By Adam Varenhorst
The 2018 alfalfa weevil populations were relatively low compared to previous years. Based on the 2018-2019 winter and the spring conditions that we are currently facing, it is possible that alfalfa weevil populations may be low again in 2019. However, it is always best to be prepared and scout alfalfa regularly during the spring to prevent alfalfa weevils from devastating a field. During heavy infestations, alfalfa weevil adults and larvae both feed on the alfalfa plant. The larvae cause the greatest injury by feeding on the terminal buds of the plant.
Identification
LARVAE
- 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 3-pairs of legs near their head.
Figure 1. Alfalfa weevil larvae.
ADULTS
- 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).
Figure 2. Alfalfa weevil adult.
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 (i.e., 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 1-4 contain threshold information for alfalfa weevils based on plant height and the number of larvae present.
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.
| 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 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.
| 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 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.
| 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 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.
| 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 |
Management
If thresholds are exceeded, there are several insecticides available for management. Please refer to the 2019 Alfalfa and Oilseeds Pest Management Guide for labeled insecticides and their associated rates.