Alfalfa Weevil (Kansas Alfalfa)

Scouting time

Early Season Infestation: Alfalfa is only 3 to 7 inches tall. Treat when feeding is evident on the top inch of growth – this usually requires one to two larvae per stem, depending on expected value of the hay. Retreatment may be necessary before cutting.

Mid-Season Infestation: Alfalfa is 8 to 14 inches tall. Significant feeding damage to the top 1 to 2 inches of growth is occurring on 30 to 50 percent of the terminals. High larval populations (four or more per stem) can cause severe foliage loss in only three or four days. Stubble sprays may be necessary in addition to treatment at this time.

Late Season Infestation: Early cutting may be advisable if the alfalfa is within 10 to 14 days of cutting, the hay is expected to dry quickly, and windrows will be rapidly removed from the field so the relatively delicate larvae are exposed to bright sunlight and drying winds. However, spraying before cutting is advisable if the top 2 to 3 inches on the majority of plants is being injured and harvest cannot be done immediately, or if weather conditions favor larval survivorship and other stresses make rapid regrowth following cutting unlikely.

After Harvest: Stubble sprays may be necessary to protect regrowth if eight or more larvae per square foot of stubble are present. As few as four larvae per square foot may prevent regrowth under unfavorable growing conditions. Adult weevils may also 'debark' stalks before refoliation can occur and may require treatment, especially under conditions of drought that delay regrowth.



To decide if an alfalfa field should be treated for alfalfa weevil, the stem-count decision method is recommended. Carefully break off 30 to 50 stems selected at random from across the field, and shake them individually into a deep-sided bucket. Methods used to remove infested alfalfa stems from the plant crown can influence the accuracy of the population estimate, sometimes causing improper management decisions. <br /> &nbsp;&nbsp;&nbsp;&nbsp; The finger-shear, palm-tip, or knife methods are recommended when taking alfalfa weevil larval samples for shakebucket evaluations. In the finger-shear method, the thumb and forefinger grip the stem (held horizontally) while the middle finger breaks the stem off. In the palm tip approach, one hand grasps and encloses the tip of the stem while the other breaks the stem off. The knife method involves severing the stem with a small knife using the thumb to push the stem into the knife blade. These approaches attempt to minimize the loss of larvae before the stem reaches the bucket for shaking. Count the stems, determine the average stem height, count the larvae, and determine the average number of larvae per stem. Refer to the Alfalfa Weevil Stem Count Decision Guide (below) to determine the suggested management action. The first relationship (a) was developed for situations where alfalfa was selling for $35 per ton. As the alfalfa value increases, fewer larvae are required to reach a treatment threshold. For instance, figure (b) indicates that $70/ton alfalfa should be treated at just over two larvae per stem when the alfalfa is approximately 17 inches tall, versus requiring nearly three larvae per stem to trigger treatment when the crop is valued at $35/ton. As the crop price increases, fewer alfalfa weevil larvae may be needed to trigger treatment


Chemical control

Spray gallonage for ground equipment: 10 to 12 gallons per acre for 7-inch alfalfa; 15 to 20 gallons for 8- to 15-inch alfalfa; at least 20 gallons per acre on alfalfa more than 15 inches tall. Use 30 psi and hollow cone nozzles, and adjust spray pattern as suggested by the nozzle manufacturer to overlap near the top of the canopy

Spray gallonage for aerial equipment: The use of less than 2 gallons of spray per acre has frequently resulted in less than satisfactory control. Overall efficacy frequently increases as even more carrier is used.







Alpha-cypermethrin (Fastac EC)

 0.014 to 0.025 lb. a.i. acre (2.2 to 3.8 fl. oz. acre)


Beta-cyfluthrin (Baythroid XL)

 0.0125 to 0.022 lb. a.i. acre (1.6 to 2.8 fl. oz.) Data indicates that rates of 0.015 to 0.02 should provide the longest protection against larval damage.


Chlorpyrifos* (numerous products)

Check label, but generally 1 to 2 pints acre


Chlorpyrifos plus gamma-cyhalothrin* (Cobalt Advanced)

19 to 38 fl. oz. of product acre



Chlorpyrifos plus zeta-cypermethrin (Stallion)

 9.25 to 11.75 fl. oz. acre



Cyfluthrin (Tombstone)

0.025 to 0.044 lb. a.i. a (1.6 to 2.8 fl. oz.)


Gamma-cyhalothrin (Proaxis, Declare)

 0.01 to 0.015 lb. a.i. acre (2.56 to 3.84 fl. oz.) (Declare: 1.02 to 1.54 fl. Oz acre) A higher rate should provide the longest protection against larval damage.


Indoxacarb (Steward)

 4.0 to 11.3 fl. oz. acre



Lambda-cyhalothrin (numerous products)

 0.02 to 0.03 lb. a.i. acre. Data reviewed so far indicates that the higher rate should provide the longest protection against larval damage.


Lambda-cyhalothrin plus chlorantraniliprole (Voliam Xpress)

 6.0 to 9.0 fl. oz. acre



Methomyl (Lannate)

 LV 3 pts. acre, SP 1 lb acre



Phosmet* (Imidan)

 LV 3pts acre, SP 1 lb acre



Permethrin (multiple products)

 0.2 lb. a.i. acre. Results in Kansas research trials have been variable.


Lambda-cyhalothrin plus chlorantraniliprole (Besiege)

 5.0 to 10.0 fl. oz. acre



Zeta-cypermethrin (Mustang MAXX, etc.)

 0.014 to 0.025 lb. a.i. acre (2.24 to 4.0 fl. oz. acre)



Non-chemical controls


Content authors


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