There are killing machines on the loose in California and the entire West Coast.

They're found in packs and look for the weak but they're not the scary predator you'd imagine. In fact, they're about the size of a rice grain.

They're called bark beetles.

A June 2016 Cal Fire report stated the ongoing drought in combination with the tiny insects are responsible for killing about 66 million trees in California since 2010. That's up from 29 million trees in 2015 and 3.3 million in 2014, according to the report.

The Cal Fire report identified 10 counties in California as having the highest hazard of dead trees. The counties spanned from Placer County down through the Central Sierra to Kern County.

But what is a bark beetle and how is this insect able to wreak so much havoc?

Bark beetles are typically black or brown hard-bodied insects.

There are 600 different species of bark beetles in the U.S. according to the United States Department of Agriculture Forest Service.

About 200 species live in California and are native to North America. Each species of bark beetle usually only feeds one one type of tree and stays within one region.

Bark beetles reproduce under the bark of trees. They lay their eggs in the inner bark, or moist phloem, and feed off the inner living tissue which blocks off the tree's ability to transport nutrients.

Once a bark beetle has successfully attacked a tree, it releases a chemical that attracts masses of beetles which makes it easier to spread to other trees. The larvae become adults and the cycle continues, according to Cal Fire.

The parent beetle creates a pattern, or 'gallery', underneath the bark which is unique to each species of bark beetle. After the larvae hatch they create 'larval galleries'.

The USDA Forest Service explains, the tiny pests become a major issue during droughts because the bark beetle looks for weak, dying hosts- such as dehydrated trees- to attack and reproduce in.

A trees' only defense against bark beetles is sap, which is normally produced by a healthy tree, according to Cal Fire. The sap pushes the beetles out, keeping them from laying eggs. A tree needs water to produce sap and during a drought, it becomes difficult for a tree to defend itself from the beetles.

Since the California drought has been ongoing since 2011, the dry climate has created a haven for bark beetles and has resulted in high mortalities for trees. The U.S. Forest Service states, beetles thrive off warm weather and are usually killed off by colder weather, which is another reason why a drought can be so detrimental to trees.

Bark beetles can kill a tree in as little as two to four weeks during warmer weather, according to Cal Fire.

They're particularly a problem for coniferous forests such as those seen in the Sierras. California saw an increase in damage done by bark beetles such as the fir engraver, the Jeffrey pine beetle, and both the western and mountain pine beetle, according to a 2014 USDA aerial survey report.

Under normal circumstances, bark beetles help with population control and are an important part of the ecological system.

Although there is not much that can be done once a tree has been attacked by bark beetles, the USDA Forest Service recommends thinning trees to a wide spacing to avoid attracting mass attacks. The healthiest trees should be left up and weak or dying trees should be cut down. The agency also recommends removing trees already infested by beetles and taking precautions, such as watering trees during a drought.