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Alfalfa

Identifying Caterpillars—Summer

On this page
  • Alfalfa caterpillar
  • Cotesia medicaginis
  • Beet armyworm
  • Hyposoter larva
  • Western yellowstriped armyworm

Use the photos below to identify caterpillars that can be problematic in alfalfa. Note that young caterpillars look different than mature ones. To determine if caterpillars are parasitized, pull apart young worms to see if white or green larvae pop out. Names link to more information on identification and biology.

Click on photos to enlarge
Young larva Older larva Parasitized larva

Alfalfa caterpillar larva
Alfalfa caterpillar
identification tip: Young caterpillars are pale green and very hairy.

Older alfalfa caterpillar larva
Alfalfa caterpillar
identification tip: Full-grown caterpillars are about 1.5 inches long; they are distinguished from other caterpillars in alfalfa by their velvety green bodies and white lines along their sides.

Cotesia medicaginis
Cotesia medicaginis
If you pull apart a parasitized alfalfa caterpillar, a white, shiny Cotesia larva is revealed.

Newly hatched beet armyworm larvae
Beet armyworm
Identification tip: Newly hatched larvae are tiny and green and feed in groups.

Older beet armyworm larvae
Beet armyworm
Identification tip: Older larvae are smooth and may be olive green (to almost black) but usually have many fine, wavy, light-colored stripes down the back and a broader yellow stripe down each side.

 

Hyposoter larva
Hyposoter larva
If you pull apart a parasitized armyworm, a green Hyposoter larva will pop out.

No photo available. Older western yellowstriped armyworm larva
Western yellowstriped armyworm
Identification tip: The caterpillar is usually black, with two prominent stripes and many narrow bright ones on each side.
No photo available.

Statewide IPM Program, Agriculture and Natural Resources, University of California
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