An Arborists Glossary of Pests and Diseases

Is protecting your trees and shrubs from pests and diseases that quietly deplete your plants’ resources, and cause irreversible damage, on your radar? It’s okay if not, our arborists are here to help you identify and prevent these things so that your landscape stays as healthy as can be. In this post you’ll read about varying pests and diseases to gain knowledge on what they look like, and the damage they can cause.

Use the menu below to jump to a specific tree pest or disease:

Branded Ash Borer Beetle
Chlorosis
Douglas-Fir Tussock Moth
Elm Leafminer and Elm Beetles
Emerald Ash Borer
Evergreen Boring Pests
Fire Blight
IPS Beetles
Japanese Beetles
Thyronectria Canker & Honey Locust Borer’s

Banded Ash Borer Beetle

Banded Ash Borer Beetles are native to the forests of North America. This wood-boring beetle does harm to stressed trees by tunneling under the bark, deep into the tree where it cuts off access to nutrients and weakens limbs. If you are seeing these guys on your trees, they are generally a secondary pest and a sign that a tree is in severe decline.

Visually, it’s pretty easy to distinguish these pests from others. The adult Banded Ash Borer is dark gray or black in color, covered in fine hairs, with distinct white or yellow looped markings on its abdomen. Their cylindrical, elongated bodies ranging from 8–18 mm. Known as roundheaded borers, Banded Ash Borer larvae are off-white with a dark, round head capsule.

Understanding the banded ash borer’s life cycle is important to proactive treatment and protecting your trees and shrubs. The Banded Ash Borer’s life cycle and damage timeline:

Adults survive the cold weather months by overwintering under tree bark
Adults emerge in spring, reproduce, and lay fresh eggs on the bark
Larvae begin by feeding under the bark in the phloem and later in the sapwood
Larvae pupate and develop in the fall
Larvae are the true damaging stage, primarily in late spring and early summer
Banded Ash Borers produce one generation per year

Damage These Beetles Can Cause

Not all trees are subjected to the devastating attacks by Banded Ash Borer Beetles. In Denver, we’re seeing Ash, Oaks, Elms, and Lindens as the species being primarily targeted. Stressed trees and newly planted trees are at especially high risk for attack because they do not have the healthy system needed to fend off pests. In addition to these types of trees, newly-cut, green logs with the bark still attached are very attractive to egg-laying Banded Ash Borer females.

Here’s a brief overview from Virginia State University Extension:

Larval tunneling in standing trees may weaken limbs, making them more susceptible to breaking in high winds.
Economic damage by Banded Ash Borer larvae is largely limited to hardwood intended for lumber or firewood.
Adult Banded Ash Borer Beetles may be found in homes after emerging from firewood brought inside during the winter.

In a worst-case scenario, unhealthy trees can be outright killed by the Banded Ash Borer in a single season. More often than not though, these pests do structural damage that weakens and kills individual limbs making them an unsafe liability on your property.

Arborists Preventative Insecticide Treatments

When it comes to preventative treatment, spring is the ideal time of year to apply insecticide treatment for Banded Ash Borer protection. The timing of the insecticide coincides perfectly with the life cycle of the Banded Ash Borer Beetle.

Insecticides that can effectively control Banded Ash Borers fall into four categories:

Systemic insecticides that are applied as soil injections or drenches
Systemic insecticides applied as trunk injections
Systemic insecticides applied as lower trunk sprays

Choosing to act in Spring can successfully control the adults before any new eggs or larvae are produced. Once summer sets in it will be too late!

Chlorosis

Chlorosis simply means lack of chlorophyll due to a deficiency of one or more of these key nutrients: nitrogen, magnesium, and iron. In Colorado, the alkaline soil is very high in pH levels, which makes most of the iron in it insoluble (i.e. not usable by plants). For this reason, trees in the front range suffer specifically from iron chlorosis due to the deficiency in iron that hinders them from properly creating chlorophyll. Lack of iron in a tree may be due to a high iron need, less effective iron uptake, or insufficient usable iron in the soil.

How Does Chlorosis Affect Trees?

According to the Colorado State University Extension, iron chlorosis can affect trees in a variety of ways. First, it typically makes a leaf become yellow while the veins of the leaf remain green. The newest leaves on the ends of branches seem to be the most affected by this condition. As the tree advances into a more severe state, all the plant’s leaves become affected. In dire cases, iron chlorosis will cause plant tissue to die. This results in brown and scorched-looking areas on the leaves.

Can a tree die from iron chlorosis? In short, yes. However, arborists say that this is the worst-case scenario and happens only in the event that you take no action to save the tree. Iron chlorosis would affect the tree in the following progression:

Individual leaves will appear white, then turn brown
Whole branches will suffer from dieback
The entire tree will die after several years

The disease causes the tree leaves to lose their green pigmentation, turn white and then become brown and scorched-looking. If left completely unattended for years, trees in advanced stages of iron chlorosis can die.

A more common fate that befalls trees affected by chlorosis is winter injury. Chlorosis weakens trees over vigor and immune system, it is more prone to winter damage and injury from freezing, road salt, and storm winds.

How Arborists Identify Chlorosis

Iron chlorosis is quite common in Denver trees, but luckily very treatable. Sadly, many trees suffer simply because homeowners aren’t aware of the warning signs. Arborists suggest keeping a vigilant eye on the trees and plants you have on your property! Catching chlorosis in the early stages and having arborists help will keep your trees healthy and strong.

Hallmark symptoms of iron chlorosis, as identified by the Master Gardeners at CSU:

Iron chlorosis shows first and more severely on the newer growth at branch tips
Newest leaves towards ends of branches are generally yellowed
Veins of affected leaves remain green but leaves may be smaller than normal
On fruit-bearing trees, fruits may be small with a bitter flavor
In advanced cases, leaf edges become scorched and leaf interiors show dead brown areas as cells die
Leaves may eventually curl, dry up, and fall
Plants become unsightly and grow poorly
Symptoms may only show on a single branch or on one side of a tree

The list above is a great guide from knowledgeable arborists for keeping chlorosis on your radar, but sometimes these symptoms can be similar to those caused by other tree issues. The experts at CSU want you to know that it is easy to confuse chlorosis with the following:

Zinc and manganese deficiencies result in similar leaf symptoms. Iron chlorosis appears first on the younger or terminal leaves. Under severe conditions, it may progress into older and lower leaves. By comparison, zinc and manganese deficiencies typically appear first on older, interior leaves.
Nitrogen deficiency shows a uniform yellowing of the entire leaf (including the veins). Nitrogen deficiency shows first in the older leaves, while iron chlorosis shows first in the newer growth.
Damage from soil sterilants (i.e., Pramitol, Atrazine, Simazine, Ureabor, and Diuron) used to prevent weeds result in similar symptoms. With these weed killers, the leaf tissue along the vein remains green. With iron chlorosis, just the vein itself remains green.
Natural aging of tissues may create similar symptoms in some plants. Root and trunk damage and some virus, phytoplasmas, and vascular wilt diseases may cause similar leaf symptoms.

Denver Trees Affected by Chlorosis

Some Denver trees are more susceptible to iron chlorosis than others. Just as you would be mindful of people with allergies or pre-existing conditions when they are in your care, you should know what kind of trees you are caring for and find out if they are at risk for developing chlorosis. Any of our arborists will tell you that knowledge is the key to working on a successful preventative care plan!

Denver Maple Trees
Front Range Oak Trees
Fruit Trees

The Best Way To Treat Chlorosis

Research by the Colorado State University Extension shows that there is no “one size fits all” best practice when it comes to treating chlorosis. Treatment programs should be customized, taking into consideration environmental factors like plant and soil conditions and having arborists you can call helps you greatly!

Lowering the Soil’s pH

Due to the high pH and lime content of many Colorado soils, this approach is seldom effective and therefore rarely utilized.

Soil Iron Treatments

This approach applies a mixture of equal amounts of iron sulfate and sulfur to the soil over a period of several months to a year. Inserting the mixture in holes around the drip line of the tree, where the sulfur reacts to lower soil pH. When effective, this treatment can last up to 3-4 years.

Tree Injections

Professional arborists have trunk injection methods available for treating iron chlorosis on trees greater than 6″ in diameter. Trunk injections may last from one to five years and application methods vary depending on the specific product.

Expert arborists are your best bet to determine and execute the most effective method for treating chlorosis. Haphazard application of iron sulfate foliar spray products can stain and ruin your home exterior and sidewalks.

Douglas-Fir Tussock Moth

Homeowners around Denver are getting to know the Douglas-Fir Tussock moth, and it’s not a good introduction. Spruces, true firs, and Douglas-firs are dying around Denver as this little moth is making its rounds. Who is the Douglas-fir Tussock moth and how can we stop its destruction?

What Do Tussock Moths Do?

The caterpillars of the Douglas-fir Tussock moth are known for feeding on the needles of spruces, true firs, and Douglas-firs. (Colorado State University Extension) Their feeding typically begins at the tops of these trees with the blue spruce being its favorite target along the Front Range. More mature caterpillars can cause significant damage to a tree over the course of a growing season, weakening it to the point of no return.

The good thing is that this moth has natural enemies. Several types of wasps and tachinid fly are known predators who love the taste of Tussock moth. Of course, spiders are a natural predator as well, particularly wolf spiders. Lastly, birds of all kinds find Tussock moths to be a fine meal among the fir trees. All of these natural enemies pose a great role in limiting Tussock moth populations.

Arborists’ Advice on How to Manage and Prevent

There is no known cure for existing damage due to Tussock moths. However, there are some good preventative measures you can take to protect your trees from future infestation:

Young larvae are more easily contained than more mature caterpillars. Mid- to late May are the best times to spray right after the eggs hatch with new Tussock moth larvae.
Inspect your trees for egg masses in early spring. This will give you an idea of which trees are at risk. Make note of any vulnerable trees within the eggs’ vicinity.

If your property has spruces, true firs, or Douglas-firs, they may be the next victim of the Tussock moth!

Elm Leafminer and Elm Beetles

The Elm leafminer and Elm leaf beetle have been causing damage to Colorado elm trees for several years now. Elm leaf beetles are most commonly found on American elms, but can also be found on Siberian elms. A few key warning signs are off-color leaves, skeletonized leaves, and chewed or notched edges on leaves.

Elm Leafminer Characteristics

The Elm leafminer is a small, greenish-yellow fly that lays its eggs on the undersides of elm leaves. The larvae tunnel into the leaves, causing them to turn yellow and eventually brown. The damage from the mines is usually not serious but can make trees more susceptible to other problems.

Here are just a few important Elm leafminer facts to know from the Colorado University Extension:

Leafminers are insects that feed within a leaf, producing large blotches or meandering tunnels.
Although leafminer injuries are conspicuous, most leafminers produce injuries that have minor direct effects on plant health.
Most leafminers have several natural controls and can be further controlled with proactive actions taken by homeowners.
Insecticides applied when leafminers lay eggs are useful for the control of many leafminers.

Elm Beetle Characteristics

Elm leaf beetles are slightly larger than a grain of rice. The adult beetle is yellow with black stripes running lengthwise along the body. Underneath the wing covers are two large black spots. Where do they hide out? Adult elm leaf beetles overwinter in protected places like inside homes or beneath tree bark.

These pests do damage to trees by skeletonizing the leaves. This means that they eat the leaf tissue between the veins, leaving only the network of veins behind. Heavily infested elm leaves appear brown and dry and may drop from the tree prematurely. Luckily, Elm leaf beetles can be controlled with insecticides applied when egg masses are first noticed in spring.

Check out these fast facts on the Elm Leaf Beetle from the U.S. Forest Service:

Elm leaf beetle feeds upon all elms, particularly Siberian and English, as well as American, rock, slippery, and other native, Asian, and European species of elm.
Adults emerge from overwintering sites (usually beneath the bark of elm logs or stumps) in late April or early May and begin feeding on elm leaves.
Elm leaf beetles have from one to three generations per year, depending upon weather and location.
Elm leaf beetle feeding can greatly affect the appearance of elms, rendering them unsightly, and can reduce growth and weaken them, particularly when large beetle populations persist for several years.
Effective applications against the first generation generally suppress populations sufficiently such that a second application is not necessary. In addition, soil systemic application of appropriate insecticide can also provide effective control.

The Dangers of Infestations

Trees heavily infested with Elm leaf beetle or Elm leafminer completely defoliate by midsummer. Repeated defoliation can stress trees and make them more susceptible to other pests, diseases, or environmental problems.

Don’t forget that Denver has very dry conditions in winter. Trees that are already stressed from drought, poor drainage, compacted soils, or other factors have difficulty fending off more serious diseases and attacks from other pests. If a tree is completely defoliated more than once, it may not recover and will likely die.

All serious tree damage begins in small incremental ways. Don’t make the mistake of writing off the Elm leafminer and Elm leaf beetle as harmless. When infestations go ignored or untreated, the real damage is done.

Preventative Treatments from Arborists

The best defense against Elm leaf beetles and Elm leafminers is a healthy tree. To prevent elm leaf miner infestation in your Colorado trees:

Select resistant varieties of elm trees when planting.
Prune and destroy infested branches as soon as possible after discovery to limit the spread of the disease.
Avoid wounding the tree bark as this provides an entry point for the fungus.

Maintain the health of your trees by watering during dry periods, fertilizing as needed, and mulching around the base of the tree. Trees should be treated for Elm leafminers and Elm leaf beetles in the spring before they emerge from their cocoons. April is the best month to treat your trees in Denver. If you wait until May or June, it will be too late – the elm leaf miners will have already emerged causing damage to your trees.

Emerald Ash Borer

The Emerald Ash Borer beetle has caused devastation to ash trees in many states, and Colorado is no exception. The dry conditions this year have made the trees more susceptible to EAB infestation, so it is important to take preventative measures now. There are several things our arborists say you can do to protect your ash trees from the Emerald Ash Borer beetle.

How EAB Spreads

The EAB spreads by flying from tree to tree and by hitching a ride on infested firewood, nursery stock, or other objects that come into contact with the trees.

They can travel quite a distance in a short time period. EAB adults can fly at least 1/2 mile from the ash tree where they emerge and can fly as far as 12 miles from the nearest infested ash tree.

Their life cycle also contributes to the rapid spread of the infestation. The female EAB lays about 60 to 100 eggs during her lifespan. She lays her eggs in the cracks and crevices of ash bark, and in 7-10 days the larvae hatch and then tunnel into the tree to feed on the inner bark. This feeding disrupts the tree’s ability to transport water and nutrients, eventually killing the tree.

Evergreen Boring Pests

With Spring 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. Read to learn more about the dangers these pests pose to your trees and tips from our arborists on how to prevent an infestation.

Types of Pine Beetles

The first step in preventing these beetle attacks is to know what kinds of beetles to be on the lookout for.

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. Don’t underestimate Mountain Pine Beetles, as you can find them in every corner of the state lurking in the background. Arborists will tell you that large outbreaks are difficult to control.

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.

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.

Zimmerman Pine Moth

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

Evergreen Borer Beetles have a preference for a handful of Denver’s most lush and ornamental pine and spruce varieties. Here is a list of these trees from our arborists:

Ponderosa Pine
Austrian Pine
Spruce
Scotch Pine

Preventative Tips from Expert Arborists

A tree with an established borer beetle infestation is difficult to treat. In many cases, the tree can never be fully restored to its original healthy condition.

Safeguarding tree species that the beetles are attracted to is the best course of action. Preventative tips from expert arborists include:

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 arborists can help recommend appropriate watering levels and the right combination of fertilizer to keep your trees happy and healthy.

Arborists Treatment For Infected Trees

Trunk injection: insecticide trunk injection every two years is recommended to treat trees that have an established infestation. Trunk injection can repel the beetles and prevent further harm to the tree. This method is often preferred and is strongly recommended for high value trees as a preventative measure.
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 the 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 by professional arborists ASAP to prevent further spread of infestation and disease.

Fire Blight

Fire blight is a destructive bacterial disease. The Erwinia amylovora bacteria attacks leaves, turning them black, causes branches to develop dark cankers, and makes twigs ooze white puss. It gives the affected tree the appearance of being burned, hence the name. Left untreated, fire blight will kill branches and then the entire tree. Fire blight is not harmful to humans, so if you recently ate a sour apple or pear and are worried about infection, you can relax. Like other plant diseases, fire blight only harms plants.

Signs of Fire Blight Infection

Some types of tree disease can creep in almost unnoticed, but fire blight is not one of those. The visual effects are truly astonishing. You will begin to see outward signs of infection at the first petal fall and can include:

Water-soaked blossoms: during blossom blight infected blossoms look as though they are seeping with water and wilt right away. They do not fall, but cling to the tree, turning dark brown.
Brown or blackened leaves: when bacteria have spread through the branch and reached the leaves, they turn crisp and dry. They curl and turn a brownish-black color, looking burnt. They stay attached to the tree.
Dead branches: fire blight cankers kill tree branches. They show up as sunken, dark discolored areas, with a narrow callus ridge along the outer edge. These ridges differentiate fire blight cankers from other fungal tree cankers.
Blackened bark: fire blight affects all parts of the tree, from the ground up. Bark on the trunk of the tree cankers with infection and turns black. Inner bark that is infected may turn green or brown.
Black “shepherd’s crook” twigs: first, the bark at the base of blighted twigs becomes water soaked, then dark, sunken and dry. Infected twigs often become bent, resembling a shepherd’s crook.
Dried dead fruits: infected fruits get lesions first, then dry out. They continue to hang from the tree and do not fall.
Creamy ooze: sticky, creamy colored bacterial ooze is prevalent on my parts of an infected tree. Ooze may appear from cankers on the tree bark or diseased twigs, even on infected fruits.

Rain, wind, and pruning tools will spread fire blight from diseased to healthy trees. The bacteria survive the winter in cankers on already-infected branches. When temperatures warm up in the spring, the bacteria ooze out of the cankers. The sticky substance attracts aphids, ants, bees, beetles, and flies. The insects pick up the bacteria and carry it on their bodies to the blossoms of healthy plants, spreading the disease.

Arborists Preventative Care Options

There is no cure for fire blight. However, you can be proactive and use preventative methods to protect your landscaping. Take control and manage your property responsibly by following these steps outlined by the Colorado State University Extension:

Make sure to plant resistant varieties: Select varieties adapted to your growing area to minimize the stress that may predispose the tree to other disease-causing agents. Local weather conditions from year to year also affect the amount of fire blight found in a variety.
Use the best cultural practices: Minimizing rapid growth and succulent tissue will reduce the risk of fire blight developing on the susceptible young, succulent tissue. Annual pruning with avoidance of major cuts will help minimize tree vigor. Similarly, limiting the amount of nitrogen fertilizer will reduce twig terminal growth.
Plant Growth Inhibitors (PGRs): Another option for minimizing rapid growth is to utilizie PGRs through trunk injections to mitigate the impact of fire blight.
Prune safely: Remove all blighted twigs and cankered branches. Prune twigs and branches 8 to 12 inches below the edge of visible infection. CAUTION! After each cut, sterilize all tools used in pruning. Spreading the blight bacteria risk is lowered if pruning is delayed until mid-winter. To decrease the chance of new infections, hire our arborists to remove from the site and destroy all infected branches.

IPS Beetles

During winter months, preying insects typically disappear from sight and it feels safe to let your guard down and take a break from tree and plant care; but arborists say signs of the IPS beetle may still be present.

What is the IPS Beetle?

Often called an engraver beetle, these dark brown beetles mark and carve the wood of a tree as they feed on its phloem. Hence the name engraver beetle! IPS beetles often go undetected due to their small size. They can be reddish-brownish or black in color.

The growing number of IPS beetles in Denver is concerning. Presently, there are more than eleven species native to the area. The IPS beetle is spreading more rapidly than other pests because it can withstand the cold winter temperatures and are active from early March through November; they mainly attack spruce and pine trees.

How Arborists Identify if Your Tree is Infested

Infected trees display signs of rapid fading early in the spring, during the summer, late summer, or early fall. Fading can be at the top part of the tree or the whole tree.
Reddish boring dust can be found at the base of the tree or on the bark crevices. You can look for the boring dust at any time during the warmer months.
IPS beetles leave a path or gallery under the bark. Galleries made by the adults are very distinct, they have a “Y” or “H” shaped pattern. They are also clear of sawdust, unlike the sawdust-filled galleries of mountain pine beetles.
Woodpecker feeding on the main stem of the tree or on large branches in the canopy is a common sign.
If the whole tree has faded, the insects may have already flown. To determine whether they are still in the tree, peel back an area of bark the size of a deck of cards at eye level on the main trunk. If you see many live insects, the tree is still infested – if you find none, and find many small, circular exit holes on the outer surface of the bark, the insects have already left.

Arborists Suggested Preventative Tips

Effective Monitoring

It is important to properly monitor your property to detect the presence of pests. If you keep a close eye on your pine and spruce trees for the IPS beetle, you can detect and have arborists remove them early on. When their populations are low, they are easy to stop. Each pest lays its eggs or is particularly visible during a certain season.

Catching signs of the IPS Beetle in the winter months means you can handle the problem before spring when the beetle will spread diseases to other trees and plants.

Necessary Watering

Watering trees and plants well will help them to thrive. Receiving the proper amount of water helps plants and trees stay healthy enough to fight off infection and repel attacks made by pests. Trees that experience drought become weak and are more vulnerable to IPS beetle infestation.

Consistent Pruning

Dead leaves clinging to a plant and around its base attract unwanted insects. Don’t give pests an invitation! Mindfully incorporate pruning into your weekly plant care routine. Remove plant litter and observe the overall health of your tree or plant as you prune.

Preventative Trunk injections

Insecticide trunk injection can repel IPS Beetles and prevent further harm to the tree if your tree is already suffering from active infestation. Additionally, this method is often preferred and is strongly recommended as a regular preventative measure for high value trees as a preventative measure.

Japanese Beetles

The warmer weather also brings with it a nasty, little pest determined to feast on our fantastic flora. Even though there are many products on the market designed to help you get rid of grubs, switch up how you care for your lawn and plants as it is the best place to start.

How Arborists Spot an Infestation of Japanese Beetles

These beetles may be a menace, but, thankfully they aren’t very stealthy. There are some basic signs which strongly indicate an infestation:

Take a Closer Look

Adult beetles are easy to spot with the naked eye. If you suspect your plants are under attack, take a look around and see if you can spot any of these winged menaces.

Beetles Buzzing

If you see beetles flying around in your yard or garden, it’s a pretty good indicator you have a Japanese beetle infestation.

Chewed-Up Leaves

Japanese beetles feed on the leaves and soft tissues of plants. If you have beetles, you will most likely see large holes in the leaves of your plants. These beetles love the soft tissue of plants, so only the veins in the leaves and the stems are left behind.

How to Spot Japanese Beetle Grubs

It’s a good idea to try and spot grubs before they evolve into beetles. Thankfully, you do not have to be a super sleuth to spot and remedy an infestation of grubs. Our arborists say these are what you want to look for:

Unusual Brown Patches of Plants and Grass: Japanese beetle grubs destroy the root system of plants and grasses. As a result, affected plants and grasses will turn brown as they die.
Easily Uprooted Grass: As a result of decimated roots, infested patches of grass and plants are easily uprooted. The roots are no longer anchored in the soil.
New Scavenging Animals: Birds and many furry creatures enjoy dining on grubs. Keep an eye out for these types of animals hunting for insects on your lawn or in your garden.

How Arborists Get Rid of These Miniature Menaces

If you don’t have an unusually large infestation, you may be able to pick the beetles off of your plants by hand. If you have a large number of beetles on your property, getting rid of them may take a little more oomph.

Shaking the plant or tree is sometimes all it takes to get rid of the beetles. Place a ‘beetle receptacle’ below the plant or tree in question and firmly give it a shake. The beetles should fall off the leaves and into the receptacle.

Insecticides are possible as a last resort. However, you want to consult with one of our arborists before you begin applying an insecticide. Many of these chemicals have side effects that need consideration.

If you’ve done battle with these pesky beetles before, we have no doubt you have seen Japanese beetle traps. They typically look like large cloth bags with a yellow plastic opening at the top. You can often find these traps full of beetles. So, they do indeed work as a mechanism for trapping these unwanted pests. However, they also draw Japanese beetles into the area. It is for this reason that our arborists recommend resisting the temptation to put one of these in your yard.

Learn more about Japanese Beetles on our website.

Thyronectria Canker & Honey Locust Borer’s

If your Denver treescape includes locust trees, there is a good chance you have come across honey locust borers at some point. Cankers, in general, are a form of tree disease. A ‘canker’ is a visible injury on the outer part of the tree. They come from an open wound that is infected with fungi or bacteria. Thyronectria fungus is the cause of cankers. All types of cankers are destructive and can spread easily, but different tree species are susceptible to different kinds of cankers. Thyronectria cankers present most often on honeylocust trees. Both of these cause significant damage to trees, so it is important to be able to identify them and take steps to have professional arborists remove them if necessary.

Damage From Thyronectria

You might be wondering, what exactly does a canker look like? You don’t need a microscope or binoculars to spot Thyronectria cankers. An injured area typically has a bumpy, cushion-like texture that is light yellow-brown or reddish-brown when fresh, but blackens with time.

Diseased spots are normally located in bark openings in which raised areas of bark function as breathing pores. Patches of the tree trunk where the bark has thinned are also susceptible to cankers as well.

Canker diseases pose a serious threat to your trees and shrubs. When left unattended, Thyronectria cankers can prove to be fatal. When a tree or shrub becomes infected, the cankers deal damage by cutting off the flow of essential nutrients to branches and strangling them. Girdled and deprived of necessary nutrients, individual branches begin to suffer from dieback. If cankers spread to the main trunk of the tree, it is likely that the entire tree will perish.

That sounds like a lot of doom and gloom, but there are symptoms that should raise the alarm long before things ever get that serious! Cankers work slowly and steadily, taking months or even years to develop enough to girdle twigs and branches. To stop Thyronectria cankers in their tracks, arborists say to keep a watchful eye out for these symptoms of the disease:

Reduced foliage
Yellowing and wilting of foliage
Premature fall coloration
Early leaf drop
Yellow-brown or reddish-brown lesions
Bark injuries that are oval and slightly sunken with callus ridges
Bark splits between diseased and the healthy tissue, oozing sap or moisture
Black inner bark that emits a foul odor

How Arborists Treat Thyronectria Cankers

Research by the Colorado State University Extension admits that cankers are difficult to control, and ultimately the best way to treat them is with preventative control that keeps trees and shrubs healthy. Treatment programs should be customized, taking into consideration environmental factors like plant and soil conditions.

The CSU staff recommends the following tips and guidelines for preventing and treating cankers:

Apply a growth regulator to aid with drought tolerance. This keeps the tree healthier and, in turn, more resistant to pests.
Prioritize preventing wounds and promoting tree vigor. Proper watering and fertilization routines in all seasons help boost tree health and strength. In addition, avoid stress caused by improper planting practices, drought, overwatering and insufficient area and oxygen for root growth.
Injuries to the base of a Honey Locust provide an entry point for fungi. Be careful when operating machineries like lawn mowers and weed trimmers, as they commonly cause basal injuries. Injuries to the stem and trunk, such as those caused by squirrel gnawing, pruning, and sunscald, can be minimized by removing loose bark and allowing the wound to air dry.
Planting small trees such as 2 to 9 cm diameter trees (1 to 3 inches) will ensure successful establishment rather than planting large trees >10 cm (>4 inches). Water trees adequately with about 2.5 cm (1 inch) of water per week. Avoid over-watering trees since their roots need oxygen.
Reduce the chance of other infections from Thyronectria cankers with prompt pruning of dead or infected branches and cutting out small cankers on main stems. Prune in cool, dry weather to minimize reinfection.
If cankers are established, remove dead or dying bark and discolored wood ASAP. If the tree appears to be recovering, however, do not cut into healthy tissue. Wound dressings are not recommended.
To stop the spread of infection, disinfect all tools used to prune and cut. Dip in 70 percent rubbing alcohol or a 10 percent bleach solution and dry after each cut.
Have professional arborists remove infected trees that are past the ability to treat, because canker fungi can grow on dead wood and produce spores that can infect nearby trees. Bury removed trees in a landfill or burn them within three weeks of cutting.

Honey Locust trees are so susceptible to Thyronectria canker that expert arborists highly recommend avoiding monoculture planting by using a diversity of trees. This is an important consideration for any Denver home or business owner planning major landscaping projects that involve new tree additions.

Damage and Prevention

Once honey locust beetles have infested your trees, the larvae begin to feed on the inner bark. This feeding disrupts the tree’s ability to transport water and nutrients, which can lead to dieback in the leaves and branches. In severe cases, an infestation can kill a tree.

Here are some signs that a tree will display when infested:

progressive dieback in the canopy
small d-shaped holes in the trunk or branches
knots and swellings on the trunk
wet spots on the bark
sawdust at the base of the tree or on the ground around it
galleries of larval tunnels under the bark

If you think your trees might be infested, take action and contact any of our experienced arborists ASAP. Our certified team of experts can inspect your trees and start a treatment plan to eliminate the pests. The sooner the better! During an infestation, weeks and months can be the difference between life and death for your Denver tree.

The best way to prevent an infestation is to have healthy trees. Honey locust borers are attracted to stressed and unhealthy trees. Proper tree care is key in preventing an infestation.

Here are some tips for keeping your Denver trees healthy:

water them regularly, especially during drought conditions
fertilize them every year
prune and trim dead branches
protect the roots from damage from road salt, construction, or other sources

If you have a honey locust tree, it’s important to be extra vigilant. Check your honey locust tree for signs of borers as a regular part of your lawn and tree care routine. If you notice any, contact a professional immediately.

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