It’s Time To Start Scouting for Alfalfa Weevils in South Dakota

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

Originally Submitted: May 8, 2023

With increasing temperatures, the degree days for alfalfa weevils have been steadily increasing. As much of the State has reached or exceeded 200 degree days, it means it is time for alfalfa weevil scouting to begin. During the last few years, alfafla weevil larvae caused significant defoliation in many alfalfa fields in South Dakota. The best way to prevent serious defoliation from happening is to scout and detect alfalfa weevil populations early. Although alfalfa weevils are generally more of an issue for the first cutting of alfafla, they can also cause defoliation after the first cutting. Populations should be closely monitored prior to cutting to ensure that large populations do not persist in the field and delay the regrowth for the second cutting.

Alfalfa Weevil Identification

The larvae of the alfalfa weevil are small, green and resemble caterpillars. They have a white stripe on their backs and a dark brown/black head capsule (Figure 1). The larvae will have three pairs of legs near their head. 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 weevils (Figure 2).

Larvae

Green beetle larvae with white stripe and brown head.
Figure 1. Alfalfa weevil larvae. Courtesy: Adam Varenhorst

Adult

Brown beetle with long snout.
Figure 2. Alfalfa weevil adult. Courtesy: Adam Varenhorst

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. If you confirm alfalfa weevils are present in a field, the next step is to 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 beetles and larvae on the intact plant, because adult alfalfa weevils will fall off the plant when disturbed as a defense mechanism.

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.

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

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

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

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

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. If you observe a pyrethroid failure for alfalfa weevil management, please report it to Adam Varenhorst at adam.varenhorst@sdstate.edu.

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