Start monitoring alfalfa weevils using degree days and scouting

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Degree day accumulations for Kansas alfalfa weevils are well ahead of normal for the entire state this year (Table 1).  A similar trend occurred in 2023. There were reports of treatable infestations in south-central Kansas during the last week of March, and in mid-April of 2023, fields were being treated, or close to being treated, in the central, southeast, and northeast regions of the state.  It is recommended that scouting for weevil activity should be occurring right now in all regions of the state.


Table 1. Alfalfa weevil degree days as of March 12, 2024.  Kansas Mesonet, 2024: Kansas Mesonet Alfalfa Weevil Degree Days Accessed 12 April 2023, http://mesonet.k-state.edu/agriculture/degreedays/

Accumulated Alfalfa Weevil Growing Degree Days

 

Station

Actual

Normal

Departure

Northwest

Cheyenne

204

29

176

 

Colby

213

26

187

Southwest

Garden City

274

61

214

 

Meade

336

64

272

North-central

Hays

271

32

239

 

Osborne

244

15

229

South-central

Hutchinson

278

36

242

 

Harper

343

64

279

Northeast

Manhattan

274

28

246

 

Corning

246

13

233

Southeast

Cherokee

352

58

298

 

Woodson

312

43

269

 

Alfalfa weevil degree days are a great way to estimate what might be going on in the field and serve as a useful tool to know when to time scouting.  Alfalfa weevil eggs begin hatching after 300 degree days have accumulated (Table 2).  Since we cannot determine if eggs present were laid the previous fall or the current spring, in Kansas, scouting should start after 180 degree days have accumulated starting from January 1.
 

Table 2. Approximate degree days required for alfalfa weevil development.  Excerpt from Whitworth et. al., Alfalfa Weevils, Kansas State University, October 2022 (MF2999).

Degree Days or Thermal Units

Stage

Importance

25-300

Eggs hatch

In stems

301-450

1st and 2nd instars

Leaf pinholing – start sampling

450-600

2nd and 3rd instars

Defoliation

600-750

3rd and 4th instars

Defoliation

750+

Pupa to adult

Adults – some feeding - oversummering

 

Be aware of insecticide resistance

While warmer spring temperatures allow for faster alfalfa weevil development, be aware that dramatic temperature drops can slow down alfalfa growth, making the plants unable to keep up with feeding damage.  Treatment may be warranted in shorter fields between 3 and 7 inches tall when feeding is evident on the top inch of growth and 1 to 2 larvae are present.  If a field is treated, it is important to verify that the expected amount of control was achieved.  In 2020, populations of alfalfa weevil resistant to lambda-cyhalothrin were verified in northwest and southwest Kansas and Oklahoma.  While this resistance has not appeared to become a widespread problem for Kansas producers, a couple of fields reported in central Kansas last year where lambda-cyhalothrin had reduced efficacy. Oklahoma continues to have resistance statewide.  Numerous products are available for alfalfa weevil control in Kansas (Table 3).  When making management decisions, it is important to rotate modes of action as this is an effective way to prevent the development of resistance. 
 

Table 3. Products registered in Kansas for control alfalfa weevil. Treatments listed are mainly used for treating alfalfa weevil larvae; products with an asterisk are also recommended for adult alfalfa weevil control. For more specific information relative to any insecticide, always refer to the actual label on the product.

Trade Name

Chemical Name

Mode of Action Class

Fastac CS

Alpha-cypermethrin

3A

Baythroid XL

Beta-cyfluthrin

3A

Lorsban

Chlorpyrifos

1B

Cobalt Advanced

Chlopyrifos + lambda-cyhalothrin

1B+3A

Stallion

Chlopyrifos + zeta-cypermethrin

1B+3A

Tombstone

Cyfluthrin

3A

Proaxis

Gamma-cyhalothrin

3A

Steward

Indoxacarb

22A

Warrior II w/Zeon Tech

Lambda-cyhalothrin

3A

Beseige

Lambda-cyhalothrin + chlorantraniliprole

3A+28

Lannate

Methomyl

1A

Imidan 70-W*

Phosmet

1B

Mustang MAXX

Zeta-cypermethrin

3A

 

One update to control options to be aware of for 2024 is the reversal of the EPA’s ruling to ban chlorpyrifos (Lorsban and others).  On November 2, 2023, the Eighth Circuit issued a ruling vacating EPA’s final rule and sending the issue of chlorpyrifos tolerances back to EPA for further proceedings.  Following that, the EPA issued a technical correction in which 11 special uses of chlorpyriphos were allowed.  Among the 11 special uses pertinent to Kansas are alfalfa, cotton, soybean, and winter wheat.

For the most up-to-date alfalfa weevil degree day accumulations, visit the Kansas Mesonet Alfalfa Weevil Degree Day Calculator (https://mesonet.k-state.edu/agriculture/degreedays/).  For a complete guide to alfalfa weevil management recommendations, please refer to the upcoming 2024 Alfalfa Insect Pest Management Guide that will be available online soon.

 

 

Anthony Zukoff, Extension Entomology Associate – Garden City
azukoff@ksu.edu


Tags:  alfalfa alfalfa weevils scouting growing degree days 

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