Bark Beetles

By the end of spring in 2016, there were 66 million dead trees in California, due to drought and bark beetles. In 2014, that number was only 3 million. By 2017, that number had reached 107 million trees.

Bark beetles (United States Department of Agriculture)

Bark beetles are a normal part of forest and tree life. Healthy trees generally protect themselves from bark beetles by producing sap that pushes the beetles out. However, when trees become water stressed due to years of drought, they are weakened and unable to fight back. Many residential trees are facing the same water stress due to reduced irrigation, making them susceptible to attack by bark beetles. According to the USDA, bark beetles kill more trees than wildfire in most years. Trees are a big investment and weakened trees can destroy homes and threaten the safety of your family. Find out how to prevent bark beetle infestations to protect your trees.


Bark beetle species
 
There are over 600 species of bark beetle in North America. Bark beetles are smaller than a grain of rice. They can be red, brown, or black. With a magnifying lens, you can see that their antennae are jointed, with a club shape at the end. Larvae are tiny, off-white grubs that may have a brown head.


Damage caused by bark beetles

Normally, bark beetles attack redwoods and other conifers. But they are expanding their tastes to include California buckeye, English laurel, and fruit trees. When new adults emerge, they leave a buckshot pattern on the bark surface of infested branches or trunks. Bark beetles chew tunnels through the inner bark, damaging the phloem and the cambium layer, robbing the tree of valuable nutrients and moisture. 


Bark beetles are also vectors for disease. As bark beetle larvae feed, they also infect the tree with a fungus that slows sap production. As they move from one tree to the next, bark beetles expand their own infestation along with whatever disease they may be carrying. Bark beetles are responsible for spreading the devastating Dutch elm disease. Bark beetles are attracted to open tree wounds, such as those that occur when pruning.


Identifying bark beetle infestations

Different species of bark beetle attack different areas of their host trees. Whichever species is active, boring dust mixed with sap is usually the first visible sign. When infestations are suspected, peel off a portion of bark near a boring hole. If bark beetles are present, a winding series of tunnels will be found. Eggs may be present in some of the galleries. Many tunnels will be filled with frass (bug poop). Dead or damaged wood may also be seen. Another common sign of bark beetle infestation is called ‘flagging’, which means the ends of twigs die.


Bark beetle management

Drought-weakened trees that become infested with bark beetles are usually dead within 3-4 weeks. Since bark beetles are protected under a tree’s bark, pesticides and insecticides merely kill off the beetle’s natural enemies, making matters worse. 


To prevent bark beetle infestations, it is necessary to keep trees healthy enough to protect themselves. Proper irrigation is critical. Proper irrigation means soaking the soil to a depth of 2-3 feet at a rate appropriate to the size, age, and variety of tree. You can learn more at the UC Page on Irrigation. These tips can also help:

• Plant resistant varieties of trees.
• Ensure that trees are not planted too close together.
• Keep lawns out from under a tree’s drip line (imagine the line of outer leaves creating an umbrella).
• Remove dead trees and shrubs to eliminate breeding grounds.
• Avoid pruning water stressed trees.
• Remove and dispose of any infested trees to avoid cross-contamination of nearby trees.
• Avoid damaging tree roots with digging or rototilling.
• Preventive spraying of valuable, uninfested trees by professional arborists may provide some protection.