NEW UTAH PESTS FACT SHEETS

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• Apple
• Apricot
• Cherry
• Peach/Nectarine
• Pear
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Critical Frost Temperatures for Fruit Trees

UTAH PESTS Staff 

	Additional articles in this issue:   Powdery Mildews in Your Backyard - Part II Can Plants get “Immunized” against Disease? Tree Killers:  Bark Beetles and Their Control CAPS Update:  Light Brown Apple Moth IR-4 Successes Monitoring Insect Pests Attract-and-Kill Stations for Management of Cherry Fruit Fly News, Publications, and Websites
	In sweep sampling, swinging the net from one side of the body to the other is considered a full sweep. The net should be kept below the tops of the alfalfa plants until the end of the sweep.

Spring Into Alfalfa Weevil Monitoring

Spring is a time when Utah alfalfa growers become increasingly concerned whether the alfalfa weevil will devastate their first cutting and negatively impact subsequent cuttings throughout the growing season. The alfalfa weevil is an early season pest and the recent loss of Furadan, an insecticide commonly used as a prophylactic spray, means sampling and monitoring weevil populations is crucial to effectively time pest control measures.

The alfalfa weevil is a snout beetle with one generation per year in Utah.  In early spring, adult weevils begin depositing eggs in 3 to 4 inch-tall alfalfa stems.  Eggs can hatch within 4 days, but continuous cool weather can delay larval emergence by several days or weeks.  Small larvae (1/20” long) emerge from the eggs, exit the alfalfa stem, and climb to new alfalfa growth to feed.  Young larvae are cream-colored and become bright green as they mature and grow in size (reaching 3/8” long).  Although adult weevils and all larval stages feed on foliage, mature larvae are the most damaging and voraciously feed near the first alfalfa harvest.  Fully grown larvae move down the plant to pupate on or near the soil surface and new adults emerge in early summer.  Although these adults are active in the field they generally do not cause economic damage.  By mid-summer the adults migrate to sheltered sites to overwinter until the following spring.

The best success for weevil management is to monitor larvae regularly before the first cutting and when stems are at least 10 inches tall, using the stem or sweep sampling method.  For either method, sample within a “U” or “Z” pattern in the field so stem and sweep samples are representative of the entire field, and avoid sampling near field edges (keep 20 paces away). 

Stem sampling, or the “shake-bucket method,” more accurately detects small larvae than sweep sampling.   To sample, clip 30 to 50 individual alfalfa stems at the soil surface and carefully place stems top-side-down into a 5-gallon bucket.  Shake groups of five alfalfa stems at a time vigorously against the inside of the bucket to dislodge small larvae hidden in tight leaf whorls.  Count the total number of weevil larvae collected and calculate the average number of larvae per stem (e.g., 32 larvae collected ÷ 50 stems = 0.64 larvae per stem).   

Sweep net sampling consists of swinging a 15” diameter canvas net from side-to-side in a 180° arc.  Take a series of 10 sweeps at several locations in the field.  Count the number of larvae after every 10 sweeps, calculate the average number of larvae per sweep, and follow the recommended strategies in the table below.  Be sure to use the correct recommendations for the selected sampling method.

Adult alfalfa weevils (top left) have a characteristic snout and elbowed antennae. Eggs (top right) are small, yellow, oval and deposited in clusters, and larvae (bottom left) have a darkened head capsule and distinct white line down the center of their back. The pupal stage (bottom right) lasts 10-14 days.

-Ricardo Ramirez, Entomologist
 

References: 

Evans, E.W. 1989. The alfalfa weevil in Utah. Utah State University Extension. Fact Sheet No. 58.

Hoff, K.M., M.J. Brewer, and S.L. Blodgett. 2002. Alfalfa weevil (Coleoptera: Curculionidae) larval sampling: comparison of shake-bucket and sweep-net methods and effect of training. J Econ Entomol 95:748-53.

Townsend, L. 1998. Alfalfa weevil field sampling program. University of Kentucky Extension. Entfact-127.

Statewide IPM Program, 1981, Integrated Pest Management for Alfalfa Hay. Oakland:  UC ANR Publication 3312.