Q: What are the most common pine trees in the Conejo Valley? Why are so many of them dying? What can be done about it?

A: There are more than 100 species of pine trees in the world, but we see only a small portion of that number growing in Conejo Valley.

The most common species of pines found in Thousand Oaks are Monterey, Aleppo, Canary Island and Italian stone pines. As is the case with most other trees seen in yards and greenbelts throughout the area, none of these pines are indigenous to the Conejo Valley. In fact, most trees of all species that are found in local nurseries are foreign to Conejo Valley.

This has been a really bad year for pine tree mortality. Right now, if you drive around the area, you can see brown pine trees in every neighborhood.

When a pine tree has mostly dark green needles, it’s normal and healthy. When all the needles turn light green, it’s usually time to worry. When a pine tree changes to a yellow color, it’s too late because the next step is totally brown and dead.

In almost every case, when a dead pine tree is cut down and the bark is peeled away, it’s riddled with bark beetle galleries full of a fine dusty material called frass. Frass is what’s left behind as the bark beetle larvae excavate and eat just under the outer bark of the tree. You can tell that bark beetles are attacking a pine tree when you see the entire top of the tree turn brown seemingly overnight. The brown then methodically moves right down the tree.

Bark beetles are opportunistic creatures that usually attack older pines and those that are stressed and already declining in vigor. In other words, bark beetles are usually the secondary reason that an already declining tree dies. If a pine tree is young and vigorous, it can usually fend off the attack by bark beetles, and you may not even know that an assault took place.

There are different species of bark beetles that attack pine trees. Until last year, it seemed as though Canary Island pines were almost impervious to beetle damage compared to Aleppo and Monterey pines, which are the species most often attacked in Thousand Oaks. Then late last summer I saw about a dozen mature Canary Island pines that turned brown from top to bottom in a period of three weeks.

This year most of the dead or dying pines I see are Monterey, Aleppo and Canary Island pines.

As with so many other trees, a large portion of the blame for dying pines goes to a lack of sufficient water. I’m concerned that if the drought gripping the Southwest doesn’t end soon, we’re going to see many more dead and dying trees next summer.

The best thing you can do to improve the health of your trees is to give them infrequent but deep soakings of water to at least 12 inches in depth. Put a soaker hose or other drip irrigation system out near the drip line of the tree to concentrate the water where it’s most effective.

Beyond that, let’s all pray for a lot of rain this coming winter.

David D. Mortimer is a certi fied arborist with more than 30 years’ experience in the tree-care industry. E-mail questions to dmortimer@theacorn.com.