator Brad Carlson was on this week’s program to discuss this topic and what it means for managing crops throughout the remainder of the 2024 growing season.

While the non-legume crops grown in Minnesota all require nitrogen amendments, a significant portion of a crop’s needs are supplied through a microbe-driven process called mineralization. In a series of temperature-dependent chemical reactions, some of the nitrogen contained in organic matter in the soil transforms from being in a plant-unavailable form to a form that can be taken up by the plant.

Routes of nitrogen loss

There are several primary means that nitrogen can be lost from crop fields, including leaching and denitrification. Because these microbial processes are temperature-driven, it is relatively safe to assume that the majority of the nitrogen already applied for the 2024 crop is in the nitrate form and is therefore subject to loss.

Recent rains have left many fields at field capacity. Field capacity is reached when all of the available pore space in the soil is replaced by water. Most of the heavier textured, soils throughout the state will hold 10-12 inches of water. Any excess will percolate through the soil profile eventually making its way out of the field through drainage tile or will pond or run off across the soil surface.

Because of its negative charge, the nitrate form of nitrogen will not adsorb to soil particles but rather can be lost through leaching, or the downward movement of nitrate dissolved in water. This can move nitrate lower in the soil profile, sometimes beyond the crop’s ability to take it up before it is lost to drainage tile. Carlson’s ‘rule of thumb’ is that on average, nitrate will move down approximately 6 inches with each inch of rain.

The other primary means of nitrogen loss from fields is a process called denitrification. Denitrification occurs in saturated soil conditions and is the microbe-driven conversion of nitrate into nitrous oxide or nitrogen gas that is subsequently lost to the atmosphere. During the first 1-2 days, little nitrogen is being lost to denitrification as the microbes that mediate these conversions take some time to ‘wake up’ and begin multiplying. The rate of denitrification will depend upon soil temperature; denitrification can take place once soil temperatures rise above 50 degrees. To complicate matters, this process doesn’t take place uniformly throughout the field, but is confined to the lower areas that tend to get and stay saturated longer.

Research has shown that in saturated soil conditions, ~6% of the nitrate can be lost in 60 degree soils saturated for a period of 4 days and ~12% can be lost under the same temperature conditions when soil is saturated for 10 days. At 70 degree soil temperatures, ~12% can be lost when soil is saturated for 4 days and ~26% can be lost when saturated for 10 days. Half of the nitrogen can be lost over a period of 10 days of saturated conditions when soil temperatures reach 80 degrees.

What this all means for nitrogen management going forward in 2024

If you sampled soil last fall to determine how much residual nitrogen was left over from the 2023 drought that could be credited to the 2024 crop, nitrogen losses that have already or will occur in 2024 due to excessive rains means that some will not be able to count on this credit. For those that did not credit residual nitrogen from 2023, much of the nitrogen lost from fields in 2024 is likely this residual nitrogen and so may not be of much consequence to your operation.

When crop plants are small, the majority of the water that is lost from the soil results from moisture evaporating directly from the soil surface. Soil evapotranspiration in spring before crop water demand increases, is ~1/10th of an inch of water per day or ~1 inch of water in 10 days. This rate doubles when corn plants have put on their sixth leaf.

If we continue to get these pretty regular rains, sampling the soil to determine the soil’s nitrogen content may be of value.

The University of Minnesota developed a decision aid to help you decide whether you need to apply supplemental N.

Audience questions addressed

LaCanne moderated audience questions for Hanson and Carlson.

Insect questions. What about rootworms in 2024? The mild 2024 winter without significant, abrupt periods of very cold weather that reached down into the soil profile likely point to the 2023-24 winter not doing much to degrade rootworm egg viability. If you had significant rootworm challenges in your area in 2023, there is very little reason to expect that similar problems will not occur in 2024. However, rootworm larvae are susceptible to drowning in saturated soil conditions and so if mother nature continues to bring significant and regular rains to the region, the best we can hope for is some casualties from drowning or fungal infection.

Watch out also for pea aphid. Pea aphid infestations have been significant through regions of Iowa and so while scouting for weevils, also look for pea aphid. If you find them, more detailed information about how best to scout and treatment pea aphid can be found here.

Nitrogen questions. When should one time side-dress applications to occur? Those that had planned on splitting their nitrogen applications should get their side-dress application on just as soon as soil conditions allow.

Should one consider tissue testing to address nitrogen deficiencies and if so, what would you recommend? Unfortunately, research has shown that there are large differences among hybrids for tissue nitrogen content and so unless you have access to what would be considered ‘sufficient’ in each hybrid you test, tissue testing will be of limited value. While tissue testing may be able to tell us whether nitrogen is deficient or sufficient, it cannot provide us a rate recommendation. In-season soil testing for nitrogen has been studied by UMN Extension nutrient management specialists Fabian Fernandez and Dan Kaiser. They found that if one samples soil 1 foot deep in a standing crop and finds at least 26 ppm nitrate, there is adequate nitrogen for the crop. Determining what nitrogen rate would be needed if there is less than 26 ppm of nitrate is not as clear at this point. If one farms on heavier soils in a field with tiling spaced 50 feet apart, 0.75 inches of water can leave tile lines in 24 hours. Knowing how much rain fell and monitoring tile lines to see how well water is flowing can help one to get a feel for how much nitrogen is being lost through leaching.

How does one approach supplementing nitrogen in areas where nitrate has been lost to denitrification? Areas of the field that tend to have saturated conditions for longer periods of time also tend to be those areas of the field where the corn crop tends to get drowned out. Few will be heading back into fields to replant those areas given the current calendar date and soil conditions and so supplementing nitrogen in those areas would not be necessary. But if one can replant those areas, a high rate of starter fertilizer applied at planting will likely be adequate to supplement for any losses.

Join us Wednesday!  

Join us Wednesday, June 5 when we welcome Extension weed management specialists, Dr. Debalin Sarangi, U of MN, and Dr. Joe Ikley, NDSU, to discuss optimizing postemergence weed control with proper weed identification and application timing. Visit for more information.

Thanks to the Minnesota Soybean Research and Promotion Council and the Minnesota Corn Research and Promotion Council for their support of this program.

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