Montana Alfalfa Seed Management

This on-line guide is meant to provide a quick and easy reference to commonly encountered insect and weed pests in seed alfalfa fields in Montana. Our hope is that this information proves to be a valuable decision-making tool for the alfalfa seed producer.  Each species entry includes a brief life history and Auto-Montage image showing important diagnostic features. Pest species entries include appropriate chemical and cultural control measures. For insect pests, economic treatment thresholds and specific chemical rates are also provided, as well as notes on particular usage restrictions. Reference to commercial products or trade names does not imply discrimination or endorsement by the Montana State University Extension Service.

About Insects in Alfalfa:

  • For pest insect control in seed alfalfa, timely use of cultural and chemical control strategies is essential. 
  • Sample fields regularly to track numbers of developing pest insects. 
  • Consider non-target effects on beneficial insects when selecting a course of chemical treatment. 
  • Alfalfa leafcutting bees are highly vulnerable to residues of many insecticides. Check labels for bee hazard warnings. 
  • Generally, pest insects are most effectively controlled early in the life cycle. 
  • Superior insect control requires matching the target species with the appropriate chemical agent and rate needed. 

About Weeds in Alfalfa:

  • In seed alfalfa, control of annual and perennial weeds is vital. 
  • The most critical time for weed control in alfalfa is during establishment or renovation prior to seeding. 
  • Selective removal of certain weeds is often not possible 
  • Superior control of certain weeds often requires a specific herbicide choice. Care must be taken to match the weed species and weed size with the herbicide and rate needed for effective control. 
  • Repeated herbicide applications may be necessary. 
  • Carefully consider crop rotation restrictions associated with a particular herbicide. For some, certain crops may not be planted for up to 40 months after application. 
  • Herbicide choices vary depending upon which of four major growth stages the stand is in: establishment, seedling, fall/spring dormant, or post-cutting. 
  • Because herbicide application rates are dependent upon factors such as soil texture, soil pH, and stand stage, specific application rates are not provided in this guide. Consult the latest herbicide label to determine rates and timings based upon local geography and soil attributes. 

Sweep Sampling Method for Assessing Insects:

Sweep Method

Collect sweep samples on a weekly basis to monitor for detrimental insects. Sweep upper canopy in alternating arcs as you walk forward along a transect. Take twenty sweeps per sample, and collect three 20-sweep samples per 20-acre field. Collect samples in warm, calm conditions, as samples are less accurate when weather is very cool or very windy. Evert the net contents into a sealable bag, and freeze the sample for a few hours. The insects can then be emptied into a shallow pan for examination. Consult the images in this guide to identify what you have collected and read information about them.


For information on heavy-duty sweep nets with 15-inch hoops:
Gempler's or BioQuip