2018 Alfalfa Weevil Activity Prediction and Scouting Recommendations Back »

Written collaboratively by Adam Varenhorst, Amanda Bachmann, Laura Edwards, Philip Rozeboom, and Patrick Wagner.


It has been a slow start to the 2018 growing season, and alfalfa development is also behind schedule. One of the major insect pests of alfalfa is the alfalfa weevil. For the past few years, this insect has been reported as having large populations throughout much of South Dakota. The alfalfa weevil defoliates alfalfa as both adults and larvae, with the larvae causing the majority of damage. One of the ways that we can determine when to start scouting for alfalfa weevils is by calculating degree days. In South Dakota, alfalfa weevils overwinter as adults. However, there are two strains of the alfalfa weevil present in South Dakota that overlap in most of the state. They are the Eastern strain and Western strain. Although the two strains are nearly identical in appearance, they do differ in their spring emergence patterns, with the Eastern strain emerging earlier in the season than the Western Strain. As a result, it sometimes appears that there are two generations of alfalfa weevils early in the season.

The equation for degree days is:

(Maximum daily temperature - Minimum daily temperature) ÷ 2 - The Developmental Threshold

For alfalfa weevils, the developmental threshold is 48° F. This equation is used for days that the maximum temperature exceeds 48° F. In cases where the minimum temperature is lower than 48°F, the actual temperature is substituted by the 48°F developmental threshold since no development is occurring below it. Degree day accumulation begins on January 1. First overwintering adult alfalfa weevil activity is generally observed around 200-degree days and larvae hatch is generally around 300-degree days (Table 1).

The alfalfa weevil has four larval instars. With the 1st being the smallest larvae and the 4th being the largest. The later instar larvae will consume additional leaf tissue and will result in increased defoliation. As a result, management would ideally be done while larvae are still in the 1st and 2nd instar growth stages.

Table 1. Alfalfa weevil life stages based on degree days.

Accumulated Degree Days Alfalfa Weevil Activity
(Overwintering as Adults)
0 – 199 Limited to no activity of alfalfa weevils
200 – 299 Overwintering adult activity is observed in alfalfa fields (begin scouting fields)
300 – 371 Spring laid eggs hatch
1st instar larvae begin feeding
372 – 438 2nd instar larvae
439 – 504 3rd instar larvae
505 – 595 4th instar larvae
596 – 814 Pupation
815 – greater Adult Emergence (will overwinter)

 

For South Dakota, most of the state has now accumulated over 200-degree days and scouting for alfalfa weevils should begin. For alfalfa near Rapid City, Hot Springs, Winner, or Vermillion, scouting should be intensified as enough degree days have been accumulated for larvae hatch and larvae activity between the 1st and 2nd instars.

Table 2. Degree day accumulations for several South Dakota locations.

Location Accumulated Degree Days
Since January 1, 2018
Recommendation
Buffalo 249 Scout for adults
Newell 209.5 Scout for adults
Rapid City 329.5 Scout for adults and larvae
Hot Springs 420 Scout for adults and larvae
Lemmon 271 Scout for adults
Faith 224.5 Scout for adults
Cottonwood 280.5 Scout for adults
Mission 291.5 Scout for adults
Selby 229.5 Scout for adults
Gettysburg 219.5 Scout for adults
Pierre 278 Scout for adults
Winner 319.5 Scout for adults and larvae
Aberdeen 257.5 Scout for adults
Huron 267 Scout for adults
Tyndall 251.5 Scout for adults
Sisseton 218.5 Scout for adults
Brookings 197.5 No scouting necessary
Vermillion 361.5 Scout for adults and larvae

 

Scouting

To scout for alfalfa weevils, we recommend using a sweep net to confirm their presence, and a five-gallon bucket to count the larvae. You can scout the field by walking a W or Z pattern and randomly sampling a total of 30 plants. Plants should be hit against the inside of the five-gallon bucket to dislodge any larvae. Once completed, the plant should be measured to determine height, and the number of larvae in the bucket should be counted. Repeat this process for 30 plants and calculate the average number of larvae and the height of the plants. Tables 3-6 contain threshold information for alfalfa weevils based on plant height and the number of larvae present.

The larvae of the alfalfa weevil are small, green, and somewhat resemble caterpillars. They have a white stripe on their backs and a dark brown/black head capsule. The larvae will have 3-pairs of legs near their head (Figure 1). The adults are small brown beetles with elongated snouts. The adult alfalfa weevils are brown with a darker brown stripe that runs down their midline (Figure 2).

If thresholds are exceeded, there are several insecticides available for management. Please refer to the 2018 Alfalfa and Oilseeds Pest Management Guide for rates and labeled insecticides.

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

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

Table 3. 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.

Value of hay per ton
Insecticide application cost per acre
$8 $12 $16 $20
Total alfalfa weevil 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 4. 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.

Value of hay per ton
Insecticide application cost per acre
$8 $12 $16 $20
Total alfalfa weevil 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 5. 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.

Value of hay per ton
Insecticide application cost per acre
$8 $12 $16 $20
Total alfalfa weevil 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 6. 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.

Value of hay per ton
Insecticide application cost per acre
$8 $12 $16 $20
Total alfalfa weevil 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
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