Invasive Shot-Hole Borer and Fusarium Dieback Field Guide Identifying Polyphagous and Kuroshio Shot-Hole Borer in California

Author(s): Eskalen, Akif; Kabashima, John; Dimson, Monica; Lynch, Shannon | Abstract: The Invasive Shot-Hole Borer is a new, rapidly spreading, invasive pest threatening landscape trees and commercial avocado groves. The beetle is a vector for the plant disease fusarium dieback. First detected in the Los Angeles basin in 2012, it has quickly spread as far north as San Luis Obispo. Currently there are no control measures for the disease. While the current economic impact to crops is focused on avocados, computer models suggest almond to be a potential host. This publication explains the life cycle, biology and identification, look-alike pests, signs and symptoms of infestation and damage, reproductive hosts, and landscape and agricultural host species.


B. Beetle Biology and Identification
ISHB bore tunnels (galleries) into host trees where they lay their eggs and grow the fungi.The two beetle species are physically identical.At 1.8 to 2.5 mm long, ISHB adult beetles are smaller than a sesame seed.The adult females (B1) are larger than the adult males (B2) and are also darker (B3).Most of the beetle's life cycle, from larva to adult (B4), is spent in the galleries.Mature siblings mate with each other so females are already pregnant when they leave to start their own galleries.

C. Signs and Symptoms
Entry holes are round and about 0.85 mm wide, the size of a ballpoint pen tip (C1).The abdomen of the female beetle may be seen sticking out of the hole (C2).Tree symptoms are unique to each host species.Around the entry hole, look for dark, wet staining that sometimes dries to white or yellow (C3), thick gumming (C4), powdery white exudate (C5), or frass (C6), which resembles sawdust.
Symptoms of infection by FD pathogens include brown to black discoloration on wood beneath the bark.Scrape away bark around the entry or exit hole to reveal dark staining surrounding the gallery (C7, C8).Cross-sections of cut branches show the extent of infection (C9).Branch dieback is the result of advanced infection by ISHB's associated fungi.It may begin on a few branches (C10) and can eventually kill entire trees (C11).

D. Look-Alike Pests
Other pests may cause damage similar to that of ISHB-FD.Clues that may indicate a pest other than ISHB include • staining, gumming, or exudate but no entry hole • entry holes with an irregular shape (not round) • entry holes larger or smaller than a ball-point pen tip • the tree is not a known host of ISHB-FD (e.g., pine or most eucalyptus species) Visit the University of California Statewide Integrated Pest Management Program website, ipm.ucanr.edu, to learn more about these pests.Spread by the western oak bark beetle (Pseudopityophthorus pubipennis).Hosts: Coast live oak; stressed or dying trees.Look for beetles (D1) 1.7 to 2.3 mm long; reddish frass (D2), reddish sap, wet discoloration, and/or foamy liquid (D3) (a sign of infection) from an entry hole (1 mm) that is smaller than that of ISHB; dead tissue around entry hole beneath bark (D4).

Look-Alike Pests That May Have an Entry Hole
Fruit tree shot-hole borer (Scolytus rugulosus) (D5) Hosts: Fruit trees in Prunus genus, English laurel.Look for entry holes (2 mm) oozing sap or frass; the holes are larger than those of ISHB, with slightly rougher edges (D6.).Exit holes are sap free.

E. Reproductive Hosts
Reproductive host species support beetle reproduction and the growth and development of the symbiotic fungi.Each species is affected differently.Trees may be more susceptible if they are already under stress due to other pests, diseases, or environmental conditions or are in close proximity to an existing infestation.This list of species is not meant to be used as a do-not-plant list.Non-Native Host Species caused by Geosmithia sp.#41 ambrosia beetle, Xyleborinus saxeseni Hosts: Stressed and dying trees.Look for beetles 2 to 2.4 mm long (D10); entry holes (1 mm) smaller than those of ISHB (D11); reddish frass and/or sap; wet discoloration and/or dead tissue around entry hole and beneath bark (D12).
However, as known hosts of ISHB-FD, species on this list should be closely monitored for potential infestation.Visit the Invasive Shot-Hole Borers website, www.pshb.org,for updates.