This vista from the Yosemite Valley in the Sierra Nevada mountains is usually lush green, but now it's marred with splotches and streaks of brown — the work of the tree-killing bark beetles that are ravaging California forests and national parks.

On a recent drive along the western slope of California’s Sierra Nevada mountains, we were struck by the large brown blemishes soiling the mountainsides.

As we headed south during that October trip, we saw brown splotches, streaks and patches that seemed to stretch on forever – the result of dead trees marring the normally bright green forests that have greeted us in the past.

Unfortunately the blight has spread to Yosemite and Sequoia, two of our favorite national parks. Logging of dead trees near roads temporarily stopped traffic during recent visits to each.

A June report from the U.S. Forest Service estimated 26 million trees in California had died in the previous eight months. This was in addition to 40 million trees killed off statewide from 2010 through 2015.

Unfortunately the future of the forests isn’t promising.

“California is facing the worst epidemic of tree mortality in its modern history,” Gov. Jerry Brown wrote to the U.S. Secretary of Agriculture, seeking federal help removing dead and dying trees, in October 2015.

The culprits in the deaths of tens of millions of pine and fir trees are bark beetles that thrive in the drought-weakened forests of the Sierra Nevada. The beetles have turned swathes of mountainsides from green to brown.

Dead trees along roads are being taken down for safety reasons, but large areas of brown cover the forests in locations that are more difficult to access.

Loggers have run out of space to store dead trees. Only about two-dozen lumber mills remain in the state, and most face a surplus of salvaged wood. Some academics and scientists have suggested that energy companies be subsidized for using dead trees as fuel.

The dead trees that remain in the forests become a major fire hazard, a particular problem because an increasing number of Californians are building houses in the state’s forests. Taking down and removing large numbers of trees creates yet another concern because heavy rains across treeless soil can produce mudslides.

The tiny beetles killing the trees are each the size of a grain of cooked rice. Their numbers have multiplied due to warming temperatures in much of the country. An absence of freezing temperatures ensures their survival.

As the beetles thrive, a five-year lack of rain and mountain snowfall has weakened the trees, which are unable to generate sufficient amounts of the gooey defensive pitch to repel the beetles. Once they’ve penetrated a tree’s bark, the beetles lay eggs that hatch into larvae that feed on the tree’s tissue, inhibiting its ability to convey nutrients.

Beetles that locate a weakened tree emit pheromones that attract other beetles. The weakened tree is soon dead, at which time the mass of beetles move onto another victim.

A large number of beetles can ravage even a healthy tree.

Trees can be sprayed to protect their bark from invasion, but the cost of undertaking this on a massive scale is prohibitively expensive. Unfortunately, the only apparent solution rests with Mother Nature – an end to the drought and more days of freezing temperatures.

There is not much that Californians residents, including their governor, can do.

If the loss of California’s pines and firs isn’t bad enough, millions of oak and tanoak trees have been killed by a pathogen spreading along the state’s coasts.

One official with the Yosemite Conservancy told us that in five years, he expects the state’s forests to look very different.

Kay and David Scott are authors of “Complete Guide to the National Park Lodges” (Globe Pequot). Visit them at www.valdosta.edu/~dlscott/Scott