Aphids in Alfalfa

Pea aphid, blue alfalfa aphid, cowpea aphid, and spotted alfalfa aphid are most common in alfalfa in Colorado. 

Order: Hemiptera
Family: Aphididae

Description and Lifecycle

Aphids have piercing-sucking mouthparts that extract plant sap (phloem). They are soft-bodied insects with long antennae and have a pair of short tubes – cornicles – at the end of their abdomen. Each species common in alfalfa in Colorado has a different potential for damaging alfalfa, so it is essential to determine which aphids are present in a field.  

  • Pea aphid: Relatively large, pale green aphid with very long, black tipped cornicles and legs. Antennae have dark bands.  
  • Spotted alfalfa aphid: Pale yellow aphid with spots and very short cornicles.  
  • Cowpea aphid: Polished, black aphid with black cornicles.  
  • Blue alfalfa aphid: Large blue-green aphid with long legs and antennae. They are similar to the pea aphid but have dark tipped antennae instead of bands.  

Aphids have multiple generations each season, reproduce asexually, and give live birth – their populations can therefore increase rapidly. Winged aphids can be found when they are moving to new host plants or overwintering sites, but majority of aphids feeding actively are wingless.  

Three species of white grubs.

Clemson University, USDA Cooperative Extension Slide Series, Bugwood.org. 
Cowpea aphid.

Masked chafer adults.

Joseph Berger, Bugwood.org
Pea aphid adult and nymphs. 

Japanese Beetle Larva.

Kansas Department of Agriculture, Bugwood.org
Spotted alfalfa aphid. 

Quick Facts

  • In Colorado, the pea aphid, blue alfalfa aphid, cowpea aphid, and spotted alfalfa aphid are most common aphids in alfalfa 
  • Pea aphids and blue alfalfa aphids are generally early season pests, while cowpea aphid and spotted alfalfa aphid occur later in the season  
  • Aphid populations may increase after insecticides applied to suppress alfalfa weevil eliminate natural enemies of the aphids 



Aphid feeding can result in stunting, yellowing, and leaf curling. In addition, saliva of some aphid species (e.g., spotted alfalfa aphid) has toxic properties that can result in yellowing of the leaf veins. Aphids also secrete droplets of honeydew during feeding, an ideal substrate for sooty mold, a contaminant of alfalfa hay. Pea aphid and blue alfalfa aphids prefer cool, dry conditions and are more problematic during the first cutting. Cowpea aphid and spotted alfalfa aphids prefer hot, dry conditions and are generally a problem in later cuttings. 




Aphids become secondary pests of alfalfa following the use of broad-spectrum insecticides for alfalfa weevil control during the first hay crop. This is the most common way that pea aphids become a problem and occurs because the insecticides that produce effective alfalfa weevil control are also highly toxic to many species of beneficial insects. Reduced populations of beneficial insects, combined with warm temperatures allow aphid populations to explode in a short period of time. 

  • Scouting for aphids: To determine if aphids have reached economically damaging levels, clip several alfalfa stems at the base of plants and record length of the stem and number of aphids present. 
  • Biological control: There are numerous natural enemies that can control aphids including ladybird beetles, lacewing larvae, syrphid fly larvae, damsel bugs, big-eyed bugs, minute pirate bugs, and parasitoid wasps. If these are abundant in fields insecticide control should be delayed and aphid densities re-examined. 
  • Action thresholds vary based on aphid species: 

Treatment guidelines for aphids on alfalfa. 

Growth Stage     Pea aphid     Blue alpha aphid 

   Spotted alfalfa aphid 

  Cowpea aphid 
Seedling     5     1     1     5 
<10 inch     40     10     10     40 
>10 inches     75     30     30     75 
20 inches     100     50     100     100 

CSU Extension Fact Sheet

Download or view the CSU Extension’s PDF fact sheet for your reference.