NPS Illustration / M. Perez Sometime between May and August, adult spruce beetles (Dendroctonus rufipennis) leave their winter homes in search of new host trees during what is called ‘flight season’. Once D. rufipennis lands on its chosen host, the beetle bores through the bark and into the phloem. Phloem is the tissue under the bark where sugary sap is carried throughout the tree. It is in the phloem where female beetles construct galleries (appearing as tunnels through the phloem) to lay eggs. Once the eggs hatch, they grow into feeding larvae that continue to create galleries and disrupt the phloem—essentially choking the tree of sugar. A tree with severely damaged phloem is unable to move water to its needles or sugars towards it roots. Needles will fade from green to red or brown and fall. Without functioning green needles, a tree will eventually die. However, a tree does have some defenses against beetle attack. As adult spruce beetles bore through the bark, a healthy tree may be able to produce resin in attempts to keep the beetle out. The resin mixes with boring dust to create pitch tubes. Beetle may get caught up in the resin or pitch tubes and succumb to the tree’s defenses. Spruce beetle attacked trees do not always die. The initial health of the tree, the severity of the beetle attack, the quality of the growing site and other factors all likely contribute to whether an individual tree will die. If tree mortality does occur, it may occur by fall during the season of attack, by early summer the following year, or even 1–2 years later. It may appear as a general fade (from green to red) of all needles at once, or as ‘flagging’ of lower branches first, wherein just a few branches’ needles are completely red while the rest of the tree’s needles appear green. Some trees survive beetle attack. Click here to return back to the main page |
Last updated: April 14, 2025