Weather Watch

Bark beetles are killing trees in the Sierra Nevada at a record pace. Here's why

I was startled by the sheer number of dead trees on a recent trip to the Sierra Nevada.

A large percentage of these dead trees were caused by bark beetle infestation, and it seems to be accelerating. To explain why, here’s some background.

“Since 2010, an estimated 129 million trees have died in California’s national forests due to conditions caused by climate change, unprecedented drought, bark beetle infestation and high tree densities,” according to the U.S. Forest Service.

Evergreen needles on conifer trees take the sun’s light from the sky, carbon dioxide from the air and water and minerals from the earth and transforms these ingredients into oxygen and energy-rich organic compounds, such as sugars, that plants use for food through a miraculous process called photosynthesis.

Trees draw in water and dissolved minerals from the ground through their roots.

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A large patch of dead trees are seen in a valley in the Sierra Nevada mountains from a helicopter tour Tuesday, Dec. 1, 2015. Mostly ponderosa and sugar pine trees are dying off in large numbers around Bass Lake and throughout the Sierra Nevada due to a bark beetle infestation brought about by years of extreme drought in California. CRAIG KOHLRUSS

The xylem, which is made of vascular tissue within the tree, transports the water, in some cases, hundreds of feet up to the conifer’s needles. A fantastic amount of water transpires from the needles of these trees. A single large sequoia redwood can move 500 gallons of water into the air in just one summer day.

As the tree’s food is created in its needles, the phloem, which is located just below the tree’s bark, transports the sugars and other essential molecules to every living cell in the tree.

Unfortunately for the tree, other creatures can feed on the phloem. I still remembered Euell Gibbons asking TV viewers, “Ever eat a pine tree? Many parts are edible,” during a Grape-Nuts cereal commercial. The tree’s phloem may not remind us of “wild hickory nuts,” but bark beetle depends on it for survival.

Below-average precipitation for most of this decade combined with above-average temperatures from climate change has cause this grain-of-rice-sized insect’s population to explode, and here’s why.

Typically, these beetles spend most of their lives in the tree’s inner bark, feasting on the phloem, breeding and laying eggs. After hatching, thousands of larvae that resemble small fly maggots carve out galleries and destroy the tree’s vascular system as they eat.

To make matters worse, they also carry fungi in their mouthparts, and when they chew the phloem, the fungi quickly multiplies, reducing the amount of pitch the tree can release. This eventually cuts off the flow of nutrients to the tree’s living cells and kills it.

You see, when a pine tree is healthy, it can expel bark beetles from their first bores with copious secretions of sticky sap and pitch. However, during droughts, these trees can’t produce enough resin to kill these beetles.

If the trees are too close together, there may not be enough water in the ground to stage a robust defense; in other words, too many straws drinking from the same milkshake.

Dead trees, killed by the drought, line a mountain road in the Sequoia National Forest in this photo from November 2016. U.S. Forest Service

Once the insects are grown, they fly away in search of new trees to infect.

Their life cycle takes about a year to complete; but at higher elevations with cooler temperatures, it may require two years.

When the beetles find a weak target, they send out chemicals called aggregating pheromones to tell others of an easy meal. Bees and wasps use the same chemicals to attack intruders in deadly swarms.

When large groups of these tiny-wing beetles fly in, it’s called a mass attack and can spill over to other nearby trees, producing a group kill. If the temperatures are warm enough, this vicious cycle can be repeated several times a year, increasing the number of dead trees and risk of wildfires.

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John Lindsey’s column is special to The Tribune. He is PG&E’s Diablo Canyon marine meteorologist and a media relations representative. Email him at or follow him on Twitter: @PGE_John.
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