by Diane G. Alston, James D. Barbour, and Shawn A. Steffan, published online December 2007
Prionus californicus Motschulsky (Coleoptera: Cerambycidae)
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.
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 more prominent of a pest of fruit trees, including sweet cherries, in the Intermountain West region in the last 10-15 years.
Eggs are cream to yellow-brown in color and about 3/16 inch long and elongate: about twice as long as wide.
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.
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).
Adults are large reddish brown beetles (1 to 2 inches in length) with smooth and shiny elytra. Adults are sexually dimorphic with males being smaller (1 to 1-3/4 inches) than females (1-1/2 to 2-1/4 inches) and having antennae that are much more strongly serrate than those of females.
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 which was retracted before the ovipositor was withdrawn. 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 1/2 to 1-1/2 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.
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 of smaller larvae (< 1-1/2 inches in length) in roots (72%) and a predominance of larger larvae (1-1/2 – 3 inches) in crowns (59%) 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.
Adult prionus can be monitored with 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 early August.
Females are known to produce a volatile pheromone that attracts males for mating. Research is underway to identify the female sex pheromone, which may provide a more efficient monitoring tool for prionus adults, and perhaps a means for managing California prionus infestations using mating disruption or mass trapping techniques.
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.
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.
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