It’s a pleasant summer day in the Cypress Hills, the air is warm, you take a hike. Everything around you looks beautiful: the mountains, the green pine trees, the reddish pine trees… the reddish pine trees? That’s a bit unusual, eh? And you’re right. Stain-coloured needles of conifer trees are not normal and not a good sign. Chlorotic trees (an adjective used to describe sick reddish-brown trees) are a sign of mountain pine beetle infestation, most of the time associated with blue stain fungi, which act together to kill coniferous trees.
Mountain Pine Beetle (MPB)? Never heard of it. Mountain pine beetles (Dendroctonus ponderosae Hopkins) are small, black, bark beetles about 4.0 to 7.5 mm long (approximately the size of a grain of rice). They mostly feed, live, and reproduce on Lodgepole pines (Pinus contorta), but can also invade other species of pine within their range. Usually after one year (or two at higher elevations) of their lifecycle, adults emerge in midsummer to attack green trees. When MPB populations are small, their preferred meals are stressed and mature trees of 80+ years old, but as populations grow, they will invade younger, healthier trees of over 12.5 cm in diameter.
Before we forget. Adult MPBs transport spores of blue stain fungus* to new trees within specialized sacs called mycangia*, creating spread of fungal infections, and eventually resulting in tree mortality. These fungi are believed to stop water transport in the stem and kill trees due to desiccation* (i.e. dehydration).
Houston, we’ve got a problem. What now? Luckily, Lodgepole pine trees have several defences that can repel or kill attacking beetles at the time of colonization. One example is the elaborate networks of ducts and glands that store large amounts of the monoterpene, oleoresin*, which helps with anatomical and chemical defence. This resin can physically enclose or delay attacking beetles while delivering secondary metabolites (SMs*) for herbivore deterrence that can negatively affect the critters. There are other cool tree defences in addition to monoterpenes*, like diterpenes* and other compounds in conifer phloem* that protect against stress (flavonoids, vanilloids, hydroxycinnamic acids, lignans, condensed tannins, etc.). Some flavonoids* have shown to directly affect beetles by acting as anti-feedants, meaning, the part of the tree from which the beetles are feeding on becomes unpalatable, forcing them to stop consuming it, yuk!
Something very important to remember! Relationships between MPBs and blue stain fungus involve complex feedback as some bark beetle symbionts* can get around this anti-fungal activity by converting phenolics* into carbon sources for larvae in the dying phloem. In other words, more resources for the beetles!
Good or bad?
Although this seems like a violent back and forward fight between trees, beetles, and fungi, it is really a balanced interaction that occurs in all levels of nature. Beetles consuming older trees will essentially create room for the growth of new trees, which leads to a healthy forest succession, and at the same time, beetles themselves are resources to other animals, like woodpeckers and larger insects. The real problem comes into play when human disturbance/interaction disrupts the ecological balance in the environment. Potentially, we could start thinking about tree vaccines to counter the spread of disease in the wild? Maybe, but we will touch base on that one another time! (wink, wink).