Tree Fruit Research & Extension Center

Orchard Pest Management Online


California prionus

Prionus californicus Motschulsky
(Coleoptera: Cerambycidae)

-- Diane G. Alston, James D. Barbour, and Shawn A. Steffan
  (Published online August 2015)


The California prionus is widely distributed in western North America from Baja California and Mexico to Alaska. The majority of the three to five year life cycle is spent underground as larvae, feeding on the roots of trees and shrubs. The larvae are often referred to as round-headed borers because of their cylindrical body shape. The adults are commonly known as long-horned beetles because of the extended length of their antennae. Infestations can cause direct or indirect death of fruit trees due to girdling of the root cambium and introduction of secondary pathogens that lead to decay. Once an orchard is infested, it is difficult to prevent increase and spread of the beetle to nearby trees. Insecticides registered for stone and pome fruits may provide incidental suppression of adults, but will not suppress larval populations on the roots. The female sex pheromone was recently identified, and is now available as a lure to use in monitoring traps and for mass trapping of males.


California prionus has a broad host range that includes most deciduous trees and shrubs found in urban and natural landscapes and some conifers and brambles. It also attacks a number of perennial agricultural crops including, grapes, hops, fruit trees, and caneberries. California prionus has become a more prominent pest of fruit trees, including sweet cherry and peach, in the Intermountain West region in the last 10-15 years.

Life Stages

Egg: Eggs are cream to yellow-brown in color and about 3/16 inch long and elongate: about twice as long as wide.

Prionus root borer larvae showing range of sizes (instars) (S. Steffan)

Larva: Larvae are cream to brown in color and grow from less than 1/4 inch to 3 inches in length over a three to five-year period. Body segmentation is strongly evident and the head is dark brown with large chewing mandibles. The number of larval instars is not known.

Pupa: Mature larvae pupate in cells constructed from soil and lined with root material. Pupae look like pale, mummified versions of the adult. They are 1 to 2 inches long, cream colored with legs and other appendages free (not glued to the body).

Adult: Adults are large reddish brown beetles (1- 2 inches in length) with smooth and shiny elytra. Adults are sexually dimorphic with males being smaller (1-1 3/4 inches) than females (1 1/2 - 2 1/4 inches) and having antennae that are strongly serrated.

Prionus root borer pupation cell (J. Barbour)

Prionus root borer male and female (J. Barbour)

Life History

Adults emerge from pupae in the soil from June to early August and do not feed. The beetles fly at night in search of mates. Females appear to be more sedentary than the males, as many more males than females are captured in light traps. Females also produce a volatile pheromone that attracts males for mating. Studies characterizing the calling behavior of P. californicus females found they typically lowered their heads and raised their abdomens while extending their ovipositors. In some cases females everted a membranous, cylindrical sac from the dorsal surface of the ovipositor. This eversible sac has not been reported for a cerambycid species, and is likely to be involved in production and/or release of pheromone.

Eggs are laid singly to 1 inches below the soil surface near the trunks of host trees soon after the female is mated. A single female can lay 150-200 eggs during her 10-20 day lifespan.

Prionus root borer damage (tunneling in roots) (S. Steffan)

Shortly after eggs hatch, the larvae seek out host roots. Larvae typically chew deep, spiraling furrows into the roots and may tunnel completely within a root. Larvae can be found in roots from 2 to over 10 inches below the soil surface. Prionus larvae usually kill the apical regions of roots as they feed upward and inward to encounter new root tissue. Samples collected from an infested sweet cherry orchard in Utah found a majority (72%) of smaller larvae (< 1 inches in length) in roots, and a predominance (59%) of larger larvae (1 - 3 inches) in crowns of trees (the crown is the region of the trunk at or near the soil surface where roots transition into the above-ground stem) . The age distribution data suggests that young larvae tunnel into deeper, smaller diameter roots and move upward into larger roots and the crown as they mature.

Mature larvae pupate near the soil surface. The life cycle can require three to five years to complete; thus, the vast majority of its life is spent in the larval stage.


Larval feeding injury can girdle and kill host roots and injure the tree crown resulting in decreased nutrient uptake, water stress and reduced growth. Less severe infestations can result in wilting and yellowing of leaves. Severe infestations can cause the direct or indirect death of fruit trees. Invasion of bacterial and fungal pathogens into damaged roots can promote decline and mortality of trees. Severe infestations of sweet cherry trees have been associated with well-drained, sandy soils along the mountain benches of the Intermountain West.

Prionus root borer larva inside root (S. Steffan)

Prionus tunnels in crown of cherry tree (S. Steffan)


Light trap for Prionus root borer adult beetles (J. Barbour)

Adult prionus can be monitored with pheromone-baited or light traps (UV and incandescent). The beetles typically fly soon after sunset. Trap-catch declines after midnight, presumably due to colder temperatures. Adults may be active from late June to September.

Females produce a volatile pheromone that attracts males for mating. Studies in northern Utah have found that hanging the pheromone lure over a funnel placed in a buried bucket can be an attractive trap for male beetles. Studies in sweet cherry found that the commercially available Contech (30 mg) and Alpha Scents (10 mg) lures were highly attractive to male beetles, and last at least four weeks in the field.

Larvae can be monitored by digging into the soil (6-10 inches deep) around a tree trunk and looking for darkened, soft areas around the crown. Larvae can be found by probing into the dark, rotting tissue of the crown. Alternatively, soil can be removed to search for infestations in roots. Usually, afflicted trees will show signs of wilted and yellowed leaves and canopy dieback during hot spells.

Biological Control

General soil-dwelling predators and pathogens such as ground beetles and fungi may contribute to natural population suppression. Nocturnal vertebrates such as rodents may consume adult beetles. However, no natural enemies providing effective biological control of California prionus have been identified.


Management options in bearing fruit orchards are limited, so avoidance and prevention are the best strategies. Tree stress should be avoided. Symptoms to watch for include canopy dieback or sudden loss of tree vigor. Avoid planting trees into a site known to be infested with prionus. Fallow an infested field for two or more years before planting an orchard crop. Plant annual (non-host) crops as ground cover during the land resting period and till under the crops each year to stimulate microbial activity and prionus larval population decline. An alternative is to fumigate the soil before planting, but the effectiveness is unproven and it is costly.

Insecticides registered for stone and pome fruits may provide incidental suppression of adults. Neurotoxic insecticides (organophosphates, carbamates, and pyrethroids) may kill adults and inhibit egg-laying when fresh residues are present on the lower trunk and soil, but will not suppress larval populations on the roots. Systemic insecticides applied through chemigation to the root zone may suppress younger larvae, but efficacy against larger larvae within a season was not confirmed in a research trial in Utah. Several years of use of a systemic insecticide may be necessary to suppress a local population.

Research trials in Utah suggest that pheromone-baited traps can be used for mass-trapping to reduce populations of the beetle. In a sweet cherry orchard, trap catch declined from 265 to 2 male beetles following five years of deploying pheromone-baited traps. Additionally, testing of an experimental mating disruption dispenser showed an average of 94% reduction in trap capture in disrupted compared to non-disrupted orchards; however, a commercial product is not yet available. In a mating disruption-treated sweet cherry orchard, male trap counts declined from 234 to 32 after two years of use. Given the lack of other effective management strategies for California prionus, pheromone tools offer promise.

References Consulted

Barbour, J. D., Cervantes, D. E., Lacey, E. S. and Hanks, L. M. 2006. Calling behavior in the primitive longhorned beetle Prionus californicus Motts. Journal of Insect Behavior. (online:

Bishop, G. W., Blackmer, L., J. L., and Baird, C. R. 1984. Observations on the biology of Prionus californicus Mots. on Hops, Humulus lupulus L., in Idaho. J. Entom. Soc. British Columbia 81: 20-24.

Cervantes, D. E., Hanks, L. M., Lacey, E. S., and Barbour, J. D. 2006. First documentation of a volatile sex pheromone in longhorned beetles (Coleoptera:Cerambycidae) of the primitive subfamily Prioninae. Ann. Entomol. Soc. of Amer. 99: 718-722.

Linsley, E. G. 1959. The ecology of the Cerambycidae. Annu. Rev. Entomol. 4: 99-138.

Linsley, E. G. 1962. The Cerambycidae of North America Part II. Taxonomy and classification of the Parandrinae, Prioninae, Spondylinae and Aseminae. University of California Press, Berkeley, CA.

Linsley, E. G., and Chemsak, J. A. 1997. The Cerambycidae of North America, Part VIII: Bibliography, index, and host plant index. Univ. Calif. Publ. Entomol. 117: 1-534.

Solomon, J. D.. 1995. Guide to insect borers in North American broad-leafed trees and shrubs. USDA AH-706. 735pp.

Steffan, S. and Alston, D.. 2005. Prionus root borer (Prionus californicus). Utah State University Extension Fact Sheet HG/Orchard/2005-01. (online:


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