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Published June 29, 2008, 12:00 AM

Watch alfalfa for weevils and pests

Two observant farmers in East Polk County recently called my attention to a potential pest problem common in alfalfa, the alfalfa weevil.

By: Jim Stordahl, DL-Online

Two observant farmers in East Polk County recently called my attention to a potential pest problem common in alfalfa, the alfalfa weevil. Since the first crop was delayed by cool, wet weather, these small green caterpillars may have flourished while the alfalfa was delayed. Generally, the alfalfa weevil — or its look-alike cousin, the clover leaf weevil — are not a major problem and are typically destroyed by the first harvest.

Larvae (the caterpillars) can usually be found in the field for one to two months, during May and June in normal years. Mature larvae move down the plant or drop to the ground and spin a fragile, lacelike cocoon attached to debris or the plant. A new adult emerges after 10 to 12 days. Adults can be found in the field until early fall, when they move to their over-wintering sites.

Both adults and larvae were observed in sweep net samples, it is just that there were large numbers of young larvae still present. The younger, yellowish-green larvae and the older, dark green larvae feed on the leaves, leaving a ragged or “skeletonized” appearance.

The field takes on a grayish or whitish cast when infestations are great and feeding is extensive. Mature larvae are dark green, 3/8-inch long, with a black head and a white stripe down the back. The cloverleaf weevil is similar, but with a tan head.

Cutting can be an alternative to insecticide application if an economic infestation is found during the bud or early bloom stage. If the cutting cannot be made within the week or so, then the insecticide treatment could probably be made.

Cutting before the optimum growth stage can reduce dry matter yields, although hay quality may be increased. Many larvae are destroyed by the cutting process, and others are left exposed to their natural enemies, high temperatures, direct sunlight and lack of food.

Survival in stubble can be reduced by removing windrows and bales quickly, as these provide some protection for the larvae. Green-chopping alfalfa is also an effective way to reduce alfalfa weevil populations.

To determine the need for treatment, assess the severity of an infestation by selecting 50-100 alfalfa stems (10 to 20 randomly selected stems from each of 5 locations) and examine for signs of feeding damage in the leafbuds and growing tip leaves. Divide the number of stems with recent tip injury by the total stems collected, convert to a percent, and compare with the threshold.

Alfalfa Weevil thresholds before cutting is 35 percent (weak stand) plants with feeding damage or 40 percent (vigorous stand) plants with feeding damage and/or 2 live larvae/stem.

After cutting management: It is important, when cutting alfalfa that has weevil larvae feeding, to assess the need for post-harvest weevil management. Monitor regrowth for potential stubble infestations, particularly beneath windrows. After the hay has been picked up, sample the stubble and early regrowth in 20 one square foot samples, 4 chosen randomly from 5 locations.

The failure of a field to “green up” after seven to 10 days with adequate moisture is another sign that a stubble treatment is needed. When regrowth after harvest is sufficiently tall, go back to monitoring tip injury. Economic threshold for after cutting is 8 or more larvae per sq. ft. (six or more on sandy soil) or when larvae are suppressing regrowth. In most years, the alfalfa pest concerns shift from alfalfa weevil to potato leafhopper after the first cutting which will be the topic for next week.

For more information on this, or other topics, contact me at the Polk County office in McIntosh or at the Clearwater County office on Wednesdays. Our toll free number is 800-450-2465. If e-mail is your thing, contact me at stordahl@umn.edu. Source: Phil Glogoza

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