Managing bark beetles: protecting valuable conifers in Yavapai County

Although the recent rains have a tendency to make us forget, Arizona has experienced long-term drought conditions for several decades. This persistent lack of precipitation causes several challenges, but one that is never far from my mind is the dynamic between our majestic conifers (generally evergreen plants that reproduce by cones and have needle- or scale-like leaves) and the native bark beetles that prey upon them.

Which trees are threatened and by what?

In Yavapai County, our major conifer groups include ponderosa and piñon pines, several species of junipers and cypress. Bark beetles are often host-specific. with Piñon Ips primarily affecting piñon pine, several species of Ips engraver beetles associated with ponderosa pine, and beetles from the genus Phloeosinus attacking our Junipers and cypress. 

These hardy trees have evolved in our arid climate, allowing them to survive long periods with limited precipitation, but that does not mean it is ideal. For example, upon attack by bark beetles, healthy conifers use copious amounts of resin – also known as pitch – to either push out or trap and kill the insects, preventing them from establishing themselves under the bark. Under normal circumstances, this dance is a balanced one, with native populations of various bark beetles culling trees that have been weakened by age or damage, while the strongest specimens are largely able to repel these attacks.

Drought conditions, however, upend this equilibrium. When conifers are stressed due to lack of water or higher than normal density (see decades of fire suppression), their defenses are compromised and even healthy trees may be overtaken. This becomes particularly problematic when large stands of dead trees create a significant and highly flammable source of fuel for forest fires, or if single trees are killed, creating falling hazards to life or property.

How to protect your trees

While we cannot control the weather – and saving every tree is neither practical nor desirable – we can take steps to improve the odds for our most valuable trees. If you have several conifers on your property or significant acreage, trees of particular value should be prioritized. These include trees that increase property value or those which could be hazardous if they fell. This management might include supplemental irrigation (deep, infrequent applications), thinning of over-crowded stands, removing infested trees and slash from thinning projects, preemptive insecticide applications, or the placement of verbenone, an anti-aggregation pheromone.

If you choose to use insecticides, the most important point to understand is that these treatments are only found to be effective if they are applied preventatively. If a tree is already infested, it is unlikely that chemical applications will save the tree.

Regarding specific products, there are several on the market and different application methods. Bark sprays containing products such as carbaryl or pyrethroids work by killing or repelling beetles when they contact the treated bark. Emamectin benzoate is a systemic treatment that is injected into the tree, providing protection from within. Importantly, many tree insecticides sold for homeowner use are applied to the soil and taken up by roots; however, these products have not shown the same level of effectiveness against bark beetles as preventive bark sprays or trunk injections and should be used for insect species that ingest sap, such as aphids, scales and mealybugs.

Finally, be sure that the product chosen is labeled for use on the species of tree that you are treating and that it is effective against the pest you are managing. The label is the law, so in many cases it may be preferable to hire a certified, professional arborist to select the product and perform the application.

Classes available in Yavapai County

For those interested in all things gardening, registration has opened for our home horticulture class. This Zoom class is a condensed version of the Master Gardener training program and will meet 6-8 p.m. every Thursday starting September 17. For more information and registration, go to: extension.arizona.edu/yavapai

Matt Halldorson is the director of Yavapai County Cooperative Extension and an Agriculture and Natural Resources associate agent.