Anthony Hanson (hans4022@umn.edu), Extension IPM Regional Educator, and Bruce Potter, Extension IPM Specialist
Mid-May to June is typically the time to begin scouting for insect problems in alfalfa. After our cold spring of 2023, alfalfa weevil adults and larvae are beginning to be found in the southern half of the state. Even if larvae are present, insecticide applications may not be needed if populations are not yield limiting or when alfalfa fields can be harvested soon. Cutting exposes the larvae to weather and can greatly reduce numbers. Remove windrows as quickly as possible. The larvae are concentrated there, and heavy feeding can delay regrowth. (Fig 1). Scouting through June will determine if control surviving larvae need to be controlled. Also remember that insecticide options are more limited compared to previous years with chlorpyrifos (e.g., Lorsban) applications no longer being allowed.
Problem 1: Extended weevil feeding season
Table 2. Conventional insecticide options for alfalfa weevil. This is not an exhaustive list and does not endorse specific products or trade names.
For more information on alfalfa insect management and how to determine if insecticide application is warranted for other pests such as potato leafhopper, visit:https://extension.umn.edu/forage-pest-management/alfalfa-insects-what-look-and-how-scout.
Mid-May to June is typically the time to begin scouting for insect problems in alfalfa. After our cold spring of 2023, alfalfa weevil adults and larvae are beginning to be found in the southern half of the state. Even if larvae are present, insecticide applications may not be needed if populations are not yield limiting or when alfalfa fields can be harvested soon. Cutting exposes the larvae to weather and can greatly reduce numbers. Remove windrows as quickly as possible. The larvae are concentrated there, and heavy feeding can delay regrowth. (Fig 1). Scouting through June will determine if control surviving larvae need to be controlled. Also remember that insecticide options are more limited compared to previous years with chlorpyrifos (e.g., Lorsban) applications no longer being allowed.
Farmers have been seeing a two-pronged challenge to managing alfalfa weevil in recent years: 1. An extended larval feeding season. 2. Reports of pyrethroids failures for alfalfa weevil. Each of these problems on their own have some solutions, but the compounding effect of both of these is leaving farmers with limited options. This article will dive into general alfalfa weevil management and what adaptations are needed as the dynamics of this pest change.
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| Figure 1. Bare strips from alfalfa weevil feeding occurred under windrows in a problem field near Morris in mid-June 2022, MN. Photo: Anthony Hanson. |
Alfalfa weevil identification and development
Alfalfa weevils overwinter as adults in nearby grassy areas and go through one generation per year. Eggs are laid in the stem of alfalfa plants in spring typically around mid-May. The larvae, rather than adults, are the stage that can cause economic injury to alfalfa.
The feeding of alfalfa weevil larvae causes round holes in the upper foliage that can skeletonize leaves so only leaf veins remain. From a distance, the defoliation causes a gray or frosted appearance of heavily infested fields. This damage window typically coincides with the end of first cutting and the start of regrowth for the second cutting. 1st instar larvae are yellow to olive with 2nd through 4th instar larvae becoming green in color with a white stripe along their back. All stages have distinctive black heads. Often, growers become aware of the larvae when they find them on their mower during the first cutting of the year (Fig. 2). If populations were high, larvae that survive mowing can end up concentrated in the windrows and feed on regrowth if hay isn’t quickly removed.
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| Figure 2. Alfalfa weevil larvae 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 instead have a brown or tan head capsule. Photo: Anthony Hanson. |
Economic crop injury ends when the larvae pupae in loose, round cocoons on the ground. While they may briefly feed, the emerging adults enter a summer dormancy (Fig. 3). Typically, these adults will be present in fields and nearby grassy areas, but will not lay eggs until the following year.
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| Figure 3. Alfalfa weevil life cycle. The timing for egg hatch can vary significantly in some years. Photo: Mimi Broeske, Nutrient and Pest Management Program, University of Madison-Wisconsin. |
Similar to crop development, alfalfa weevil development is affected by temperature (Fig 4). Larvae that would be found so far in May would have either just hatched or else very small early-stage larvae. Especially during 2021 and 2022, there were reports of economic damage in west-central fields for second cutting (even into early July) in what appeared to be delayed development of larvae that would normally have developed into adults. This could have resulted in more reported cases of heavy feeding under windrows (Fig.1).
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| Figure 4. Eastern strain alfalfa weevil degree-days as of May 25, 2023. Updated forecast maps can be viewed at https://vegedge.umn.edu/alfalfa-weevil. |
While the exact cause isn't confirmed yet, one possible cause for extended feeding later in the season is a different strain or biotype of alfalfa weevil. The eastern strain has historically been assumed to be the predominant strain in Minnesota, and that is what alfalfa weevil degree-day models are based on. However, there is also a western strain that has about a 1-2 week delayed development. In maps from 2005, the western strain was projected to be just west of the Minnesota and Dakota borders, so it would not be surprising to see it farther east into Minnesota at this point. It's possible the western strain has become more predominant in some areas, and has been reported to be spreading in more southern states. There may be other differences between the strains that cause differences in timing and larval peaks during the growing season.
Another potential cause of extending the weevil season, though in this case earlier in the year, has been noticed during warm winters in more southern states where adults may lay eggs during the previous fall. Normally alfalfa weevil eggs cannot survive winters in northern states, but it may be possible during a warm winter or one with heavy snow cover for insulation. If those eggs can survive, there may be earlier feeding than expected that alters the normal seasonality of alfalfa weevil with staggered generations of both larvae and adults present at the same time of year. Most reports in recent years have been of later rather than early feeding, but scouting may need to occur earlier if when we experience a very warm winter..
Regardless of the cause, growers are increasingly having to manage alfalfa weevil as a two-harvest pest rather than only having to primarily manage for a single cutting. This means that while the current degree-day models can be good indicators of when initial larval feeding begins, they may not be as accurate in some areas for predicting the end of alfalfa weevil season (Fig. 4) With a good integrated pest management program and scouting, it's possible to address this change, but the challenge is compounded by what options growers have left for managing the pest across multiple cuttings.
Scouting and management
Different parts of the United States use a variety of methods to determine whether economically damaging levels of alfalfa weevil are present. Scouting should occur in mid-May through June by using a sweep net to determine if larvae are present. Sweep nets are not as accurate for determining treatment thresholds, but can be a good indicator to take a closer look if larvae are easily found in sweeps. Remember not to count adults in scouting tallies as they only cause very minimal feeding damage.
Monitoring tip injury is often recommended as a pre-harvest method. This method is relatively simple and generally adequate for management purposes. It provides a good rule of thumb estimate for the alfalfa weevil’s pre-harvest damage potential.
- Collect 50 to 100 alfalfa stems (10 to 20 randomly selected stems from five locations).
- Examine whether they show obvious feeding damage – pinhole or more severe feeding – in the rapidly growing tip leaves and leaf buds.
- Divide the number of stems with recent tip injury by the total stems collected to convert it to a percent.
- Consider treating before first cutting when 35% of tips (weak stand) or 40% of tips (vigorous stand) plants show feeding and two or more live larvae/stem. Consider cutting if near bud stage or mowing would occur anyways prior to the pre-harvest interval.
A second method of determining economic thresholds better incorporates economics and includes the number of larvae, alfalfa growth stage, and hay price to determine the potential for crop injury. Select and cut 30 plants across the field at ground level. Record each plant’s height and shake in a 5-gallon bucket to determine the average number of larvae per stem in the field and if weevil counts exceed economic thresholds (Table 1). What tactic is best to use will depend on when the next cutting of hay will occur and what insecticide options are available.
Table 1. Economic thresholds for alfalfa weevil based on hay value. PHI = pre-harvest interval. Reviewed by Pellissier et al. 2017 and modified from NDSU Extension.
Historically in Minnesota, most crop injury occurs in late first cutting and occasionally in second cutting regrowth, so the likelihood of a return on investment from insecticide application after the first cutting of the year is often reduced. This is in part because mowing, crimping, and exposure from lack of cover kill many larvae from regular harvest. Mowing may also be the only option when considering any pre-harvest intervals for insecticide use. However, crop loss from alfalfa weevil and other insects, such as variegated cutworm, can still occur under windrows where larvae are protected (Fig. 1). This damage is most pronounced when hay cannot be baled shortly after mowing. Because of protected larvae and unhatched eggs, growers should plan to scout at least once after the first cutting.
| Hay value ($/ton) | |||||||||
|---|---|---|---|---|---|---|---|---|---|
| Treatment | $50 | $75 | $100 | $125 | $150 | $175 | |||
| Stem height | Management tactics | cost/acre | Average larvae per stem | ||||||
| 10-15 inches (mid-vegetative) |
Long or short PHI insecticide |
$7 $8 $9 $10 $11 $12 |
3.6 4.1 4.7 5.3 5.9 6.4 |
2.2 2.6 3.0 3.4 3.7 4.1 |
1.5 1.8 2.1 2.4 2.7 3.0 |
1.1 1.4 1.6 1.8 2.1 2.3 |
0.9 1.1 1.2 1.4 1.6 1.8 |
0.7 0.8 1.0 1.2 1.3 1.5 | |
| 16-20 inches (late vegetative) |
Short PHI insecticide or mow early |
$7 $8 $9 $10 $11 $12 |
3.8 4.4 4.9 5.5 6.1 6.7 |
2.4 2.8 3.2 3.6 4.0 4.4 |
1.8 2.1 2.4 2.6 2.9 3.2 |
1.4 1.6 1.8 2.1 2.3 2.5 |
1.1 1.3 1.5 1.7 1.9 2.1 |
0.9 1.1 1.2 1.4 1.6 1.7 |
|
| >20 inches* (early bud) |
Mow early | $7 $8 $9 $10 $11 $12 |
4.0 4.6 5.2 5.8 6.3 6.9 |
2.7 3.1 3.5 3.8 4.2 4.6 |
2.0 2.3 2.6 2.9 3.2 3.5 |
1.6 1.8 2.1 2.3 2.5 2.8 |
1.3 1.5 1.7 1.9 2.1 2.3 |
1.2 1.3 1.5 1.6 1.8 2.0 | |
*If >50% of plants are at bud stage, it's more beneficial to mow.
Historically in Minnesota, most crop injury occurs in late first cutting and occasionally in second cutting regrowth, so the likelihood of a return on investment from insecticide application after the first cutting of the year is often reduced. This is in part because mowing, crimping, and exposure from lack of cover kill many larvae from regular harvest. Mowing may also be the only option when considering any pre-harvest intervals for insecticide use. However, crop loss from alfalfa weevil and other insects, such as variegated cutworm, can still occur under windrows where larvae are protected (Fig. 1). This damage is most pronounced when hay cannot be baled shortly after mowing. Because of protected larvae and unhatched eggs, growers should plan to scout at least once after the first cutting.
Stubble scouting
For post-harvest, monitor regrowth and potential stubble infestation. After the hay has been picked up, search the stubble and early regrowth in 20 samples (four randomly chosen, 1-square-foot samples from each of five locations). Because harvesting often concentrates weevil larvae in the windrow areas of the field, it may be useful to sample first (or only) from windrows. Consider insecticide treatment if 8 or more larvae per square foot are found, (6 per square foot on sandy soil), or larvae are suppressing regrowth. When regrowth after harvest is sufficiently long to make square-foot counts too difficult, revert to sampling plants instead.
For post-harvest, monitor regrowth and potential stubble infestation. After the hay has been picked up, search the stubble and early regrowth in 20 samples (four randomly chosen, 1-square-foot samples from each of five locations). Because harvesting often concentrates weevil larvae in the windrow areas of the field, it may be useful to sample first (or only) from windrows. Consider insecticide treatment if 8 or more larvae per square foot are found, (6 per square foot on sandy soil), or larvae are suppressing regrowth. When regrowth after harvest is sufficiently long to make square-foot counts too difficult, revert to sampling plants instead.
Insecticides
Alfalfa weevil is typically the first pest insecticides may be used for in alfalfa during the year. Insecticides labeled for alfalfa weevil include pyrethroids, organophosphates, and oxadiazine. Parasitoid wasps and other natural enemies help suppress alfalfa pest populations, but they are also susceptible to these insecticides, so flare-ups of alfalfa weevil or other pests such as pea aphid can occur after treatment. Weevil adults can remain in fields feeding throughout the summer, though they generally do not cause economic damage. However, this also means they can be exposed to insecticides used for later-season pests, such as potato leafhopper. In addition to further reducing natural enemy populations, multiple applications in a field can also increase the likelihood of alfalfa weevil becoming resistant to those insecticides, which further illustrates the need for careful use of currently available control options (Table 2). Use high labeled rates as low rates can speed up the development of resistance.
Alfalfa weevil is typically the first pest insecticides may be used for in alfalfa during the year. Insecticides labeled for alfalfa weevil include pyrethroids, organophosphates, and oxadiazine. Parasitoid wasps and other natural enemies help suppress alfalfa pest populations, but they are also susceptible to these insecticides, so flare-ups of alfalfa weevil or other pests such as pea aphid can occur after treatment. Weevil adults can remain in fields feeding throughout the summer, though they generally do not cause economic damage. However, this also means they can be exposed to insecticides used for later-season pests, such as potato leafhopper. In addition to further reducing natural enemy populations, multiple applications in a field can also increase the likelihood of alfalfa weevil becoming resistant to those insecticides, which further illustrates the need for careful use of currently available control options (Table 2). Use high labeled rates as low rates can speed up the development of resistance.
Table 2. Conventional insecticide options for alfalfa weevil. This is not an exhaustive list and does not endorse specific products or trade names.
| Group | Class | Active ingredient | Trade Names |
|---|---|---|---|
| 1A | Carbamate | Methomyl | Lanate |
| Carbaryl* | Sevin | ||
| 1B | Organophosphate | Malathion† | Malathion |
| Phosmet† | Imidan | ||
| 3A | Pyrethroid‡ | Alpha-cypermethrin‡ | Fastac Mustang-Maxx, |
| Beta-cyfluthrin‡ | Baythroid | ||
| Cyfluthrin‡ | Tombstone | ||
| Gamma-cyhalothrin‡ | Declare, Proaxis | ||
| Lambda-cyhalothrin‡ | Warrior and generics | ||
| Permethrin‡ | Arctic, Permethrin, others | ||
| Zeta-cypermethrin‡ | Mustang Maxx | ||
| 22 | Oxadiazine | Indoxacarb | Steward |
*Carbaryl can burn regrowth if applied after cutting.
†Single group 1B products tend to only have ~50% control, but are closer to 100% efficacy if in a mixture (e.g., malathion + phosmet).
‡Resistance to pyrethroids appears likely, especially in western and central MN, but is not yet officially confirmed. If you have what appears to be an insecticide failure in a field, do not use the same group twice in a row.
†Single group 1B products tend to only have ~50% control, but are closer to 100% efficacy if in a mixture (e.g., malathion + phosmet).
‡Resistance to pyrethroids appears likely, especially in western and central MN, but is not yet officially confirmed. If you have what appears to be an insecticide failure in a field, do not use the same group twice in a row.
As an attractive crop to pollinators when in bloom, following threshold and integrated pest management recommendations for alfalfa can also act as protection for pollinators. If a field is near bloom stage, it is often too late for an insecticide application, but mowing followed by scouting can often provide the most economical decision as well as avoid risking application when pollinators would be attracted to the field.
Since adults can remain in the field later in the season, an insecticide with the same group number should not be used again on the same field that year in order to reduce the risk of insecticide resistance. For instance, if a field is treated with a carbamate (group 1A) or oxadiazine (group 22) for alfalfa weevil, but needs to be treated later in the year for potato leaf hopper, a pyrethroid (group 3A) would normally be considered to maintain a rotation of insecticide modes of action. Carefully read labels as some common products, such as bifenthrin, may only be available for seed alfalfa production and not for feed use.
Problem 2: Possible pyrethroid resistance
In 2022, many reports came in from western Minnesota and other parts of the state of pyrethroid failures for alfalfa weevil control. Some of these reports may have been application issues such poorly timed applications, improper rates, or as lack of coverage. However, the volume of these reports and confirmed pyrethroid resistance in western US states does make resistance a suspect here in Minnesota. While resistance could not be confirmed in 2022, there are efforts underway to test field samples in 2023 in the state. With relatively few options available already (Table 2), loss of pyrethroids only leaves organophosphates and oxadiazine groups for insecticide rotation.
A major change to recommendations where pyrethroid resistance has been documented in western states to avoid pyrethroids if resistance is suspected in a field, and do not apply the same group within three years (e.g., year 1 - organophosphate, year 2 = pyrethroid, year 3 = oxadiazine). The three-year rotation will be difficult to follow if pyrethroids are no longer an option, but it is also to protect the remaining options, especially group 22 products such as Steward. However, this highlights the current challenging alfalfa insect management situation and why we need to utilize all the integrated pest management tools we have in the toolbox to preserve insecticide options. If growers can avoid an insecticide application each year or multiple applications within a year, management options will be more flexible in other years.
One class of insecticides not listed in Table 2 are the spinosyns (group 5). These insecticides are available for organic alfalfa growers and would theoretically help with an additional group to rotate with pyrethrin insecticides (same group as pyrethroids and likely would face similar resistance concerns). However, efficacy for these products is only around 70% for alfalfa weevil. For this reason, labels for these products (e.g., Entrust SC) will specifically state the product is only useful for suppression, not control. Spinosad products can also be expensive at around $35 per acre for even low application rates.
Another option that may help hay producers with weevil issues is to transition away from planting solely alfalfa. Alfalfa-grass mixes can reduce weevil densities, but this option is going to be highly dependent on livestock needs. With the other primary options being limited insecticides and timing mowing for management, producers may need to weigh this option more heavily if they continue to face alfalfa weevil issues.
The landscape for alfalfa weevil management does seem to be changing between needing to monitor this pest for a longer portion of the growing season while trying to managing it with limited insecticide options. Alfalfa weevil issues alone make a good case for being very selective about insecticide use, but remember that many of the same insecticides are also used for pea aphid, plant bugs, and potato leaf hopper. Additional research is needed in the state to reassess how best to manage the changing alfalfa weevil situation, so for now, the guidance in this article covers the best practices we have to date.
For more information on alfalfa insect management and how to determine if insecticide application is warranted for other pests such as potato leafhopper, visit:https://extension.umn.edu/forage-pest-management/alfalfa-insects-what-look-and-how-scout.
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