How to Beat Evergreen Borer Beetles Before They Attack Your Property

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Spring is nearly here, and with it comes the inevitable spread of pest infestation and tree disease. Evergreen Borer Beetles are on the rise in Denver, attacking some of your favorite pine and spruce trees. Follow our guide to learn more about the dangers these pests pose to your trees and how to prevent an infestation on your property.

Types of Evergreen Borer Beetles Local to Denver

The first step to fighting off Denver’s local evergreen boring beetles is to know how to identify them. Read up with fast facts on each from the experts at Colorado State University Extension, who are trying to spread awareness among Denver residents. 

Mountain Pine Beetle

Historically one of the most dangerous insects to trees on Denver’s front range, the Mountain Pine Beetle’s impact has lessened only slightly over the past 5-8 years. Although their threat is often overshadowed by the IPS beetle, Mountain Pine Beetles should not be underestimated. They can be found in every corner of the state, lurking in the background,  and large outbreaks of Mountain Pine Beetles are difficult to control


  • Mountain Pine Beetles are just  ¼ of an inch long
  • Their tiny white larvae are the size of a grain of rice
  • Adult beetles are rusty brown or black in color

Fast Facts:

  • Native Colorado bark beetles that predominately infest ponderosa pine, lodgepole pine, and limber pine
  • Adults typically fly to seek new hosts in July through August
  • MPB complete a generation within one year from egg to adult
  • Attacking females initially prefer large-diameter trees until they are depleted from the forest

Pine Sawyer Beetle

This evergreen boring beetle is the primary carrier of the highly damaging Pine Wilt Disease. They are not very picky and will seek out and attack all sorts of Denver evergreen trees.


  • Adult beetles can grow to be 14-24 mm in length
  • Pine Sawyer Beetles are black or brownish grey with white markings
  • Spines on the exterior form a sort of armor
  • Many Pine Sawyer Beetle varieties have horns

Fast Facts:

  • Pine Sawyer Beetle is notorious for spreading Pine Wilt disease
  • The Pine Sawyer Beetle is a carrier for the native strain of nematode during the hot summer months
  • This beetle variety loves high altitudes
  • Ponderosa Pines and Douglas Firs are its preferred habitat
  • Beetles feed on the sapwood and heartwood of recently killed trees

White Pine Weevil

These overwhelming pests typically target spruce trees, and unlike most Evergreen Borer Beetles, they are active during winter months. Timing is everything when trying to stop the White Pine Weevil from infesting your trees.


  • Adult beetles have a very distinct snout and are about .2 inches in length
  • The larvae are long, with no legs,  and are yellowish-white in color
  • The beetle’s exterior is brown and scaly with white spots and wings

Fast Facts:

  • Injury by white pine weevil is almost entirely limited to higher elevation areas of the state and foothill areas
  • The standard approach to manage white pine weevil is to spray insecticides in spring
  • A closely related species, Pissodes terminalis, causes similar injury to lodgepole pine in Colorado. This insect is sometimes called the lodgepole pine terminal weevil

Zimmerman Pine Moth

The Zimmerman Pine Month does not discriminate between Denver evergreen species but deals serious damage to all trees in its path.


  • Larvae are approximately an inch long and are an off-white color
  • Adult moths are medium-sized
  • Adults have grey and reddish wings with a zigzag design 
  • Adult moths are rarely sighted

Fast Facts:

  • Caterpillars of the Zimmerman pine moth damage pine trees by tunneling into the trunks and branches, often causing branches to break
  • Scotch and Austrian pines are particularly susceptible to injury
  • Evidence of injury are pale yellow, popcorn-like masses of sap that develop at in response to feeding wounds
  • Preventive insecticide sprays are effective when applied during periods when the caterpillars are exposed on the surface of the tree

Denver Trees Most Likely to Be Affected by Evergreen Beetle Borers

Evergreen Borer Beetles have a preference for a handful of Denver’s most lush and ornamental pine and spruce varieties. Worried about your trees but not quite sure which species are on your property? Below is a list of the pests’ absolute favorites, with physical descriptions from the National Parks and Forests Services. Read up and see if you can identify any of these varieties in your yard or neighborhood. You’ll want to keep a vigilant eye on them!

Ponderosa Pine 

With tall, straight, thick trunks, textured in scaled, rusty-orange bark that has split into big plates, the Ponderosa Pine is pretty easy to spot. Think you found one? Sniff around, their bark smells like vanilla or butterscotch! Examine and feel the needles. They are approximately  4-8 inches long and should feel thick and flexible. Typically there are three to a bundle, and the needles hang gently from their branches.

Austrian Pine

The first thing you’ll notice about the Austrian Pine is its shape. Young trees are pyramidal in shape, but become oval with age and, on some sites, flat-topped. Its rich green needles are dark and eye-catching. They contrast most of the other foliage around them.  Austrian pine needles feel stiff to the touch. They have very little bend are usually 2 to 4 inches long growing in groups of 2. 


Most fir and pine trees have downturned branches. In contrast, you’ll notice right away that  Spruce trees have upturned branches. They also grow in a uniquely full and bushy shape. Another identifying feature is its vibrant and sought-after blue-green color.

Scotch Pine

Scotch Pines can range from bluish-green to medium green to yellow-green in color. They feature relatively short needles that are firm to the touch and grow in bundles of two. The needles stand out because of their strange twisted shape and bluish color. The entire tree itself is known to grow at contorted or odd angles.

Even with a handy guide, tree identification can be tricky. Our friendly and expert arborists can conduct a quick consultation on your property to identify your tree species and inspect for any signs of beetle infestation!

Signs and Symptoms of Evergreen Borer Beetles

Now that you can identify some of the Evergreen Borer Beetles’ favorite trees, it’s important to recognize the first signs and symptoms of infestation. If caught very early on, much of the damage these pests create can be mitigated. Colorado State University Extension offers these warning signs and symptoms that you should be aware of, especially with spring fast approaching!

Mountain Pine Beetle

  • Needles on infested trees typically turn ‘rust’ colored 
  • Needles typically drop from branches the second summer after the tree has been infested
  • Boring dust in bark crevices and on the ground immediately adjacent to the tree base is also a sign of bark beetles.
  • Popcorn-shaped masses of resin called “pitch tubes,” are caused by beetle tunneling and often found on the trunk 
  • Pitch tubes may be brown, pink or white 
  • Larvae on infested trees attract woodpeckers
  • Check for woodpecker damage: stripped portions of the bark and accumulations of bark at the base of trees 
  • Small exit holes on the bark surface may be seen after the adult beetles emerge from infested trees

Pine Sawyer Beetle and Pine Wilt Disease

  • Infection and colonization of pines by the PWN start in June or July but observable symptoms don’t normally appear until late summer or fall
  • Needles to initially turn grayish turn from gray-green to tan and eventually brown due to lack of water 
  • Dead needles remain attached to the tree through the winter 
  • On Scots pine, the entire tree usually wilts and dies within a few months
  • Wilting on Austrian pines may at first be restricted to a portion of the tree and the wilting process may be prolonged
  • When cut, diseased wood is very dry to the touch
  • Trees killed by pine wilt also develop blue stain in the wood

White Pine Weevils

  • The white pine weevil causes visible injury in late spring
  • Visible signs include new growth wilts and dieback
  • The affected shoots wilt into a ‘shepherd’s crook’ form 
  • Needles often become lighter-colored before they ultimately die and drop
  • Affected trees have stunted growth and develop a bushy appearance with multiple leaders

Zimmerman Pine Moth

  • Damage is caused by injuries made when the developing caterpillars tunnel into the trunk and branches
  • Tunneling produces irregular gouging wounds under the bark. 
  • Popcorn-like masses of sap ooze and collect on the surface where these wounds occur 
  • Sap is soft and can be easily pulled away from a recent injury
  • Underneath an injury, you’ll find the tunnel of the feeding caterpillar.  
  • Damage usually occurs in the upper part of the tree
  • Feeding is often concentrated in the area where branches meet the trunk.  
  • Injury may result in limbs dying or breaking from the tree

If you observe any of these warning signs it is critical that you reach out to our tree health experts ASAP. The life of your tree could depend on it!

Get An Accurate Diagnosis From An Arborist You Can Trust

With more information than ever online and at our fingertips, we’ve become accustomed to diagnosing all kinds of things – our failing AC unit, a hot water problem, even the rash that developed from yesterday’s yard work.

But tree health is not something that the average homeowner can diagnose on their own. In fact, timing is everything and if you waste precious time and misdiagnose your tree it can mean the difference between life and death for that plant.

Let our expert arborists evaluate your trees with a trained eye and make an educated recommendation for an effective treatment plan!

Preventative Action Denver Homeowners Can Take 

Effectively treating a tree with an established borer beetle infestation is incredibly difficult and in many cases, the tree can never be fully restored to its original healthy condition.

The best course of action is to safeguard tree species that the beetles are attracted to. Preventative measures include:

Soil injection 

Soil injection promotes overall tree health and limit stress drought. Beetles are attracted to stressed trees, but strong well-cared-for trees can effectively fight off an attack by pests. Fielding Tree and Shrub Care recommends repeating soil injection treatment twice per year, in the Spring and Fall. 

Regular Watering and Fertilization

Beetles are drawn to trees that are already injured or drying. One of the best ways to keep your trees safe is to schedule routine watering and fertilization throughout the year. Our experts can help recommend appropriate watering levels and the right combination of fertilizer to keep your trees happy and healthy.

Treatment For Infected Trees 

  • Trunk injection: insecticide trunk injection every 2 years is recommended to treat trees that have an established infestation. Trunk injection can repel the beetles and prevent further harm to the tree.
  • Insecticide spray: An annual application of insecticide spray is a great treatment option depending on the size and condition of the tree. If the beetles are only just arriving and are topical, the spray can be very effective.
  • Tree removal: in dire cases, if an infestation is well-established, the tree must be removed. This is for safety of all your other trees and your neighborhood. Beetles spread rapidly from one tree to the next. If their threat cannot be neutralized, the tree should be removed ASAP to prevent further spread of infestation and disease.

Get ahead of the Evergreen Borer Beetles this spring. Team up with the expert arborists to protect your trees and fight off infestation before it begins. Start by scheduling your complimentary on-site inspection with one of our certified arborists!