Fact Sheets And Publications
Alfalfa Weevil Control in Alfalfa
March 2025 | Written by: David Owens, Ben Sammarco, Joanne Whalen
Description
Adult - ¼ inch long, mouth part “snout” like, brown with a dark stripe down its back (fig. 1)
Larva - up to 3/8 inch long, yellowish–green with a white stripe down the middle, black head (fig. 2)
Egg - orange, laid in clusters of 2-25 in alfalfa stems (fig.3)
Life History and Phenology
The alfalfa weevil (AW) overwinters in both the adult and egg stages. Eggs are laid in Fall and Spring in alfalfa stems. Embryo development occurs when temperatures are above 48 degrees F (fig. 3). AW lays more eggs in new growth (6-7 inch alfalfa) compared to bud stage and stubble (Dively 1970). Egg hatch occurs at around 300 degree days (base temperature 48 F), but as eggs laid in the Fall partially develop before winter, egg hatch can occur very early in the spring. Warm fall and winter temperatures can result in egg hatch in late February or early March, and economic populations as early as mid-March.
Alfalfa weevils generally have one generation per year. A weevil larva passes through four larval stages in approximately 3 weeks, then spins a cocoon on the leaves of alfalfa plants or on the ground (fig. 4). Eggs laid in the spring emerge as adults around mid-June and feed for a few weeks, but do not produce a second larval generation. Alfalfa weevil can feed on other legumes such as clovers but will prefer alfalfa for feeding and reproduction (Pellissier et al. 2017).
Damage
Alfalfa weevil is primarily a pest of the first cutting, but high populations at the time of the first cutting can damage the regrowth of the second cutting. Larvae feed on the tips of plants and on leaves, resulting in a skeletonized plant (fig. 5). Severe damage causes the leaves to turn gray and appear “frosted”. Damage from larval feeding results in reduced yield and quality of the first cutting, and extensive damage to the first cutting may result in season-long yield loss and reduce the life of the stand.
Adults also feed on leaves but are less likely to cause economic loss. If alfalfa is less than 12 inches tall when the weevil population peaks, fewer than 2 larvae per stem can defoliate a crop. In comparison, alfalfa greater than 18 inches tall can tolerate significantly higher populations. Note: overwintered egg hatch can result in highly damaging populations before alfalfa has any significant spring growth.
If fields are monitored routinely and sprayed only when economic levels are present, natural enemies may be able to suppress weevil populations below an economic level in subsequent years.
Sampling and Decision Making
Begin sampling in late February - early March and continue on a weekly basis until the first cutting. Fields should also be checked within one week of the first cutting of both larval and adult damage to the re-growth. During the first visit, examine 5-10 stems for damage and larvae. A full stem sample is not needed until damage or larvae are found on the plants. If alfalfa is less than 6 inches tall and half of the stems exhibit signs of fresh leaf feeding damage, spray as soon as possible (Laub 2017)
If leaf feeding is present, randomly collect 30 stems throughout the field. Grasp stems at the base and place each stem upside down in a bucket. After collecting the stems, separate them into 3 or 4 bundles and beat them against the inside of the bucket to dislodge larvae from the stems. Count and record all larvae found per 30 stems. Note that small larvae may be hiding deep in stem terminals and may not dislodge. Measure 10 of the 30 stems and record the average stem height. Also, note if buds or flowers are present to determine the percentage of plants in the bud or flower stage.
If alfalfa is in the full-bud stage and economic levels are present, early harvest is an alternative to spraying. If harvest is not possible within 3 days and populations are increasing, use a short residual insecticide. If economic levels of alfalfa weevil are present before harvest and you decide to cut instead of spray, be sure to check fields within one week of cutting for damage to the re-growth.
If temperatures remain cool after cutting, there is often not enough “stubble heat” to control populations with early cutting. In some cases, damage to re-growth can be significant. A stubble treatment will be needed if you find 2 or more weevils per stem and the population levels remain steady. Early harvesting should only be done once in the season, and subsequent cuttings should be done after 10% bloom.
Use the following tables to determine the proper threshold for the number of weevils per 30 stems.
Pure Stand Alfalfa
| Value of Hay ($/ton) | Plants 12-18 inches AND Control costs ($/acre) | Plants 18-24 inches AND Control costs ($/acre) | Plants 24-30 inches AND Control costs ($/acre) | |||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| $14 | $16 | $20 | $25 | $14 | $16 | $20 | $25 | $14 | $16 | $20 | $25 | |
| $300 | 32 | 36 | 47 | 59 | 35 | 40 | 51 | 64 | 37 | 42 | 53 | 66 |
| $320 | 30 | 34 | 44 | 55 | 32 | 38 | 48 | 61 | 35 | 40 | 49 | 62 |
| $340 | 28 | 32 | 41 | 52 | 31 | 36 | 45 | 57 | 33 | 37 | 47 | 58 |
| $360 | 26 | 30 | 39 | 49 | 29 | 34 | 43 | 54 | 31 | 35 | 44 | 55 |
| $380 | 25 | 28 | 37 | 47 | 27 | 32 | 41 | 51 | 29 | 33 | 42 | 52 |
| $400 | 24 | 27 | 35 | 44 | 26 | 30 | 39 | 49 | 28 | 32 | 40 | 50 |
| $420 | 23 | 26 | 34 | 42 | 25 | 29 | 37 | 47 | 27 | 30 | 38 | 47 |
| $440 | 22 | 25 | 32 | 40 | 24 | 28 | 35 | 45 | 25 | 29 | 36 | 45 |
| $460 | 21 | 23 | 31 | 39 | 23 | 26 | 34 | 43 | 24 | 28 | 34 | 43 |
| $480 | 20 | 22 | 29 | 37 | 22 | 25 | 33 | 41 | 23 | 26 | 33 | 41 |
| $500 | 19 | 22 | 28 | 36 | 21 | 24 | 31 | 40 | 22 | 25 | 32 | 40 |
| $520 | 18 | 21 | 27 | 34 | 20 | 23 | 30 | 38 | 22 | 24 | 31 | 38 |
| $540 | 18 | 20 | 26 | 33 | 19 | 23 | 29 | 37 | 21 | 24 | 29 | 37 |
| $560 | 17 | 19 | 25 | 32 | 19 | 22 | 28 | 36 | 20 | 23 | 28 | 36 |
| $580 | 16 | 19 | 24 | 31 | 18 | 21 | 27 | 34 | 19 | 22 | 27 | 34 |
| $600 | 16 | 18 | 24 | 30 | 17 | 20 | 26 | 33 | 19 | 21 | 26 | 33 |
| $620 | 15 | 17 | 23 | 29 | 17 | 20 | 25 | 32 | 18 | 21 | 26 | 32 |
| $640 | 15 | 17 | 22 | 28 | 16 | 19 | 25 | 31 | 18 | 20 | 25 | 31 |
| $660 | 14 | 16 | 22 | 27 | 16 | 18 | 24 | 31 | 17 | 19 | 24 | 30 |
If alfalfa is under drought stress or growing very slowly, reduce thresholds by 0.5 to 1 weevil per stem.
Mixed Stand Alfalfa
The number of weevils per 30 stems in the table below should be adjusted based on the % of alfalfa in a mixed stand. For example, if a threshold below 40 weevils, but alfalfa only comprises ¾ or 75% of the hay, divide the threshold number by the proportion of alfalfa (ex 40 ÷ 0.75 = 53 larvae per 30 stems). Weevil control is generally not justified once the proportion of alfalfa in a mixed stand is below 50%.
| Value of Hay ($/ton) | Plants 12-18 inches AND Control costs ($/acre) | Plants 18-24 inches AND Control costs ($/acre) | Plants 24-30 inches AND Control costs ($/acre) | ||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| $14 | $16 | $20 | $25 | $14 | $16 | $20 | $25 | $14 | $16 | $20 | $25 | ||
| 120 | 79 | 191 | 114 | 143 | 87 | 100 | 124 | 152 | 91 | 105 | 130 | 163 | |
| 140 | 68 | 77 | 99 | 123 | 75 | 86 | 107 | 134 | 78 | 90 | 112 | 140 | |
| 160 | 60 | 68 | 86 | 108 | 65 | 75 | 93 | 116 | 68 | 79 | 98 | 123 | |
| 180 | 52 | 60 | 77 | 97 | 58 | 67 | 84 | 105 | 61 | 70 | 87 | 109 | |
| 200 | 48 | 54 | 69 | 86 | 52 | 60 | 76 | 95 | 55 | 63 | 79 | 99 | |
| 220 | 43 | 49 | 63 | 79 | 47 | 55 | 69 | 87 | 50 | 57 | 72 | 91 | |
| 240 | 40 | 45 | 58 | 73 | 43 | 50 | 63 | 79 | 46 | 53 | 66 | 83 | |
| 260 | 37 | 42 | 54 | 68 | 40 | 46 | 59 | 74 | 43 | 49 | 61 | 77 | |
| 280 | 34 | 39 | 50 | 63 | 37 | 43 | 55 | 69 | 40 | 45 | 56 | 70 | |
| 300 | 32 | 36 | 47 | 58 | 35 | 40 | 51 | 64 | 37 | 42 | 53 | 67 | |
| 320 | 30 | 34 | 44 | 55 | 33 | 38 | 48 | 61 | 35 | 40 | 49 | 62 | |
| 340 | 28 | 32 | 41 | 51 | 31 | 36 | 45 | 57 | 33 | 37 | 46 | 58 | |
| 360 | 26 | 30 | 39 | 49 | 29 | 34 | 43 | 54 | 31 | 35 | 44 | 55 | |
| 380 | 25 | 28 | 37 | 46 | 27 | 32 | 41 | 52 | 29 | 33 | 42 | 53 | |
| 400 | 24 | 27 | 35 | 45 | 26 | 30 | 39 | 49 | 28 | 32 | 39 | 49 | |
Scout again one to two weeks after an insecticide application, as spring-laid eggs and late pupae may have avoided contact with the application and emerged afterward, requiring a second application.
Chemical Control
Once thresholds have been exceeded, pay attention to weather conditions before and after an application. Delay application a couple of days after cold or freezing weather to allow weevils time to warm up and resume activity and come into contact with insecticide residue. Weevil activity resumes once temperatures are in the high 40s.
| Insecticides Labeled for Alfalfa Weevil Product MOA Active Amount PHI REI (hrs) Remarks Parasitoid/ | |||||||
|---|---|---|---|---|---|---|---|
| Group | Ingredient | product per acre | PHI Days | REI (hrs) | Remarks | Parasitoid/ pollinator toxicity | |
| Carbaryl 4L Lannate LV | 1A 1A | Carbaryl Methomyl | 1.5 qt 3.0 pt. | 7 7 | 12 48 | CAUTION May cause temporary, non yield-limiting bleaching, does not control adults. DO NOT apply more than once per cutting DANGER | L-H/H H/H |
| Lannate LV Imidan | 1A 1B | Methomyl Phosmet | 3.0 pt. 1.0 to 1.3 | 7 7 | 48 120 | DANGER Do not apply when temps >50F WARNING, DO NOT apply | H/H |
| 70-W Fastac EC | 3 | Alpha-cyperm | lb 2.2 to 3.8 | Cutting/ | 12 | more than once per cutting, Do not not use stickers, extenders or latex, pineolene based adjuvant DANGER | |
| Baythroid | 3 | ethrin Beta-cyfluthrin | fl. oz. 1.6 to 2.8 | grazing: 3 Seed:7 7 | 12 | DO NOT apply more than once per cutting WARNING | H/H |
| XL Tombstone | 3 | Cyfluthrin | fl. oz. 1.6 to 2.8 | 7 | 12 | DANGER | |
| Tombstone 2EC | 3 | Cyfluthrin | 1.6 to 2.8 fl. oz. | 7 | 12 | DANGER | |
| Declare | 3 | Gamma-cyhalo thrin | 1.02 to 1.54 fl. oz. | Forage harvest: 1 Hay harvest: 7 | 24 | CAUTION DO NOT apply more than once per cutting | |
|---|---|---|---|---|---|---|---|
| Warrior Il or OLF | 3 | Lambda-cyhal othrin | 1.28 to 1.92 fl. 0z | Forage harvest: 1 Hay harvest: 7 | 24 | WARNING; DO NOT apply more than once per cutting | H/H |
| Pounce 25WP or OLF | 3 | Permethrin | 6.4 to 12.8 fl. oz. | ≤ 0.1 lb. AI/A (6.4 oz): 0d > 0.1 lb. AI/A: 14d | 12 | CAUTION; A maximum of 12.8 fl.oz may be applied per cutting DO NOT apply to mixed forage | H/H |
| e | 3 | Zeta-cypermet hrin | 2.24 to 4.0 fl. oz. | 3 | 12 | WARNING | M/H |
| Steward EC | 22 | Indoxacarb | 6.7 to 11.3 fl. oz | 7 | .12 | GENERAL USE A maximum of 11.3 fl oz may be applied per cutting | L/H |
| W | 1B | Malathion | 2.0 pt/A | 0 | 12 | GENERAL USE | H/H |
The label is the law. Always consult product label for most up to date application guidance. Check EPA bulletins for live changes to product restrictions. Natural enemy and pollinator impact based on University of California IPM: https://ipm.ucanr.edu/agriculture/alfalfa/relative-tox icities-of-insecticides-and-miticides-used-in-alfalfa-to-natural-enemies-and-honey-bees/#gsc.tab=0. Insecticides not rated are generally assumed to have high impact to both natural enemies and pollinators. When populations are well over threshold Steward is recommended to bring populations back under control. Results of successful control should be apparent by a drop in population 5 days after treatment. Steward has been the most consistent product tested in UD spray trials, many of which can be found at https://www.udel.edu/academics/colleges/canr/coo perative-extension/sustainable-production/pest-mana gement/insect-management-reports/. Please note that often winter annual weeds are highly attractive to pollinators and thus there may be considerable pollinator and beneficial insect activity present in a field at the time of application. Use lower impact insecticides if possible and treat late in the day or evening if possible to reduce impact on beneficials.
Biological Control
Alfalfa weevils are the target of several braconid parasitoids in the genus Bathyplectes (introduced in 1911), which form distinctive football-shaped cocoons with a lateral white band. Weevil larvae are preyed upon by generalist predators such as lady beetles, lacewings, and damsel bugs, but these predators may opt to feed on aphids instead of weevils if present (Pellissier 2017). The most successful Bathyplectes species, B. anurus, requires approximately 21-25 days to complete larval development (Bartell and Pass 1980). Wasps oviposit primarily in 2ndand 3rdinstar weevil larvae. These wasps are generally believed to keep AW in check in large areas of the Eastern United States. Parasitoid peak emergence from overwintered cocoons occurs between 350-400 degree days, approximately when alfalfa is 10-15 inches high (Eklund and Simpson 1977, Barney 1977). Recent AW outbreaks on Delmarva begs the question if overwintered late-winter larval hatch occurs before parasitoid activity resumes in spring (hypothesized by Kuhar et al. 1999 in Virginia) or if current insecticide use patterns suppress parasitoids more currently than previously.
Under certain conditions, a fungal pathogen, Zoopthora phytonomi, can destroy a large portion of AW larvae which may reduce AW pressure in the following year (figure 6). Other parasitoids were introduced, targeting eggs, larvae, and adults (Cornell Biological Control).
References
Dively, Galen P. 1970. Overwintering Alfalfa Weevil Eggs in Three Stages of Alfalfa Growth in New Jersey. Annals of the Entomological Society of America 63: 1213-1216.
Pellissier, Mackenzie E., Z. Nelson, and R. Jabbour. 2017. Ecology and Management of the Alfalfa Weevil (Coleoptera: Curculionidae) in Western United States Alfalfa. Journal of Integrated Pest Management 8(1): 1-7.
Whitworth, J.R., H. Davis, and B. McCornack. 2011. Alfalfa Weevils. Kansas State Research and Extension\
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