Written collaboratively by Adam Varenhorst, Philip Rozeboom, Patrick Wagner, and Brad McManus.
Alfalfa weevil larvae are active across South Dakota. Although they had a somewhat slower start this year, we are now observing all instars of the larvae in alfalfa fields. These instars vary in size and how much leaf tissue the consume each day. Although both the alfalfa weevil adults and larvae feed on alfalfa, the larvae cause more defoliation. The larvae also feed on the terminal buds, which can cause stunting of the regrowth. The best way to prevent alfalfa weevils from causing substantial defoliation is to routinely scout fields.
Identification
Larvae
The alfalfa weevil larvae are green and small, and vary in size based on the the stage or instar. These larvae may resemble caterpillars but they do not have abdominal prolegs. They do have six legs that are located near the head capsule. The larvae have an obvious white stripe located on the middle of their backs and a dark brown/black head capsule (Figure 1).
Adults
The adult alfafla weevils are small brown beetles with a darker brown stripe that runs down their midline. The 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
| 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
| 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
| 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
| 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
