Polyphagous Shot Hole Borer and Fusarium Dieback in California

Abstract

The Polyphagous Shot Hole Borer (Coleoptera: Curculeonidae: Scolytinae), Euwallacea sp. near fornicatus, is an ambrosia beetle native to Asia that has been introduced into Israel, California, and South Africa. The beetle maintains a symbiotic relationship with three species of fungi that it vectors between host trees by carrying spores within a mandibular mycangium. These ambrosial fungi are inoculated into host trees and are the sole nutritional source for the adults and the larvae. Unfortunately for forest and resource managers, one of the fungi, Fusarium euwallaceae, is a moderately virulent pathogen and is responsible for causing a dieback disease in susceptible hosts. High levels of infestation of susceptible host trees have resulted in high levels of mortality. The currently recognized host range for the beetle-fungus complex includes more than 200 tree species that can be attacked by the beetle, more than 100 species that can support growth of the fungus, and 37 species that can be used as a reproductive host by the beetles. Many of these reproductive hosts are important agricultural crops, components of the California urban forest, or components of native riparian forest communities. Management is currently focused on monitoring using visual inspections or trapping, sanitation using solarization or chipping, and direct control using contact or systemic insecticides. Future management approaches will include planting resistant or unsuitable host trees and biological control. As more information about the biology and ecology of the insect-fungus complex is developed, it may be possible to develop strategies for limiting spread within and among regions of the world with Mediterranean climates.