Pine beetles are small insects, a type of bark beetle, that pose a severe threat to pine trees across North America. These beetles, like the Mountain Pine Beetle or the Southern Pine Beetle, bore into the bark to reproduce, often leading to the death of the host tree. Identifying a beetle-infested tree involves looking for a sequence of signs, starting with subtle physical indicators on the trunk and progressing to noticeable changes in the tree’s canopy. This article guides readers in recognizing the damage left by these destructive insects.
Primary Indicators on the Tree Trunk (Pitch Tubes and Frass)
The most immediate signs of a beetle attack appear on the trunk, where the tree attempts to defend itself. Pitch tubes are masses of hardened resin or sap expelled by the tree as it tries to push the boring beetle out of the bark. These protective masses resemble popcorn and range in color from creamy white or pale yellow to reddish-brown, depending on the success of the tree’s counterattack.
A healthy tree with strong resin flow may successfully “pitch out” the beetle, resulting in a lighter-colored pitch tube with a dead beetle trapped inside. Conversely, a successful beetle attack often results in a reddish-brown pitch tube, a color derived from the mixing of the tree’s resin with the beetle’s boring dust.
This reddish-brown sawdust, known as frass, is the second indicator and is composed of excavated bark and wood material. Frass is easily identifiable as a fine powder that accumulates in the bark crevices or collects around the base of the tree. The presence of this dust confirms that the beetle has bored through the outer bark and into the inner layers. If pitch tubes are small or absent, often due to a tree being stressed or in a drought, frass alone indicates a successful infestation.
Canopy Changes and Tree Discoloration
While trunk signs indicate an active attack, changes in the tree’s canopy signal that the infestation is progressing and the tree’s internal systems are failing. The first visible sign from a distance is a gradual discoloration of the pine needles, typically starting with a pale green or yellowish-green fading in the lower crown or at the top of the tree.
This color change progresses to the “red-top” stage, where the entire crown turns a rust-red or reddish-brown. This shift occurs roughly eight to ten months following a successful beetle attack and results from the tree’s inability to transport water and nutrients. By the time a tree displays this red color, it is dead or dying, and the adult beetles have often completed their life cycle and moved to a new host tree.
The final stage of discoloration sees the needles turn a dull gray as they begin to drop off, which can take three to four years after the initial attack. This mass, uniform discoloration should be distinguished from the natural, seasonal shedding of older needles, which typically affects only the inner-most needles. A completely red or gray crown indicates widespread systemic failure caused by the beetle’s activity beneath the bark.
Internal Signs of Infestation (Galleries and Fungi)
Confirmation of pine beetle activity often requires peeling back a small section of bark to reveal the distinctive internal damage. Just beneath the outer bark, etched into the sapwood, are the galleries—tunnels created by the adult beetles and their larvae. The pattern of these tunnels is unique to the species, providing a definitive identification sign.
For example, the Mountain Pine Beetle typically excavates a vertical, I- or J-shaped egg gallery, with smaller, horizontal larval tunnels branching off at right angles. The Southern Pine Beetle creates winding, serpentine, or S-shaped galleries that crisscross and effectively girdle the tree. These galleries disrupt the flow of the phloem, the layer responsible for moving food from the leaves to the roots.
Accompanying the galleries is the presence of blue stain fungi, such as Grosmannia clavigera, which the beetles carry and introduce to the tree. This fungus rapidly colonizes the sapwood, the water-conducting tissue, causing the wood to take on a characteristic blue-gray coloration. The fungi’s thread-like growth blocks the tree’s vascular system, preventing water transport and hastening the tree’s death.