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Prionus Root Borers

Prionus spp. (e.g., Prionus californicus, the California prionus)[4]

Prionus root borers are very large brown longhorned beetles whose cream-colored grubs spend three or more years tunneling in roots; the California prionus attacks fruit trees, cottonwoods and other woody plants.[1][2]

Group
Insect
Order
Coleoptera
Family
Cerambycidae
US records
43 states
Sources
4 cited
Updated
2026-10-15
Large dark reddish-brown California prionus beetle beside a penny for scale
Large dark reddish-brown California prionus beetle beside a penny for scale. Photo: Robbie Hannawacker, public domain (CC0), via iNaturalist
On this page
  1. Identification
  2. Where it lives
  3. Biology
  4. Damage and signs
  5. Health risk
  6. Control
  7. Common questions
  8. Sources

How to identify it

  • Adults are large, reddish-brown to nearly black beetles; CSU gives 35-50 mm for the California prionus and USU 1 3/4 to 2 1/4 inches, with long, deeply notched or toothed antennae that are larger in males.[1][2][3]
  • Larvae are legless, whitish roundheaded borers with powerful dark jaws, up to about 4 1/4 inches long.[1][2]

Where it lives

The genus occurs widely; CSU lists eight species in Colorado, some developing on grass or rangeland shrub roots, while USU covers the California prionus in Utah orchards and landscapes.[1][2]

US map: public records from 43 states since 1990 (GBIF).

Biology and life cycle

Females push eggs into the soil near host plants; larvae move to roots and feed for about 3 to 5 years, working upward toward the root crown.[1][2][3]

Mature larvae pupate in a soil cell near the roots; in Utah, adults fly from July to September.[1][2]

Damage and signs

  • Larvae carve deep, sometimes spiraling furrows in roots and the root crown, cutting water and nutrient uptake, girdling plants and opening them to root rot.[2][3]
  • Symptoms include branch dieback and loss of vigor; damage is worse in sandy soils.[2]
  • Hosts of the California prionus include sweet cherry, peach, apricot and other stone fruits, cottonwood and poplar, willow, oak, pines and other woody plants.[1][2]

Health risk

None; adults are not harmful to people or pets.[1]

Control

Prevention

Keep trees healthy and unstressed and watch for canopy dieback or sudden decline.[2][3]

Watch for adult beetles from early July; pheromone and light traps are used for monitoring.[2][3]

Pesticides

USU's ornamental guide lists trunk-base and soil sprays against egg-laying adults and systemic root-zone treatments for young larvae; its fruit sheet notes no insecticides are registered specifically for prionus in stone fruit.[2][3]

Common questions

What is the giant brown beetle with long antennae I found in July?
Possibly a prionus beetle, one of the largest longhorned beetles; adults fly in summer.[1][2]
Can prionus kill a tree?
Heavy root feeding can girdle trees, cause dieback and invite root rot, especially on sandy soils.[2][3]
How long do the grubs live?
About 3 to 5 years in the roots before pupating.[1][2]

Sources

  1. Colorado State University Extension (Colorado Insects of Interest, W. Cranshaw), “Prionus Longhorned Beetles (Colorado Insects of Interest).” wci.extension.colostate.edu/wp-content/uploads/sites/14/2017/03/PrionusLonghorns.pdf (accessed 2026-10-15)
  2. Utah State University Extension (Integrated Pest Management), “Prionus Borers (Ornamental Pest Guide).” extension.usu.edu/planthealth/ipm/ornamental-pest-guide/arthopods/wood-borers/prionus-borers.php (accessed 2026-10-15)
  3. Utah State University Extension (Integrated Pest Management), “Prionus Root Borer.” extension.usu.edu/planthealth/ipm/notes_ag/fruit-prionus-root-borer.php (accessed 2026-10-15)
  4. iNaturalist, “Prionus taxon page (order and family).” www.inaturalist.org/taxa/59893 (accessed 2026-10-06)

Updated 2026-10-15. Built from sourced records. Spot an error? Tell us.