PSHB Infestation at Local Park.

California native sycamore showing rusty stains and small holes indicative of the Polyphagous Shot Hole Borer. 

A large local park in the San Gabriel Valley has an extensive infestation of PSHB  that has killed or damaged what appears to be 10% of its tree canopy. The two most affected plants are California sycamore (Platanus racemosa) and White alder (Alnus rhombifolia). The extent of this infestation here in the San Gabriel valley is quite significant and could change the the way we use trees here.

Alder bark showing PSHB hole and staining.

One of the things that might be exacerbating this infestation is the long period of drought we are currently in. There has been only one year of normal rainfall here in the L.A. area since 2005. Ground water levels are low and this can put huge amounts of stress on large trees that rely on groundwater such as Alders, Sycamores and Oaks. Extra water may not be the answer because it might not be possible to supply the amount needed for large trees solely through irrigation.

White alder 3/4 dead from PSHB

One of the things that might have to change is the tree density here. If you look at historic photos of Los Angeles you notice large areas of treeless plains. There is a reason for this; these areas were mostly grasslands and could not support large trees without irrigation.

California native sycamore infested with PSHB

Cottonwood tree growing in undeveloped river bottom showing possible PSHB infestation.