Prionus californicus Motschulsky
-- Diane G. Alston, James D. Barbour, and Shawn A. Steffan
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)
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- 2¼ 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)
Eggs are laid singly ½ to 1 ½ 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)
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.
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)
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.
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.
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: http://www.springerlink.com/link.asp?id=p12214434w5h3313).
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.
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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: http://extension.usu.edu/files/publications/publication/HG_Orchard_2005-01.pdf).