1. Harvest early instead of treating with insecticides if the crop has attained sufficient growth before larval feeding damage becomes severe.
2. Avoid insecticide applications except in field situations where thresholds have been exceeded and early harvest is not possible.
3. If sprays are required, leave untreated portions of fields as a nursery to maintain beneficial insects for the next cutting or next year.
Another cultural practice is late fall removal of the last crop for hay or by grazing. This removes overwintering egg-laying sites for the adult weevils and helps to reduce the early season brood of weevil larvae.
Begin sampling for weevil larvae during middle to late April, depending on the arrival of warmer weather in the spring. With earlier, warmer springs, sampling must start in late March to early April in the Delmarva production areas. Each field must be sampled weekly or more frequently to properly apply insecticide when necessary. Randomly walk through the field in a “W” or zigzag pattern and collect 30 stems. Record the number of stems that show weevil-feeding injury. Place all stems top first in a bucket. Shake the stems individually against the sides of the bucket, and count the number of healthy larvae. Measure 10 stems to get an average plant height for the field.
The general rule is to treat the field when 50 percent or more of the tips show weevil-feeding injury before full-bud stage. For more precise decisions, use the tables accompanying this section to make a spray decision. Select the table that fits the plant height category of the field being sampled. Estimate the cost of the spray per acre and the value of the crop in dollars per ton of hay equivalent. In the appropriate table, find the number of larvae that corresponds to the value of the crop and cost of spray. If the field population of weevil larvae per 30 stems exceeds the level in the table, then an insecticide application would be profitable. If the population falls below the tabulated level, then control is not required, but resampling may be necessary in 5 to 7 days. If the crop is in full bud and the Category III table indicates to spray, it may be more cost-effective to harvest early rather than to spray. However, if harvesting is impossible for at least 3 days or more and the feeding injury is increasing, then spray with a short-residual insecticide. Remember that the days-to-harvest restrictions may dictate which insecticide can be used.
If the weevil population is above the threshold when the first crop is harvested, check the field in the same way as before, 4 to 5 days after cutting. If regrowth does not occur within 4 or 5 days after cutting, a stubble spray is warranted if carryover populations exceed 2 larvae or adults per crown. An alternative decision method is to calculate the number of days of complete regrowth defoliation (D) that can be tolerated using the equation D = A/(B x C), in which A = insecticide plus application cost ($/acre), ex., $7.00; B = value of hay ($/ton), ex., $100.00; and C = loss factor (1st bloom harvest = 0.0198; 28-day harvest = 0.0345). An example for a 1st bloom harvest of the new crop is calculated as follows: 7.00/(100 x 0.0198) = 7.00/1.98 = 3.5 days. For this example, a stubble spray is warranted if carryover populations are expected to defoliate the regrowth for more than 3.5 days.
When sprays are properly timed, usually only one application is required for alfalfa weevil control. All insecticides listed below can injure honeybees and parasites of the alfalfa weevil. Avoid spraying when alfalfa or weeds are in bloom, or apply treatments only in the evening or early mornings when bees are inactive. Good weed control will greatly reduce the hazard to honeybees.
| Insecticide and formulation | Rate of active ingredient per acre | Rate of formulation per acre | Time lmits: Days before harvest | Remarks |
|---|---|---|---|---|
| chlorpyrifos (Lorsban 4E) |
0.5-1.0 lb | 1.0-2.0 pt | 1 pt:14, >1 pt:21 | Restricted Use: Some temporary yellowing may occur after application but will disappear within a week. |
| indoxacarb (Steward EC) |
0.065-0.11 lb | 6.7-11.3 oz | 7 | |
| lambdacyhalothrin (Lambda-Cy 1EC) |
0.02-0.03 lb | 2.56-3.84 oz | 1 for forage, 7 for hay | Restricted Use: |
| lambdacyhalothrin (Warrior II) |
0.02-0.03 lb | 1.28-1.92 oz | 1 for forage, 7 for hay | Restricted Use: |
| methomyl (Lannate 2.4LV) |
0.9 lb | 3.0 pt | 7 | Restricted Use: Do not apply to dormant or semi-dormant alfalfa when minimum daily temperature is 50 degrees or lower. |
