In a natural forest, engraving bark beetles play a healthy role in selectively removing weak and overly mature trees.

Healthy Pines trimmed at the correct season

This natural process rejuvenates the forest by allowing new species to emerge. Healthy trees are able to fight-off the attacking beetles by smothering them with kino (sap). Unfortunately, due to less than accommodating planting situation, many trees in our urban forest are chronically stressed and pre-disposed to beetle infestation.

Our urban forest, although fairly healthy, is living in less than adequate living arrangements. We typically plant our trees in areas that are too small for their root system, water them to frequently with a low volume of water (only wetting the surface), over fertilize, compact the soils and remove the organic materials around them, plant them in the wrong area (full sun, full shade, etc.), over prune (remove more than 25% of the live canopy), and improperly prune (large wounds, topping, shaping). These factors cause our urban forest to be stressed.

Bark beetles love stressed trees. Beetles recognize these trees as lacking the ability to smother them while invading the tree and therefore are seen as a good host. Bark beetles that attack pine species are actively searching for a tree host from late spring to early fall. At this time, our pine trees are most susceptible to infestation.

What can we do to help prevent them from becoming hosts? Increasing the overall health of our trees by proper cultural practices is the best and easiest answer. This will provide the tree with more stored energy to fight off invading beetles. There is another very important action that we can take (or not take) to prevent infestation – trimming.

Western Pine Beetle Entry Hole

Trimming our trees, if performed correctly, can be very beneficial, but it creates wounds. In nature,tree wounds are created from branches breaking or splitting during events like wind and storm. Trees that break during storms become stressed and are great hosts for beetles. Therefore, when we trim our urban pine trees during the active beetle season (May – Nov), we’re sending out the wrong signal, “hey- over here! Make my tree your new home.” So, be nice to your pine trees and trim them from December through April.

Learn more about the correct season to trim individual tree species in San Diego – Multi Year & Seasonal Tree Care Management Plans.