Prionus californicus
Pest description and crop damage A very large (up to 3.5 inch body), brown native long-horned beetle with elongate serrated antennae and large mandibles (jaws). Three lateral spines are present on each side of the thorax. The adult beetles are capable of vocalizing when disturbed. The adults fly close to the ground at night and are attracted to lights, making them familiar to residents of the Pacific Northwest. Adult beetles do not feed. The larvae, or grubs, are also quite large, and range from 0.75 inch when young to more than 4 inches when mature. The white, legless larvae are the damaging stage as they destroy roots, typically starting as young larvae at the tips, and increasing in size as they mature and progress toward the root crown. This may cause symptoms of stress in the tree canopy that manifest as dieback, reduced growth/poor vigor or reduced nutrient status in plant tissues. Prionus can also damage subsurface drip irrigation lines and damage to anchorage roots can predispose trees to toppling in storms. Populations can build to high levels in mature orchards and growers sometimes suffer heavy tree mortality when replanting an orchard with high numbers of grubs remaining in the soil. Consider allowing a fallow period before replanting heavily infested orchards. Lighter soils may be more conducive to this pest.
Biology and life history Eggs are laid just below the soil surface in summer. Larvae emerge from eggs and begin seeking out roots. As described above, the youngest larvae are found on more distal roots, and the older and largest larvae are typically found within the crown. Mature grubs are sometimes found inside the trunk just above the root crown and soil level. Larvae girdle roots and hollow them out with their tunnels. The larvae can develop for up to five years before pupating close to the soil surface and emerging as adults in mid-summer. Because they don't feed, the adults are not damaging to the orchard, and they can live for close to a month producing up to 200 eggs per female. Adults may burrow during the day and are rarely seen in the orchard outside of traps.
Pest monitoring Pheromone traps are available to monitor adult beetle populations. The traps capture male beetles using a female-produced sex pheromone. Typically, a panel trap with large surface area is most appropriate for capturing beetles with the pheromone lure. Light traps can also capture adults. Larvae may be discovered by removing soil and examining roots for feeding damage. Trees with poor vigor or signs of canopy stress may help narrow down the search for larvae.
Management-biological control
Very little is known about potential biological controls. Soil-dwelling predators may cause some level of larval mortality. Entomopathogenic nematodes may attack grubs. Birds and small mammals may prey on adults.
Management-cultural control
Minimize drought stress and manage plant nutrition to prevent attack.
Management-behavioral control
There is potential for mass trapping adult males using the commercial pheromone lure. Enough male beetles can be caught to reduce mating success and lower the number of eggs laid into the soil, reducing the local population over time. Cross-vein panel traps are hung very low in the canopy and baited in the center. They should be deployed at a high rate such as 1 trap per 2 acres to maximize trap captures. Because the larvae develop in the soil over a period of years and adults emerge each year, it is important to maintain the trapping program for several years to see any results. Mating disruption also shows promise but remains in the research phase of development. Light traps can be effective.
Management-chemical control: COMMERCIAL & HOME USE
No insecticides are labeled against Prionus in hazelnuts. Soil-applied imidacloprid targeting aphids may kill some of the younger grubs.