Polyphagous Shot Hole Borer
Polyphagous Shot Hole Borer (PSHB) is an exotic ambrosia beetle that was first detected in 2003 in Southern California.
It is currently found in L.A., Orange, San Bernardino, western Riverside, and San Diego Counties. The “ambrosia” name refers to a symbiotic fungus that is carried by the female in special organs in her mouth parts. The fungus is grown in the beetle galleries and both the adult beetles and larvae feed on the fungi. The adult beetles are very small, ranging from 0.05 to 0.1 inches in length. They come in a range of shades between black (females) and brown (males) coloring. While this beetle attacks a large number of plant species, the majority of which are hardwoods, it can only reproduce in 31 species including maples, sycamore, oaks, willows, alders, and avocado.
The characteristics of PSHB attack and fungus infection differ among tree species. The beetle commonly attacks the main stem and larger branches of trees and shrubs, but injury can be found on twigs as small as 1 inch in diameter. The beetle produces a very precise, perfectly round, tiny (< 0.1 inches in diameter) entry hole in most trees. Wet staining and discoloration on the bark of the main stem and branches are early symptoms of beetle attack. Depending on the tree species attacked, PSHB injury can be identified either by staining, gumming, or a sugaring response on the outer bark. Infection with the fungus can cause leaf discoloration and wilting, dieback of entire branches, and tree mortality.
TREE-äge provides 2 years of control of the beetle, while Propizol will protect trees from the fungi introduced by the ambrosia beetle. Use a soil surfactant, such as NutriRoot, as a sub-surface injection or drench, in combination with watering to encourage root growth, increase penetration of water into the soil, and to assure better translocation of the formulations throughout the tree.
The most significant damage occurs from May through October so if PSHB is found near or on trees under your care, treatments should be made immediately.
Generally, the best seasons for injection are spring and fall, since the best uptake occurs when trees are actively transpiring, but treatments can be made at any time of the year when there is good soil moisture to encourage translocation of the systemic formulations.
Trunk injections with insecticide or combined with fungicide will distribute upward in the tree within 4-6 weeks with adequate soil moisture. For trees in natural, non-irrigated areas with questionable soil moisture, consider using NutriRoot to enable trees to extract water from the soil, reducing the need for watering, and resulting in faster and better movement of systemic products even in drought conditions.
Main photo taken by Stacy Hishinuma, UC Davis, Department of Entomology
Attack point taken by Don Grosman, Arborjet
Bark staining taken by Don Grosman, Arborjet
Damage taken by Don Grosman, Arborjet