It’s Time To Start Scouting for Alfalfa Weevils

Written collaboratively by Adam Varenhorst, Philip Rozeboom, Patrick Wagner, and Brad McManus.

Originally Submitted: May 17, 2024

Although the degree day accumulation in 2024 began relatively slow, things are speeding up across the state due to warmer weather. Each week, we put together a degree day estimation article in the spring to provide a rough idea of when alfalfa weevil larvae will be active. This week, alfalfa weevil larvae have been spotted in the fields. It’s important to remember that all instars of alfafla weevil larvae are capable of causing defoliation injury to alfalfa, but the amount of defoliation caused by later instars is generally noticed more often. Routine scouting is the best method to ensure that alfalfa isn’t heavily defiolated by this pest. In addition to growth stage, the population size of the alfalfa weevils in a field determines just how much defoliation will occur. Large infestations cause more rapid defoliation. Both the adults and larvae of alfafla weevil feed on alfalfa. However, the larvae cause the greatest damage by feeding on the terminal buds, which can cause stunting.

Identification

Larvae

Green beetle larvae with white stripe and brown head.
Figure 1. Alfalfa weevil larvae. (Photo: Adam Varenhorst, SDSU Extension)
 

Alfafla weevil larvae are small, green, and resemble caterpillars. They have an obvious white stripe on their backs and a dark brown/black head capsule (Figure 1). The larvae will have 3-pairs of legs near their head.

Adults

Brown beetle with long snout.
Figure 2. Alfalfa weevil adult. (Photo: Adam Varenhorst, SDSU Extension)
 

The adults are small, brown beetles with a darker brown stripe that runs down their midline. Alfalfa weevil adults also have an elongated snout that is characteristic of the weevil family. (Figure 2).

Scouting

The best way to determine if alfalfa weevils are present in a field is to use a sweep net and a five-gallon bucket. This method of scouting can be done quickly and provides a yes or no regarding infestation. If alfalfa weevil larvae are caputred, the next step is determine their population density. To do this, we recommend walking in a “Z” pattern and randomly sampling a total of 30 plants (for example, sample approximately 10 plants per leg of the “Z”). Each plant that is selected needs to be pulled out gently and then hit against the inside of the five-gallon bucket to dislodge any larvae that are present. We recommend this method versus trying to count the larvae on the intact plant the larvae can be hard to find.

Once completed, the plant should be measured to determine height, and the number of larvae in the bucket should be counted. Repeat this process until a total of 30 plants have been sampled. Lastly, calculate the average number of larvae and the height of the plants. Tables 1 through 4 contain threshold information for alfalfa weevils based on plant height and the number of larvae present.

Management

If thresholds are exceeded, there are several insecticides available for management. Please refer to the latest Alfalfa and Oilseeds Pest Management Guide for labeled insecticides and their associated rates. In recent years, there have been an increase in reports of pyrethroid failures when these insecticides were applied to manage alfalfa weevils in other states. If you observe a pyrethroid failure when managing alfalfa weevils please report it to Adam Varenhorst.

Economic Thresholds

Table 1. Alfalfa 0 to 12 inches tall. Economic thresholds for alfalfa weevils
based on larvae per stem and calculated from the total larvae in a 30-stem sample.
 
Insecticide application cost per acre
 
$8
$12
$16
$20
Value of hay per ton
Total alfalfa weevil larvae per 30 stems
$60
84
127
169
211
$80
63
95
127
158
$100
51
76
101
127
$120
42
63
84
105
$140
36
54
72
90
$160
32
47
63
79
$180
28
42
56
70
$200
25
38
51
63
$220
23
35
46
58

Source: SDSU Extension

Table 2. Alfalfa 12 to 18 inches tall. Economic thresholds for alfalfa weevils
based on larvae per stem and calculated from the total larvae in a 30-stem sample.
 
Insecticide application cost per acre
 
$8
$12
$16
$20
Value of hay per ton
Total alfalfa weevil larvae per 30 stems
$60
91
137
183
225
$80
68
102
136
171
$100
54
81
95
137
$120
45
68
91
114
$140
39
59
77
99
$160
34
51
68
86
$180
30
45
60
77
$200
27
41
54
69
$220
25
37
49
63

Source: SDSU Extension

Table 3. Alfalfa 18 to 24 inches tall. Economic thresholds for alfalfa weevils
based on larvae per stem and calculated from the total larvae in a 30-stem sample.
 
Insecticide application cost per acre
 
$8
$12
$16
$20
Value of hay per ton
Total alfalfa weevil larvae per 30 stems
$60
99
149
199
240
$80
75
113
150
186
$100
62
90
120
149
$120
50
75
100
124
$140
43
64
86
107
$160
37
56
75
93
$180
32
50
67
84
$200
29
45
60
76
$220
26
41
55
69

Source: SDSU Extension

Table 4. Alfalfa 24 to 30 inches tall. Economic thresholds for alfalfa weevils
based on larvae per stem and calculated from the total larvae in a 30-stem sample.
 
Insecticide application cost per acre
 
$8
$12
$16
$20
Value of hay per ton
Total alfalfa weevil larvae per 30 stems
$60
104
156
182
260
$80
78
117
137
195
$100
63
94
110
156
$120
52
78
105
130
$140
45
67
90
112
$160
39
58
79
98
$180
35
52
70
87
$200
31
47
63
79
$220
28
42
57
72

Source: SDSU Extension

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