Alfalfa Weevil, Hypera postica.
The following text on alfalfa weevil management is greatly abbreviated. For more detailed information please see KSRE publication MF2999: Alfalfa Weevil.
Alfalfa weevil adult.
Alfalfa weevil larva.
Alfalfa weevil larval defoliation.
The adult weevil is 3/16-inch long and light brown with a dark mid-dorsal line extending down the middle of the back. Adults possess a distinctive snout and readily fall to the ground when disturbed. Eggs are laid inside alfalfa stems in fall or spring. Small, light green, black-headed, legless larvae have a distinct white stripe down the center of the body. Larvae feed on the terminal and upper leaves of the plant early in the spring, reaching a quarter-inch in length in about three weeks. Larvae spin loosely-woven, silken cocoons, generally found on the ground among the leaf litter. Most damage occurs before the first cutting, but damage by larvae and adults can suppress yields by delaying regrowth after the first cutting.
Destruction of fall-, winter-, and early spring-laid alfalfa weevil eggs through grazing, burning or crushing the stems with a heavy roller may reduce spring larval populations. Studies suggest that flail mowing or propane-fueled flaming during alfalfa dormancy may lower weevil populations and delay development of significant damage in some fields. When populations are dense, survivors, or insects hatching from spring-laid eggs deposited after flaming or burning, may cause enough damage that treating with insecticides will still be recommended.
Predicting years in which adequate suppression is provided through dormant-season habitat manipulations is not yet practical. Timing is important with all forms of population suppression. Long-term effects on plant stand survival and productivity may be questionable when some of these strategies are employed. Tiny parasitic wasps and fungal pathogens also may help limit damage. In the future, highly weevil-resistant varieties of alfalfa may become available. However, at present, insecticides and timely cutting remain the most widely used emergency intervention tools to prevent economic damage.
To decide if an alfalfa field should be treated for alfalfa weevil, the stem-count decision method is recommended. Carefully break off 30 to 50 stems selected at random from across the field, and shake them individually into a deep-sided bucket. Methods used to remove infested alfalfa stems from the plant crown can influence the accuracy of the population estimate, sometimes causing improper management decisions. <br /> The finger-shear, palm-tip, or knife methods are recommended when taking alfalfa weevil larval samples for shakebucket evaluations. In the finger-shear method, the thumb and forefinger grip the stem (held horizontally) while the middle finger breaks the stem off. In the palm tip approach, one hand grasps and encloses the tip of the stem while the other breaks the stem off. The knife method involves severing the stem with a small knife using the thumb to push the stem into the knife blade. These approaches attempt to minimize the loss of larvae before the stem reaches the bucket for shaking. Count the stems, determine the average stem height, count the larvae, and determine the average number of larvae per stem. Refer to the Alfalfa Weevil Stem Count Decision Guide (below) to determine the suggested management action. The first relationship (a) was developed for situations where alfalfa was selling for $35 per ton. As the alfalfa value increases, fewer larvae are required to reach a treatment threshold. For instance, figure (b) indicates that $70/ton alfalfa should be treated at just over two larvae per stem when the alfalfa is approximately 17 inches tall, versus requiring nearly three larvae per stem to trigger treatment when the crop is valued at $35/ton. As the crop price increases, fewer alfalfa weevil larvae may be needed to trigger treatment
Early Season Infestation: Alfalfa is only 3 to 7 inches tall. Treat when feeding is evident on the top inch of growth – this usually requires one to two larvae per stem, depending on expected value of the hay. Retreatment may be necessary before cutting.
Mid-Season Infestation: Alfalfa is 8 to 14 inches tall. Significant feeding damage to the top 1 to 2 inches of growth is occurring on 30 to 50 percent of the terminals. High larval populations (four or more per stem) can cause severe foliage loss in only three or four days. Stubble sprays may be necessary in addition to treatment at this time.
Late Season Infestation: Early cutting may be advisable if the alfalfa is within 10 to 14 days of cutting, the hay is expected to dry quickly, and windrows will be rapidly removed from the field so the relatively delicate larvae are exposed to bright sunlight and drying winds. However, spraying before cutting is advisable if the top 2 to 3 inches on the majority of plants is being injured and harvest cannot be done immediately, or if weather conditions favor larval survivorship and other stresses make rapid regrowth following cutting unlikely.
After Harvest: Stubble sprays may be necessary to protect regrowth if eight or more larvae per square foot of stubble are present. As few as four larvae per square foot may prevent regrowth under unfavorable growing conditions. Adult weevils may also 'debark' stalks before refoliation can occur and may require treatment, especially under conditions of drought that delay regrowth.
Regrowth damaged by adult weevils.
Close-up of adult weevil 'debarking' damage.
Spray gallonage for ground equipment: 10 to 12 gallons per acre for 7-inch alfalfa; 15 to 20 gallons for 8- to 15-inch alfalfa; at least 20 gallons per acre on alfalfa more than 15 inches tall. Use 30 psi and hollow cone nozzles, and adjust spray pattern as suggested by the nozzle manufacturer to overlap near the top of the canopy
Spray gallonage for aerial equipment: The use of less than 2 gallons of spray per acre has frequently resulted in less than satisfactory control. Overall efficacy frequently increases as even more carrier is used.
Please refer to the most recent version of the Alfalfa Insect Management Guide for control options.
Page last updated 11/07/2013 by J.P. Michaud.