‘Signs” has a particular meaning in the world of plants and trees. Looking at a large Monterey pine recently, showing symptoms of stress – fewer and paler needles than in previous years and a lack of new growth this spring – I also saw “signs” of an insect pest: reddish granules collecting in bark crevices on the lower trunk.
For Monterey pines here in Napa Valley, that sign says: “Impending Death.” It is “frass,” boring particles about the size of coffee grounds, produced by certain insects. In this case it is an engraver beetle, the five-spined Ips. The adult beetles “engrave” a wishbone-shaped gallery in the cambial zone under the bark. Then the larvae branch out, feeding on the living tissue.
If you have the opportunity to pry the bark off of a cut pine log, you may see the galleries, and even the larvae still feeding there. They can emerge as young adults from cut logs.
I have been checking up on this tree periodically for several years. Back then, it showed only fair vigor. On my advice, the owner has been providing it with summer irrigation to prevent drought stress that contributes to the decline in warm, dry summer locations.
Over the intervening years the first “sign” of trouble appeared: open “pitch tubes” on the base of the trunk. The openings are about the diameter of a wood pencil, surrounded by a mixture of grayish-white pitch and reddish frass. It is direct evidence of red turpentine beetle infestation (Dendroctonus valens). This beetle tunnels in the bark and the larvae feed there. The damage is harmful but not lethal. The adults are attracted to stressed pines, so the appearance of pitch tubes indicates the tree is stressed.
If you have been following the extensive die-off of drought-stressed pines in the Sierra, you are probably aware of the Western pine beetles (Dendroctonus brevicomus and others) as the agents that kill the stressed trees. Making matters worse, a buildup of their population can affect even the healthy trees.
There is a principle at work here: susceptibility induced by stress. It is evident in many tree species. To name a few: cedar and cypress bark beetles, shothole borers in fruit trees and various shade trees, bark beetles in elms that in turn infect them with Dutch Elm Disease, Western oak bark beetles and ambrosia beetles, and bronze birch borer in white birch.
The principle also applies to opportunistic diseases. The first one that comes to mind is a fungal canker disease,
Botryosphaeria. It causes dieback in giant Sequoias and sometimes in coast redwoods, madrone, and many others. “Diseases of Trees and Shrubs” (Sinclair, Lyon and Johnson) lists about 130 susceptible trees, shrubs and vines.
Now for the practical part. First comes stress prevention. For Monterey pines that means growing them where the environment resembles that of their native territory: Cool coastal portions of Mediterranean climate zones. That is a matter of planning and planting for the future. If you need an evergreen conifer for a relatively warm dry zone around the Napa area, consider the true cedars (Cedrus): Deodar, Atlas and cedar of Lebanon if there is ample space. Visit ”SelectTree” online for good suggestions.
Prevention includes anticipating problems with respect to the tree and the site where it is living. It goes along with stress reduction. Irrigation is the single most effective and manageable practice. Apply water as needed for the tree and the conditions on the site. Don’t guess. Use some sort of soil probe to get a feel for the moisture conditions. Check for moisture content before and after watering and before repeating. Too much can be as harmful as too little.
In some cases, protective insecticidal sprays or systemic insecticide injections will help save highly valued trees.
Weigh the costs and the benefits, then make a well-informed decision.