Managing alfalfa weevil in 2026

 Anthony Hanson, Regional Extension Educator - Field Crops Integrated Pest Management

Both as an entomologist an alfalfa producer, May through June is a busy time of year. Alfalfa weevil is often the driver of that busy schedule as we get into June. It is becoming an increasingly difficult pest to manage for farmers across the Upper Midwest (Fig. 1), especially as farmers have to weigh economic costs of agronomic decisions, inputs, and preventing pest damage in financially challenging times.

Figure 1. Alfalfa weevil larvae under a magnifying scope collected from Morris, MN showing the range in size due to age.

Farmers have been seeing a two-pronged challenge to managing alfalfa weevil in recent years: 1. An extended larval feeding season. 2. Reports of pyrethroid 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 is a deep-dive into alfalfa weevil management and what adaptations are needed to manage this pest long-term.

Alfalfa weevil identification and development

While some adult alfalfa weevil overwinter in alfalfa, most find shelter in nearby dense grass and or wooded areas. Based on pest forecasts from this winter, soil temperatures likely did not get cold enough across much of the state to cause significant weevil mortality. Alfalfa weevil goes through one generation per year. Eggs are laid in the stem of alfalfa plants in spring, typically in early to mid-May. The larvae 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, leaving only leaf veins. From a distance, heavily defoliated fields have a gray or frosted appearance. The damage window typically coincides with the end of the 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 first become aware of the larvae when they find them on their mower during the first cutting of the year (Figs. 1-2). If populations are high, larvae that survive mowing can end up concentrated in the windrows and feed on regrowth if hay isn’t quickly removed.

 

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.

Mature larvae pupate in loose, round cocoons on the ground.  While they may briefly feed, the emerging adults enter a summer dormancy (Fig. 3), so scouting ends when larvae have developed into pupae. These adults will be present in fields and nearby grassy areas, but eggs are typically laid the following spring. Typically adults are not managed because they are mobile, and trying to reduce adult populations does not necessarily mean there will be fewer egg-laying adults in your field next year.  

Figure 3. Alfalfa weevil life cycle. Mimi Broeske, Nutrient and Pest Management Program, University of Madison-Wisconsin

 

Problem 1: Extended weevil feeding season

Similar to crop development, alfalfa weevil development is affected by temperature (Fig. 4). Current larvae appear primarily to be large third and fourth instars with additional development and feeding time needed farther north. Early larval detection isn’t the only issue though for timing. 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 by then.

Figure 4. Eastern strain alfalfa weevil degree-days as of June 2, 2026. 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 had 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 1998, 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 variation between the strains that cause differences in timing and larval population peaks during the growing season, such as fall egg laying behavior in the western strain. Normally alfalfa weevil eggs cannot survive winters in northern states, though in southern states with milder winters, larvae from these eggs can emerge and feed earlier than eggs laid in spring.

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. 5). In general, plan on at least 1-2 weeks of additional scouting after these maps indicate eastern strain larval development is finished. 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 integrated pest management

Different parts of the United States use a variety of methods to determine whether economically damaging levels of alfalfa weevil are present. Scouting is the cornerstone of any integragated pest management (IPM) program and occur in mid-May through June by using a sweep net to determine if larvae are present. While not highly accurate for determining treatment thresholds, sweep net samples can be a quick indicator of weevil presence to indicate follow up with more accurate sampling methods. Remember not to count adults in scouting tallies as they 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.

1. Collect 50 to 100 alfalfa stems (10 to 20 randomly selected stems from five locations).      
2. Examine whether they show obvious feeding damage – pinhole or more severe feeding – in the rapidly growing tip leaves and leaf buds.      
3. Divide the number of stems with recent tip injury by the total stems collected to convert it to a percent.  
4. 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). Remember that economic thresholds are the trigger-point for making an insecticide decision and there is some buffer time before reaching the the break-even point for yield loss vs. insecticide cost or the economic injury level. Treating at populations below the values in Table 1 means you are spending more money on insecticide application than what you'd get in return. 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.

		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.

Mowing as a management tactic

Historically in Minnesota, most crop injury occurs in late first cutting and 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. Mowing, crimping, and exposure from lack of cover due to regular harvest can kill many larvae. Mowing may also be the only option when considering any pre-harvest intervals for insecticide use. University field trials in Wyoming examined the efficacy of early-mowing for alfalfa weevil management and found early was as effective as using an insecticide from an economic perspective. Mowing strategies should be the first consideration before exploring insecticide options.

 

Similar to how even insecticides will not always reduce pest populations to zero, crop loss from alfalfa weevil and other insects, such as cutworms, can still occur under windrows after mowing where surviving larvae are protected (Fig. 5). This damage is most pronounced when hay cannot be harvested shortly after mowing after high population densities.

 

Figure 5. Bare strips from heavy alfalfa weevil feeding occurred under windrows in a problem field near Morris in mid-June 2022, MN. Photo: Anthony Hanson.   

 

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 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. Up to the 1970s and 80s, parasitoid wasps were released by the USDA the appeared to be well-established and controlling alfalfa weevil. There has been discussion that regular use of broad-spectrum insecticides may have reduced parasitoid numbers in the Upper Midwest and made alfalfa weevil flare-ups more common, so formal reassessment of those natural enemy populations in our region is needed. In addition to further reducing natural enemy populations, multiple applications in a field per year 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).

 

Table 2. Conventional insecticide options for alfalfa weevil. This is not an exhaustive list and does not endorse specific products or trade names. If using an insecticide, use high labeled insecticide rates as low rates can speed up the development of resistance.

Group	Class	Active ingredient	Trade Names
1A	Carbamate	Methomyl	Lanate
		Carbaryl*	Sevin
1B	Organophosphate	Malathion†	Malathion
		Phosmet†	Imidan
		Chlorpyrifos**	Pilot 4E, Chlorpyrifos 4E, Lorsban (no longer registered)
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 have variable control, but are closer to 100% efficacy if in a mixture (e.g., malathion + phosmet). 
‡There have been reported failures of pyrethroids in MN; resistance is possible, but has not yet been officially confirmed as the cause. If you have what appears to be an insecticide failure in a field, do not use the same group twice in a row. 

**Very few chlorpyrifos products are registered with the Minnesota Department of Agriculture this year for food or forage crops, and many are being discontinued. Other previously registered chlorpyrifos products (e.g., Lorsban) cannot be applied to field crops in 2026 unless registered with MDA.

Newly emerged adult weevils can be found in alfalfa during the summer but seldom cause economic injury. However, this also means they can be exposed to insecticides used for later-season pests, such as potato leafhopper. 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 (group 3 pyrethroid), may only be available for seed alfalfa production and not for feed use.  

 

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 anyways, 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.

Problem 2: Possible pyrethroid resistance

Pesticide resistance is always a concern in pest management. In 2022, many reports came in from western Minnesota and other parts of the state of pyrethroid failures for alfalfa weevil control, and this has continued through 2025. 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 raises the possibility of pyrethroid resistance here in Minnesota. With relatively few options available already (Table 2), loss of pyrethroids only leaves organophosphates and oxadiazine groups for insecticide rotation, so using scheduled or "insurance" insecticide applications rather than IPM and economic-threshold based applications means not only unneeded inputs, but the likely accelerating the loss of insecticides in future years.

 

Some laboratory insecticide tests of field-collected alfalfa weevil in west-central Minnesota have been done in collaboration with NDSU to attempt to confirm resistance. In 2025, larvae were collected two Minnesota locations: the UMN West-Central Research and Outreach Center at Morris and a farm near Brooten that has not applied pyrethroids in the past 10 years. Larvae were placed in glass vials coated with various concentrations of a pyrethroid (group 3) to determine what concentration was needed to kill 50% of larvae as a common measure of efficacy. For the pyrethroid test using lambda-cyhalothrin, LC50s were 0.028 ug/cm2 for Morris and and and 0.084 ug/cm2 for Brooten locations meaning that the Morris location had higher pyrethroid efficacy or less insecticide needed to reach 50% mortality. 

 

If you calculate what the equivalent field rate for that pyrethroid would be in a vial, the LC50 for the Morris population was only 8% of field rate, but the Brooten population appeared to have more resistance with an LC50 that's 24% of the field rate. Those results were also surprising for Brooten since foliar insecticides had not been needed on that farm since at least 2015, but could be due to insect movement from other fields in the area. With the Brooten population as an example, this shows there is not much buffer between the amount needed to kill alfalfa weevil and the maximum amount that can actually be applied in a field. In the field, applications are never perfect as there could an issue with the sprayer, weather, lack of canopy penetration, etc. that reduces how much insecticide actually reaches the insects. If populations with this level of resistance are more widespread, it may explain the variable reports of efficacy issues across the state. 

 

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. 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. 

 

While using different insecticide group numbers in the same tank-mix might be marketed for increased efficacy, this is not recommended because it will increase the rate of resistance even more and whittle down already limited insecticide options.

 

Alternative insecticides

Questions often are asked about creative alternatives to add to an insect application with fewer insecticides currently available. One is using insecticide synergists like piperonyl butoxide (PBO) to increase efficacy of pyrethroids. Insecticide trials from Oklahoma for alfalfa weevil including PBO in insect applications. Efficacy was variable in some cases, but in general, plots using PBO did not have an increase in efficacy for pyrethroids or other individual insecticide groups. More telling though was that there were no statistically positive increases in yield when adding PBO to the tank in these alfalfa weevil trials. In one case with a indoxacarb application (e.g., Steward) yield was actually lowered when PBO was added. In general, do not expect PBO to make pyrethroid use more feasible in areas when efficacy is already in question.

 

Growers also sometimes ask about adding sugar or other similar products such as milk to sprays as another creative way to supposedly control alfalfa weevil. There will be a future article addressing this topic for insects more broadly, but in short, there are no known field trials in Minnesota or surrounding states that show that applying sugar to alfalfa will reduce alfalfa weevil numbers either by directly affecting them or by drawing in natural enemies. Field research from Utah did examine the effects of sugar sprays on on alfalfa weevil management, and it found that while parasitoid wasp populations were increased in sugar-treated plots, alfalfa weevil numbers still remained unchanged. 

Spinosyn insecticides (group 5) are not listed in Table 2.  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 spinosyn 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. Spinosyn products can also be expensive at around $35 per acre for even low application rates with reports from agronomists suggesting costs are approaching $70 to $80 per acre.

Avoiding pure-stand alfalfa if possible

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.

 

Outlook

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 manage it with limited insecticide options. Alfalfa weevil issues alone make a good case for being very selective about insecticide use and using IPM, but remember that many of the same insecticides are also used for pea aphid, plant bugs, and potato leaf hopper. Spraying for any one of these insects can make alfalfa weevil management next year, so pay careful attention to how you manage insects across the entire growing season. 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. If you have questions on alfalfa insect IPM, feel free to contact Dr. Anthony Hanson at hans4022@umn.edu.

 

This work was supported by a federal grant from USDA National Institute of Food and Agriculture: Crop Protection and Pest Management Extension Implementation Program (grant # 2024-70006-43568).