Utah Pests News Spring 2011

Utah Pests News

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The following can be found on our Web site:

Backyard Orchardist Series:

Critical Frost Temperatures for Fruit Trees


Diane Alston 

Ryan Davis
Arthropod Diagnostician 

Marion Murray
IPM Project Leader
Editor, Utah Pests News

Claudia Nischwitz
Extension Plant Pathologist

Ricardo Ramirez
Extension Entomologist

Cory Vorel
USU CAPS Coordinator

Utah Plant Pest Diagnostic Lab
BNR Room 203
Utah State University
5305 Old Main Hill
Logan, UT 84322

Utah Pests News is published quarterly.

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All images © UTAH PESTS and USU Extension unless otherwise credited



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.

Recommendations for stem sampling (shake-bucket method).
Avg. larvae/
Alfalfa height Strategy  
Less than 2 --- Sample 1 week later  
2 or more 10-14 inches Control recommended  
2.5 or more 15-18 inches Control recommended  
3 18+ inches Control recommended  
Recommendations for sweep sampling.  
Avg. Larvae/sweep Strategy  
Less than 10 Sweep 1 week later  
10-19 Sweep 3-5 days later  
20+ Control recommended  
Currently, chemical control and early cutting are the two main strategies for alfalfa weevil control.  When larvae are not at destructive levels or destructive levels are just reached during the final 2 weeks before expected cutting (refer to Table 1 and 2), strongly consider an early cutting as an alternative to chemical spray. It is important to monitor re-growth and stubble infestations post-harvest particularly when using early-cutting for management.  Newly emerged adults and weevil larvae may be concentrated in windrows and have the potential to reduce re-growth.  It is recommended that a post-harvest treatment be done when adults are found feeding on 50% of crowns and normal green-up is prevented for 3-6 days or when larvae from 30 stems exceeds 20, 30, and 45 larvae on alfalfa stems 2, 4, and 6 inches tall, respectively, 5-7 days after harvest.  When adults and larvae are below these levels and normal green-up is present no treatment should be necessary.

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


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.


Featured Picture of the Quarter



The adult male western black widow (Latrodectus hesperus) is unlike the adult female in that it lacks the red abdominal markings, is brown in color, and is smaller.  A bite from a male black widow is somewhat harmless whereas a bite from an adult female injects a hefty dose of latrotoxin, a neurotoxin that causes latrodectism (muscle pain followed by severe cramping).  The widow name is now considered a misnomer.  Although females in captivity have been seen consuming males after mating, in the wild, males typically retreat before being consumed. 

-Image by Jabe Huber, Utah IPM Program