Get Your Evergreen Trees Ready for Tree Insects Common to the Denver Area

Today, many homeowners try to fix, treat, and diagnose many things using the internet, whether it is a pulled muscle, an appliance, or a lawnmower. But the management of the Evergreen tree insects discussed below is complex because it depends on the insect’s lifecycle and the number of products available for control. Also, many of the treatments start at the crown of a tree. Most homeowners do not have the equipment to apply a control product on top of a four-story tall Pine or Spruce tree. So unless you are an entomologist or have a bucket truck, why waste time and money trying to figure it yourself.

Ross Tree’s motto is Integrity in Action, which means we’re committed to minimizing the impact on the environment when using products to control tree insects. We are a pioneer in tree injection and a go-to-company for Plant Health Care in the Denver area. Ross Tree has three Qualified Supervisors (QS) licensed by the Colorado Department of Agriculture to handle pesticide products. We promise only to use what is necessary to treat your trees and nothing more.

According to our three ISA Certified Arborists, the best strategy for many of these insects is prevention. A hardy tree is likely to withstand the onslaught of many of the insects discussed in this Handbook. The following practical tree practices will keep yard trees healthy:

  • Thin out any overgrown Pine, Spruce, or Fir thickets
  • Hire a tree service company to prune high-value trees to promote healthy growth.
  • Plant native trees with natural resistance to many of these tree insects.
  • Add Deciduous trees to the landscape to increase tree diversity.

We used the Colorado State Extension Service, the Colorado State Forest Service, and in house expertise at Ross Tree Company to create this Handbook. Below is a survey of nine tree insects that affect Evergreens in the Denver area. They are:

  1. Ips Beetle
  2. Pine Sawyer Beetle
  3. Mountain Pine Beetle (MPB)
  4. Douglas Fir Tussock
  5. Ponderosa Pine Sawfly
  6. Spruce Beetles
  7. Pine Needle Scale
  8. Striped Pine Scale
  9. Zimmerman Pine Moth


Ips Beetle

ips beetle

Ips Beetles are bark beetles that develop under the bark of Pine and Spruce trees. They damage the trees by producing tunnels that girdle the tree. Eleven species of Ips Beetles reside in Colorado. Ips beetles attack stressed trees. However, they will feed on healthy Pines and Spruces during outbreaks or droughts. Small round exit holes in the bark of infested trees indicate the beetles have completed their development and left to find new trees to attack.

Just the Facts

  • Ips are also called engraver beetles.
  • Ips beetles are 1/8” to 3/8” reddish-brown to black beetles.
  • Several generations of Ips can grow each summer.
  • Ips Beetles possess a unique cavity at the rear end with three to six pairs of tooth-like spines, depending on the species.

Ips Beetle Management

Tree pruning and thinning that promotes vigorous growth is the first step in controlling Ips Beetles. Newly transplanted trees, trees suffering root injuries from construction, and trees exposed to large Ips infestations are at risk and benefit from preventive insecticide applications. The choice of control products for large outbreaks is extensive and situational. It is best to contact a professional tree service for treatment.


Pine Sawyer Beetle

sawyers beetle hostThe Pinewood Nematode (PWN), Bursaphelenchus xylophilus, is native to North America and is found in stressed or dying pines where it feeds on Blue Stain Fungi. The nematode cannot travel outside of the tree independently, so it uses the Pine Sawyer Beetle as its host or vector. PWN spreads when the Sawyer Beetle mates and lays eggs on trees or recently cut logs. After hatching, the beetle larvae tunnel into the wood and pupate. If the tree is infected, the microscopic nematodes invade the thoracic spiracles and tracheae of young beetles. When the Sawyer Beetles start to feed, it injects the PWN into trees. Once infected, the PWN grows exponentially, stopping the flow of resin and causing wilt. PWN infection starts in June or July with observable symptoms appearing later in the summer or fall.

Just the Facts
pinewood nematode

  • The large Spotted Pine and White Spotted Sawyer Beetles transmit the pinewood nematode in Colorado.
  • Non-native Scots, Austrian and Mugo Pines are susceptible to PWN
  • In the summer, Pine Wilt causes rapid change in needle color from green to gray-green to brown or tan.
  • On Scots pine, the entire tree usually wilts and dies within months of infection.
  • Dead needles remain attached to the tree through the winter.
  • The wood in affected trees is dry, brittle and stained blue.
  • Newly planted Pines or trees stressed by root damage or drought make are susceptible.

Pine Wilt Management

All trees killed by Pine Wilt and any downed logs on the property need to removed before the emergence of Sawyers Beetles in late May to reduce breeding opportunities. Also, do not stack pine firewood near Scots, Austrian, and Mugo Pines since they are highly susceptible to PWN. Tree service companies treated PWN by tree injection.


Mountain Pine Beetle (MPB)

mountain pine beetle MPBIn some areas in the Rockies, the damage caused by the Mountain Pine Beetle, Dendroctonus ponderosae, is as far as the eye can see. The rust-colored trees are called “beetle kill.” MPB lives in Ponderosa, Lodgepole, Scotch, and Limber Pine. Outbreaks sometimes reach from wilderness areas into mountain subdivisions and city back yards. Natural controls of MPB are woodpeckers, Clerid beetles, and extremely cold weather. Many Coloradans wish for one or two frigid winters to control this pest. The best approach to MPB treatment is prevention because once infected; nothing can save the tree.

Just the Facts:

  • Adult Mountain Pine Beetles are between 1/8” to 1/3” long.
  • Their white larvae are about the same size as the adult.
  • Adult beetles are brown or black.
  • Needles on infected trees turn rust-colored.
  • Needles drop from branches the second summer after infection.
  • Look for woodpeckers around affected trees, since they tear off bark to feed on the beetles.
  • Look for resin masses or pitch tubes on the side of trees.

Mountain Pine Beetle Management

Mountain Pine Beetles breed well in old and dense forests. So, thinning out overcrowded Pines and planting other types of trees creates diversity and less opportunity for the MPB. For prevention, sprays are applied to high-value Pines in the early summer to kill or deter the attacking beetles. Selecting and using the right control product at the right time is complicated and best left to a professional tree service company with a competent Plant Health Care department.

Douglas Fir Tussock

Douglas Fir Tussock caterpillar

The Douglas Fir Tussock moth, Orgyia pseudotsugata, is native to Colorado. These fuzzy caterpillars devastate Fir and Spruce trees. Outbreaks tend to be abrupt, causing widespread tree defoliation. The larvae start munching at the top and work their way down each tree, looking for food. Subsequent attacks on individual trees will eventually kill them. After their eggs hatch in late May or early June, these highly mobile caterpillars spread out attacking adjacent trees.

Just the Facts

  • During outbreaks, the larvae strip Spruce and Fir trees of all of their foliage in a single season.
  • Females of the Douglas-fir tussock moth (top) are wingless.
  • Males are winged.
  • The caterpillars at hatch are tiny with long thin hairs and can be dispersed by wind.
  • Full-grown caterpillars leave the host plant and pupate in the near vicinity, attaching cocoons and, later, laying eggs nearby.

Douglas Fir Tussock Management

Proper tree thinning promotes healthy growth, which helps trees withstand the moth and its larvae. The choice of control products for large outbreaks is extensive and situational. Since treatment starts at the crown of trees, it is best to contact a professional tree service company with the right equipment and expertise to treat for Douglas Fir Tussock.


Ponderosa Pine Sawfly

ponderosa pine sawfly

The Pine Sawfly, Neodiprion autumnalis, made its presence known in Colorado by defoliating Ponderosa Pine forests in Elbert County. The small wasps emerge in the fall and deposit eggs on pine needles to overwinter. Pine Sawfly infestations are hard to miss with large populations of larvae feeding on pine needles.

Just the Facts

  • Adults begin to emerge in mid-September when they mate and deposit rows of eggs in slits on pine needles.
  • Defoliation starts at the top of trees with large numbers of larvae eating their way to the base of trees in search of food.
  • Pole-sized Pines growing on dry sites with poor soils tend to be most susceptible.
  • The adult Sawfly flew north into the Denver Metro Area in 2019.

Ponderosa Pine Sawfly Management

Sawfly populations are controlled in nature by weather, disease, parasitic wasps and flies, and cocoon predation. Direct control occurs in early spring when Pine Sawfly eggs begin to hatch, and their larvae start to feed on the pine needles.


Spruce Beetles

spruce beetle denver

Spruce Beetles, Dendroctonus rufipennis, are native bark beetles that infest various Spruce in Colorado. Adults fly to seek new hosts in late May through July, targeting large trees. Needles on infected trees tend to turn a pale yellowish-green color and fall during high winds.

Just the Facts

  • Spruce Beetle flourish in warm dry winters.
  • Their lifecycle is between 2-3 years.
  • The spruce beetle threatens higher-elevation forests of Engelmann spruce.
  • Streams of sap at the base of trees are signs of a recent attack.
  • Boring dust at the base of trees and woodpecker activity indicate beetle presence.

Spruce Beetle Management

Large outbreaks of the spruce bark beetle are challenging to control. Tree management practices such as thinning and removal of distressed trees promote vigor and beetle resistance. The Colorado State Forest Service recommends treating high-value trees near homes, businesses, or recreation sites.


Pine Needle Scale

Pine Needle Scale

The Pine Needle Scale, Chionaspis pinifoliae, feeds on the needles of most species of Pines, Spruce, and Fir trees found in Denver yards. During outbreaks, the White Scale looks like spattered white paint on trees and decreases vigor, causes needle drop and dieback, and increases susceptibility of the affected trees to other insects or disease.

Just the Facts

  • The white female Pine Need Scale is about 1/8” long.
  • Scale eggs winter underneath the body of the female on a tree needle.
  • During warm winters, mother Scales continues to lay eggs.
  • Newly hatched Scales are called crawlers and start to feed.
  • Once settled, the crawlers molt and produce a golden brown hard scale. At this time, treatment becomes difficult.

Pine Needle Scale Management

Pine Needle Scale has several natural predators which check their spread. For treatment, tree service companies use dormant oils and crawler sprays depending on the season and stage of insect development.


Striped Pine Scale

Striped Pine Scale

The Striped Pine Scale, Toumeyella pini, primarily attacks Scotch Pines in the Denver area. The Scale produces large amounts of honeydew as it feeds, which coats the twigs and needles of affected trees. Striped Pine Scales overwinter as fertilized, immature females attached to the twigs of affected Pine trees. They start laying eggs in May and June. The eggs hatch and crawlers emerge in June.

Just the Facts

  • Adult tortoise-shell colored females are round with a cream-colored stripe down the center of their dorsal surface.
  • A single female lays hundreds of eggs hatching over several weeks.
  • Striped Pine Scale nymphs are generally orange to brown.
  • Males emerge to mate in late summer.
  • There is one generation per year.

Striped Pine Scale Management

House Finches and Yellow-Rumped Warblers feed on Striped Pine Scale adults. However, natural control of this Scale is not effective. For outbreaks, the management of this Scale is similar to those for Pine Needle Scale. Horticultural oils and crawler insecticides are accepted treatments. Also, systemic products successfully treat the Striped Pine Scale.


Zimmerman Pine Moth

zimmerman pine moth control denver

Zimmerman pine moth, Dioryctria zimmermani, is a wood-boring insect found in the Denver area. Austrian and Scotch Pines are particularly susceptible to this insect. Their caterpillars tunnel into the trunk and branches, wounding the tree. The wounds form masses of sap ooze that collect on the bark surface. Pulling off these sap masses exposed the caterpillar tunnels. Damage starts in the upper part of the tree and spreads to where branches meet the trunk. The gray winged adult moths are red-brown and marked with zigzag lines. It is not uncommon for Denver homeowners to open the door of their garage and find hundreds of flying Zimmerman moths in late summer.

Just the Facts

  • Zimmerman Pine Moths overwinter as first stage caterpillars, sheltered in tiny cocoons attached to the bark surface.
  • Between mid-April and early May, the caterpillars emerge and begin to tunnel into the tree.
  • Pale yellow, popcorn-like masses of sap develop at feeding wounds.
  • Zimmerman pine moths live one-year.

Zimmerman Pine Moth Management

Zimmerman Pine Moths are controlled two times during its lifecycle – early spring when the overwinter caterpillars emerge or when after the eggs hatch late in the summer. Product application timing is critical since no insecticides kill the insect after it has tunneled into a tree. Since there are several product options, it is best to contact a professional tree service company to control Zimmerman Pine Moths.

Homeowners who see clusters of larvae on their Pine, Spruce or Fir trees, click here to fill out a service request form to make an appointment with a Ross Tree Plant Health Care Specialist.