Alfalfa Weeviles, Cutworms, and Mosquitos


Spring plantings and summer season always involve insects in some form and degree.   I would just like to touch on three insects that are prevalent in Roosevelt and surrounding counties and the Montana State University Extension studies that involve them.

The first insect I would like to mention is the alfalfa weevil.   The weevil is the most damaging to alfalfa in the larval stage.   Small alfalfa larvae have a characteristic black head capsule with a white to light-yellow colored body.   As they grow, larvae acquire the more typical green body color with a white stripe down the middle of the back and still have the black head capsule.   The peak population of larvae occurs about the time of the first harvest, but monitoring for the weevil should begin any time if you have not started already.   Growing Degree Day (GDD) accumulations show that we are having a warmer spring than usual, so on sunny days we may see weevils feeding earlier than normal.   Damage in fields done by the first larval instars is not very visible and more damage is done by the larger instars.   Severely damaged fields appear white or gray, similar to having been frosted because of the extensive skeletonization caused by larval feeding.  Generally, when larvae populations average between 1.5 and two larvae per stem or 20 larvae per sweep, treatment is considered to be economical, depending on value of hay and the cost of treatment.   Also consider crop height, time to harvest, estimated yield, and stage of the weevil larvae 

Fortunately for us in Montana, the most economical treatment most of the time is early cutting.   Since larvae numbers traditionally peak about the time of first harvest, early cutting can reduce weevil larvae populations by exposing them to desiccating effects of sun and wind and disrupting their food supply.   However, early cutting does not necessarily mean that a chemical treatment is avoided.   When small larvae, which have not completed their development, are concentrated under swath rows, they are protected and may continue to feed and prevent green-up where cut forage laid before baling.

In the alfalfa stand longevity study that is being done in the county, defoliation by alfalfa weevil is being looked at as well as management practices including fall harvest timing, fall harvest management, irrigation methods, seeding practice and use of nurse/cover crop.   Environmental factors are also being considered.   These include taproot damage by clover root curculio, defoliation by alfalfa weevil, other insect and pathogen pressures, root and crown diseases, weed pressures, rainfall, temperature, and relative humidity throughout the growing seasons.   This is the first year of the three year study.

If you would like more information on alfalfa weevils or growing degree days you can contact our office for MontGuide 9602 (Alfalfa Weevil), MontGuide 200103 (Using Growing Degree Days to Predict Plant Stage) or on the internet you can access the High Plains IPM Guide crops section at MontGuide 200103 at .   You can access the growing degree days that have accumulated in your area at    This site will give you both a current and historical 30 year average to compare this year to.   Also, if you would like to have me sweep your field to check for weevil pressure, give the Extension Office a call at 787-5312.

The second insect I would like to mention is the cutworm.   There are two species of cutworm in Montana, the pale western and army cutworm.   The army cutworm larvae are greenish-brown to greenish-gray with a narrow pale mid-dorsal stripe usually present.    The head is pale brown with brown to dark brown freckles.   The army cutworm can be seen feeding above the ground where as the pale western cutworm feeds below ground.   The pale western cutworm larvae curl up into a characteristic 
C-shape when disturbed.   The general body color of the larva is pale yellowish-gray with a distinct white mid-dorsal line.   The head is yellow-brown with two distinct vertical black dashes that form an inverted V. The adults of the cutworm are what we call miller moths.  

Both pale western and army cutworms can feed on and damage wheat, barley, alfalfa, canola, peas and sugar beets.   Bare spots in the field in early spring may indicate cutworm activity.   Damaged leaves indicate army cutworm damage and dead or wilted tillers or plants indicate pale western cutworm damage.  

Risk warning models at (click on color maps) indicate that there is low danger for army cutworms throughout Roosevelt County this spring and early summer and low, moderate and high risk for pale western cutworm depending on location throughout the county. The map indicates a high risk around the Froid/Homestead area, low risk in the Wolf Point area and medium risk throughout the rest of the county.  

This risk model was developed by using pheromone traps in August and September in four locations throughout the county to count the adult insects to predict the larval pressure for the next spring.   Also, included in the model is temperature and rainfall for the area in May and June.   More than 10 wet days (i.e., days on which more than a quarter inch of precipitation falls) increase cutworm mortality: less than 10 wet days favor cutworm survival.

For more information on cutworms in Montana you can call our office for MontGuide 200005 (Pale Western and Army Cutworms in Montana) or go on line at .  

For more information on cutworm monitoring in Montana, access the internet at

The third and last insect I would like to mention is our friendly mosquito.   Mosquitoes have been in the news the last few years as carriers of the West Nile Virus in humans, horses and other animals.   There will once again be a site monitoring the West Nile virus in Roosevelt County and also at the Medicine Lake Wildlife Refuge as well as other counties in the state.  

Researchers are finding out that one genus of mosquitoes is primarily responsible for transmitting the West Nile virus.   This is the Culex genus.   This genus over winters as adult female and lays its eggs on standing water.   They may go through many generations a year and can develop from egg to adult in two weeks if temperatures are high enough.  

This is just one more reminder to eliminate standing water where possible, use larvicides where not possible, cover up in the high mosquito hours of evening and morning and use a mosquito repellent with DEET.

If you would like more information on any of the insects mentioned above, you can contact the Extension Office at 
787-5312 or access the Montana Integrated Pest Management Center at



The programs of the MSU Extension Service are available to all people regardless of race, color, national origin, gender, religion, age, disability, political beliefs, sexual orientation, and marital or family status. Issued in furtherance of cooperative extension work in agriculture and home economics, acts of May 8 and June 30, 1914, in cooperation with the U.S. Department of Agriculture, Dr. Douglas Steele, Vice Provost and Director, Extension Service, Montana State University, Bozeman, MT 59717.

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