Alfalfa Weevil Pest Management Recommendation Tables

Current Alfalfa Weevil Degree Day Totals for Kentucky

Degree Days 190-225
# Larvae / 30 stems
27
47
67
85
100
115
130
Alfalfa Height (inches)
2
3
4
5
6
7
8

Apply a long residual insecticide if there are more larvae in your 30 stem sample than the value listed in the Table for the height of your alfalfa. Sample again in 2 days if there are 15 or more larvae in the sample but fewer than the figure for the alfalfa height (expect to spray at that time). Sample again in 7 days if you find less than 15 larvae per 30 stems.

Degree Days 226-275
# Larvae / 30 stems
15
17
19
20
20
20
20
Alfalfa Height (inches)
2
3
4
5
6
7
8

Apply a long residual insecticide if the number of larvae equals or exceeds the number in the Table for the height of your alfalfa. If there are fewer, sample again in 7 days.

Degree Days 276-325
# Larvae / 30 stems
27
45
60
67
83
94
105
125
135
Alfalfa Height (inches)
4
5
6
7
8
9
10
11
12

Spray with a medium residual insecticide if the number larvae is equal to or greater than the Table value. If less, sample again in 7 days.

Degree Days 326-375
# Larvae / 30 stems
82
95
105
105
105
105
105
105
105
Alfalfa Height (inches)
8
9
10
11
12
13
14
15
16

Spray with a short residual insecticide if the larval count equals or exceeds the Table value. If less, sample again in 7 days.

Degree Days 376-525
# Larvae / 30 stems
52
60
64
68
72
76
80
Alfalfa Height (inches)
12
13
14
15
16
17
18

Spray with short residual insecticide if larval number is exceeded OR harvest if 30% or more of the alfalfa is in the bud stage.

Harvest is a good control option if 30% or more of the plants are in the bud stage. The hay should be removed as quickly as possible to allow the hot sun to kill the larvae in the stubble. Monitor the regrowth of the stand.

Evaluating Post-Harvest Alfalfa Weevil Larval and Adult Damage

Routine stubble sprays are not justified. However, weevil larvae and adults left in the field after harvest can feed on developing shoots and slow recovery of the plant. If early harvest was used as a weevil management tool, there occasionally may be sufficient larvae or newly-emerged adults present to justify an insecticide application. Watch such fields carefully for the normal green-up that indicates active regrowth. Surviving larvae may feed on developing leaves and new adults can cause "notch-like" feeding holes on leaves giving them a feathery appearance.

The table below may be used to evaluate larval numbers and plant height to determine whether or not control is necessary. Treatment may be justified if adult weevils are found feeding on 50% or more of the crowns and regrowth is prevented for 3 to 6 days. Use low rates if an insecticide application is necessary.

Assessing Alfalfa Weevil Larvae on First Cutting Regrowth (5 to 7 days after harvest)

Plant height
2"
4"
6"
8" or taller
# Larvae /30 stems*
20
33
47
60
# Larvae /30 stems**
17-20
17-32
23-46
23-59

*Spray with a short residual insecticide if the number of larvae/30 stems exceeds the number in the table above for the appropriate alfalfa height.

**Sample again in 2 days if the number of larvae/30 stems is in this range for the appropriate alfalfa height. If numbers are below this level, no treatment should be necessary.