Anthony Hanson, Nathan Drewitz, Bruce Potter, and Jared Goplen
Mid-May to June is typically the time to begin scouting for insect problems in alfalfa. However, even with our cold spring of 2022, there have already been reports of alfalfa weevil adults and possibly larvae in southwest and central Minnesota. Even if larvae are present, insecticide applications may not be needed if populations are not extreme or if alfalfa fields will be harvested soon after seeing increased populations. Scouting through early June will determine if any action is needed. Also remember that insecticide options are more limited compared to previous years with chlorpyrifos (e.g., Lorsban) applications no longer being allowed.
Alfalfa weevils overwinter as adults and only go through one generation per year. Larvae are the most damaging stage that can skeletonize leaves where only leaf veins or holes remain. 1st stage larvae are yellow to olive with 2nd through 4th stage 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. 1).
Once larvae develop into pupae, they are no longer causing feeding damage, and newly developed adults enter a summer dormancy (Fig. 2). These adults will be present in fields, but will not be laying eggs until the following year, so economic damage from alfalfa weevil ends when larvae are no longer present. This damage window typically coincides with the end of first cutting and the start of regrowth for the second cutting. Similar to crop development, alfalfa weevil development is affected by temperature. Despite warm temperatures last year during the 2021 drought, there were reports of economic damage in some fields for second cutting in what appeared to be delayed development of larvae that would normally have developed into adults. Larvae that have been found in central MN so far were very early instars, and depending on your location in the state and spring temperatures, development could range from egg hatch just beginning to just starting to see some larval feeding.
Mid-May to June is typically the time to begin scouting for insect problems in alfalfa. However, even with our cold spring of 2022, there have already been reports of alfalfa weevil adults and possibly larvae in southwest and central Minnesota. Even if larvae are present, insecticide applications may not be needed if populations are not extreme or if alfalfa fields will be harvested soon after seeing increased populations. Scouting through early June will determine if any action is needed. Also remember that insecticide options are more limited compared to previous years with chlorpyrifos (e.g., Lorsban) applications no longer being allowed.
Alfalfa weevil identification and development
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| Figure 1. Alfalfa weevil larvae can be distinguished from other weevil species by the black head capsule. Clover leaf weevil larvae can sometimes be confused for alfalfa weevil, but instead have a tan head capsule. Photo: Anthony Hanson. |
Once larvae develop into pupae, they are no longer causing feeding damage, and newly developed adults enter a summer dormancy (Fig. 2). These adults will be present in fields, but will not be laying eggs until the following year, so economic damage from alfalfa weevil ends when larvae are no longer present. This damage window typically coincides with the end of first cutting and the start of regrowth for the second cutting. Similar to crop development, alfalfa weevil development is affected by temperature. Despite warm temperatures last year during the 2021 drought, there were reports of economic damage in some fields for second cutting in what appeared to be delayed development of larvae that would normally have developed into adults. Larvae that have been found in central MN so far were very early instars, and depending on your location in the state and spring temperatures, development could range from egg hatch just beginning to just starting to see some larval feeding.
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| Figure 2. Alfalfa weevil life cycle. Photo: Mimi Broeske, Nutrient and Pest Management Program, University of Madison-Wisconsin. |
Scouting and management
Scouting should occur in mid-May through June by using a sweep net to determine if larvae are present. If present and easily found in sweeps, 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).Table 1. Economic thresholds for alfalfa weevil based on hay value.
Most damage occurs around 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 most larvae from regular harvest. Early mowing, if feasible, can also control alfalfa weevil to some degree in place of insecticide if thresholds are reached, especially when considering any pre-harvest intervals for insecticide use. However, damage from alfalfa weevil and other insects, such as variegated cutworm, can still occur under windrows where larvae are protected. 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.
Alfalfa weevil is typically the first pest insecticides may be used for in alfalfa during the year. Insecticides labeled for alfalfa weevil include pyrethroids and organophosphates. 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). 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) for alfalfa weevil, but needs to be treated later in the year for potato leaf hopper, a pyrethroid (group 3A) or oxadiazine (group 22) should be considered to maintain a rotation of insecticide modes of action. Also, carefully read labels as some common products, such as bifenthrin, may only be available for seed alfalfa production and not for feed use.
Table 2. Insecticide options for alfalfa weevil. This is not an exhaustive list and does not endorse specific products or trade names.
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.
For more information on alfalfa insect management and how to determine if insecticide application is warranted for pests such as alfalfa weevil or potato leafhopper, visit:https://extension.umn.edu/forage-pest-management/alfalfa-insects-what-look-and-how-scout.
| Hay value ($/ton) | ||||||||
|---|---|---|---|---|---|---|---|---|
| Treatment | $50 | $75 | $100 | $125 | $150 | $175 | ||
| Stem height | cost/acre | Average larvae per stem | ||||||
| 10-15 inches (mid-vegetative) |
$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) |
$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) |
$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.
Most damage occurs around 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 most larvae from regular harvest. Early mowing, if feasible, can also control alfalfa weevil to some degree in place of insecticide if thresholds are reached, especially when considering any pre-harvest intervals for insecticide use. However, damage from alfalfa weevil and other insects, such as variegated cutworm, can still occur under windrows where larvae are protected. 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.
Alfalfa weevil is typically the first pest insecticides may be used for in alfalfa during the year. Insecticides labeled for alfalfa weevil include pyrethroids and organophosphates. 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). 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) for alfalfa weevil, but needs to be treated later in the year for potato leaf hopper, a pyrethroid (group 3A) or oxadiazine (group 22) should be considered to maintain a rotation of insecticide modes of action. Also, carefully read labels as some common products, such as bifenthrin, may only be available for seed alfalfa production and not for feed use.
Table 2. 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 |
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.
For more information on alfalfa insect management and how to determine if insecticide application is warranted for pests such as alfalfa weevil or potato leafhopper, visit:https://extension.umn.edu/forage-pest-management/alfalfa-insects-what-look-and-how-scout.
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