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Bark Beetles – Tiny Pests with Enormous Harms

Bark beetles are the members of the Scolytinae family which is a subfamily of Curculionidae family. There are around 6000 species of beetles out of which 600 species are bark beetles. California alone is home to more than 200 species of Bark Beetles. They exist worldwide and are usually cylindrical in shape and range around 6 mm in height and are considered destructive. They reproduce in thousands as the ratio of male per female is 1:60, that is; one male mates with 60 females. They are black or brown in color and live inside the bark of a tree. The female constructs an egg chamber where they deposit and protects their eggs. When the eggs are hatched, each larva bore away from the egg chamber, creating a series of galleries or tunnels. These larvae emerge out as fully grown adults at the end of their tunnels and bore out of the tree bark to make their own colonies on other trees.

There are different species of bark beetles and each bark beetle species attacks a specific tree, damages its fruit, seeds, stems, and roots. They are the main reason why certain tree species suffer from deadly diseases. For example, the elm bark beetles are known for spreading fungal disease in trees known as the Dutch Elm Disease. Once the tree is affected by this disease, there is a lesser chance of that tree surviving.

When the bark is removed from the infected trees, one can see the colonies and galleries these beetles have created over time. Common bark beetle species like Dendroctonus and Ips attack pine trees.

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Bark beetles are the common pests of the conifer trees and some of them also attack broadleaf trees as well. California is prone to 20 invasive species of bark beetles. Two recently found invasive species include the redhaired pine bark beetle and the Mediterranean pine engraver. They are known for colonizing numerous Mediterranean pine trees throughout the Central Valley and the Los Angeles Basin.

List of the Invasive Bark Beetle Species and their Attributes:

1) Cypress and Cedar bark beetles

Cypress
bark beetles

Trees of Interest:Redwood, Junipers, False Cypress, Cypress, and Arborvitae

Generations per year: 1 - 2

Attributes: 

  • The galleries and colonies look like a centipede on the wood surface and the inner bark.
  • Adult beetles feed on the twigs.
  • Female beetles are attracted to the trunk of the dying or dead trees for laying eggs.

2) Elm bark beetles


Trees of Interest:Elm Trees

Generations per year: 2

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Attributes:

  • The larvae spend the entire winter feeding the inner bark.
Elm bark beetles
  • Bored holes in the bark dictate damage.
  • The females lay eggs inside the trunks and limbs of weakened or injured elms.
  • Known for spreading Dutch elm disease fungi.

3) Engraver bark beetles

Engraver bark beetles

Trees of Interest:Pine trees

Generations per year: 1 - 5

Attributes:

  • The larvae spend the entire winter feeding the inner bark.
  • Their colonies or galleries are wishbone-shaped.
  • Likes the stem of the pine trees.


4) Fir engraver

Trees of Interest:Red and White Fir

Generations per year: 1 - 2

Attributes:

  • The larvae spend the entire winter feeding the inner bark.
Fir engraver
  • The adult beetles like to dig deep and long tunnels.
  • Their colonies are generally two-armed across the grain of the sapwood

5) Jeffrey pine bark beetle 

Trees of Interest:Jeffrey pine tree

Generations per year: 1 - 2

Attributes:

  • The larvae spend the entire winter feeding the inner bark.
Jeffrey pine bark beetle
  • Prefer the mid trunks of large trees (5 - 30 ft).
  • Their tunnels are J-shaped and long.

6) Mediterranean pine engraver bark beetle


Trees of Interest:Pine tree

Generations p​​​​​​​er year: 3 - 4

Attributes:

  • Likes to feed on the trunks and limbs of large Mediterranean pines, such asAleppo pine and Italian stone pine
Mediterranean pine engraver bark beetle

7) ​​​​Mountain pine bark beetle


Trees of Interest:sugar pine, pine, and 

lodgepole trees

Generations per year: 1 - 2

Attributes:

  • The larvae spend the entire winter feeding the inner bark.
Mountain pine bark beetle
  • Prefer the mid trunks of large trees (5 - 30 ft).
  • Their tunnels are J-shaped and long.

8) Oak ambrosia beetles or Oak bark beetles

Trees of Interest:Oaks, buckeye oak

and tanbark oak

Generations per year: 2 - 4

Attributes:

  • The larvae spend the entire winter feeding the inner bark.
Oak ambrosia beetles or Oak bark beetles
  • Identifying this species is easy. They make frothy, bleeding, and bubbling holes and the bark surface is covered with boring dust.
  • Prefers damaged or stressed out oaks.

9) Redhaired pine bark beetle


Trees of Interest:Pine tree

Generations per year: 2 - ​3

Attributes:

  • Prefer the roots and lower trunk of Canary Island pine and Aleppo pine.
Redhaired pine bark beetle

10) Red turpentine bark beetle

Trees of Interest:Pine trees, larch,

spruce, and white fir

Generations per year: Half - 2

Attributes:

  • The larvae and adult both spend the entire winter feeding the inner bark.
Red turpentine bark beetle
  • Attack the large roots and lowest 2 to 8 ft. of the trunk.
  • Identification – an appearance of pitch tubes on the bark.
  • Never kills the host (tree).  

11) Shothole borer bark beetle

Trees of Interest:Several fruit trees,

hawthorn, English laurel, and other 

woody 

plants

Generations per year: 2 - 4

Attributes:

Shothole borer bark beetle
  • Identification – an appearance of boring dust and gumming of woody parts.
  • Easy to cure – simply remove and destroy affected parts of the tree.

12) Twig bark beetles


Trees of Interest:Pine tree, walnut tree

Generations per year: 2 - 3

Attributes:

  • Are deep diggers, can mine the pitch of the tree.
Twig bark beetles
  • Prefer attacking the twigs and lateral shoots of the tree.
  • Are known for causing pitch canker disease in pine trees.

13) Western pine bark beetle


Trees of Interest:Ponderosa and

Coulter pines

Generations per year: 2 - 4

Attributes:

  • The larvae feed on the inner bark, making tunnels.
Western pine bark beetle
  • Emerges out fully developed at the end of the tunnel.
  • Attack the mid trunk, and spread up inside making galleries and colonies, making the tree weak.

Characteristics of Bark Beetles

  • Bark beetles primarily feed on the inner bark of the tree. They bore into the bark and damages the tree functioning. That is; they break the flow of nutrients from the leaves to other parts of the tree.
  • There are several reasons for bark beetle outbreak. Some of them include dense forest stands, drought, and rocky/shallow soils. Human activities include past forest management practices, fire suppression, urbanization, and past grazing practices.
  • It is documented that the bark beetles can fly up to 2 miles in one go. They target the nearest suitable host to colonize.
  • The stressed-out trees tend to release certain airborne chemicals or volatile compounds. The bark beetles are evolved to an extent that they can easily detect these chemical compounds.
  • Bark beetles get active during spring and are believed to take their first flight around April/May. They can’t stand cold climate and spend the entire winter inside the bark.
  • Bark beetles are the carriers of blue-stain fungi. It is the greatest enemy of wood. However, the wood remains undamaged during the first year of infestation; but it slowly breaks down.
  • The decline rate of bark beetle-infested/killed trees depends on the location, size and the species of the beetle. It is said that the bark beetles can kill a fully grown tree in 3-4 years.     

Identification of Bark Beetles

There are three major groups of bark beetles found in nature. They are;

  • Ips Beetles
  • Black Turpentine Beetles
  • Southern Pine Beetles

Ips Beetles:

Ips beetle species vary in size but they all have the same physical appearance and habits. 

The bark beetles of this species have concave abdomens having 5-6 spines on both sides of concavity. Identifying them is very easy; their abdomen seems to be chopped off. Ips beetles are generally 1/2 inch in diameter. Their attack is limited to 5-8 feet around the basal region of the tree.

Ips Beetles

Black Turpentine Beetles:

Black turpentine beetles are one of the smallest bark beetle species found. 

They are only 1/4-inch-long and as the name suggests, are black in color. They are easy to identify due to their round abdomen. Their colonies are concentrated around the trunk of the tree up to a heigh of 7-10 feet. Their colonies are usually larger than other bark beetle colonies.

Black Turpentine Beetles

Southern Pine Beetles:

The identification of Southern Pine Beetles can be determined by looking at their colonies. 

They carve large S-shaped galleries beneath the bark. Moreover, they are also known as the carrier of blue-stain fungi. Once affected, the fungi completely destroy the wood and there is no way to recover it. They are similar to the size of Black Turpentine Beetles.

Southern Pine Beetles

Life Cycle of Bark Beetles

The bark beetles attack those trees that are injured, diseased or stressed. They lay their eggs under the tree bark and when the larvae are hatched, they further dig deep inside the trunk feeding on the inner bark. The cycle is repeated once the larvae become adults. The bark beetles release pheromones once they have attacked a particular tree. These pheromones help in attracting other beetles of the same species. Once the colony is formed, it becomes difficult for the tree to fend them off and ultimately, the tree dies or gets badly injured.

The bark beetles are active throughout the warm spring and summer and tend to dig deep inside the bark during winters.

The natural way to prevent bark beetle attack is pitch, sap or resin. It is the tree’s only protection against these beetles. The sap is produced by almost every tree. But its quantity will depend on the tree’s health. Healthy trees will produce more sap than a stressed out tree. The sap pushes the bark beetles out. Its sticky and gooey formation makes sure that the beetles are not in there for long. But when a tree is surviving in a drought-stricken area or there is a scarcity of water, then it becomes stressed. Stressed trees are known for not producing ample sap. Which in return, fails to protect itself from bark beetle invasion.

The bark beetles are so dangerous for the flora species that when they are at the epidemic level, they can even destroy a healthy tree.

What are the Indications of Bark Beetle Infestation?

The signs of infestation become pretty clear the moment the beetles attack a tree. Their activity starts from the tree’s bark and extends to its inner surface, twigs, trunks, and branches. The following are the signs that may suggest that a tree is attacked by bark beetles.

In the Needles or Leaves -

  • The leaves or needles of the trees slowly convert to brown from green. That is, its color fades away.
  • The needles on the trees like pines and other conifer trees begin to turn brownish in color. The change starts at the top of the tree and slowly moves down, covering the entire tree.
  • Weaker trees sometimes die within few days or weeks of infestation without showing any signs like a color change or something else. Certain trees can survive for years even after when the infestation is at the epidemic level.

In the Bark -

The best way to find out the bark beetle infestation on a tree is to examine its bark. Most of the trees tend to release pitch or sap whenever something like this happens. This is a defense mechanism followed by healthy trees that prevent the beetles from digging deep inside its bark. If you see reddish-brown or white sap tubes on the surface of the tree’s bark, it means that the tree is invaded by deadly bark beetles.

However, a reddish-brown sap indicates that the tree put up a fight with the beetles but failed. The beetles managed to penetrate deep inside the tree’s bark. On the other hand, if you see white sap; it means that the tree successfully repelled the bark beetles.  

The sap will be surrounded by a sawdust-like material known as frass. The appearance of frass is the indication that the bark beetles and larvae have dug deeper into the bark.

You can also look for frass on the ground or in the tree crevices. For identification, you can look for reddish-brown coffee-colored powdered substance.

If you see the constant visit from woodpeckers on one particular tree, then it means that the tree is infested with some kind of bug. If you see tiny holes or flaky barks, it means that the woodpeckers are foraging for bark beetles.

Removing the flaky barks will reveal small holes created by the beetles.

Controlling Bark Beetle Infestation

Improving the tree vigor is the only way to control bark beetle infestation.  There is nothing more one can do to stop it. The reason is that the bark beetles live beneath the bark, which makes the of insecticides very difficult. One way to control the infestation is the regularly prune the trees and dispose of the affected barks and limbs like burning. In case, if the main trunk is infested; then the entire tree should be chopped down for the sake of the other trees and plantation. Whatever you do, it is necessary that you dispose the affected parts far away from other plantation. Never pile the infested substances adjacent to other trees or shrubs.

One way you can reduce the effect of bark beetles is through cultural control

Tree Selection:

Try to plant only those species of trees that properly adapts to the environment of that particular area. Learn about the tree’s cultural requirements and provide proper care and attention to make sure they are growing vigorously. The bark beetles will refrain from attacking healthy trees as they can produce ample amount of sap.

On the other hand, in the area where for example, red turpentine beetles and engraver beetles are more prone to attack, plant nonhost trees like atlas cedars and redwoods. Moreover, planting mixture of shrub and tree species in planted landscapes ultimately reduce the mortality of the trees.

Biological Control:

The bark beetles are not the apex predators in their food chain. They too live hiding from their enemies. When they attack a tree, their enemies naturally attracted to mating and feeding the beetles. Their natural enemies are divided into two groups – parasites and predators. Predators play a crucial role in regulating the population of bark beetles. The natural enemies will not probably save an infested tree but they can help in controlling the population of the bark beetles.

Natural enemies like woodpeckers and other beetle eating beetles like black-bellied clerid and trogossitid beetle play an important role in keeping the bark beetle population at bay.

Reduce Tree Stress:

It is necessary to pay special attention to slow-growing, old, newly planted or crowded group of trees in a landscape. The transplanted trees or large nursery stock like pines and oaks are highly susceptible to bark beetles after replanting. Transplantation will only work depending on the environmental condition of that particular area, the tree species, and the transplanting site.

The trees will eventually become stressed due to poor transplanting choices like planting during the inappropriate time of the year,  poor planting techniques, lack of care or planting an inappropriate species. Everything will count when it comes to protecting the plantation from getting infested by bark beetles.

Prevention is better than cure:-

  • Make sure that the trees are healthy. The bark beetles prey on injured or stressed trees.
  • Avoid injuries to the trunks and roots
  • Avoid damage to the soil in any manner
  • Protect the newly planted trees from the direct rays of the sun
  • Make sure the trees are getting ample water during the initial stages of their growth
  • Make sure to prune down the dense stands and branches
  • Avoid frequent watering
  • Use high-quality organic fertilizers
  • Plough down the field every once a while, especially around the roots to ensure proper air circulation

Chemical Control:

Using chemical compounds as a defense for bark beetle attack is a waste if the beetles have already penetrated beneath the bark. You will have to focus on the adult beetles as they are the ones that attack the trees. You should be walking a step ahead of the bark beetles. Spaying chemical compounds on the trees once a while will act as a shield. It works in a way that when a colony of bark beetles attacks the tree in an attempt to lay eggs. The chemicals will kill them when they start digging holes inside the tree.

Behavioral Control:

The bark beetles repel, attract and locate mates and other members of the same species by discharging species-specific chemicals (airborne) knowns as pheromones. The pheromones are the chemicals that occur naturally. Scientists have used pheromones as a bait to locate and monitor bark beetle population. The farmers use the same technique to capture the invasive species of bark beetles. The professional foresters lure the bark beetles using pheromones, they capture them in boxes and kill them. This is one of the ways to control the bark beetle population. There are artificial pheromones available especially for bark beetle management. However, it can only be used by trained professionals and one should consult a landscape specialist if he/she is interested in this bark beetle management option.

Natural Enemies of Bark Beetles

The natural enemies of bark beetles are mainly classified under three superordinate groups – parasites, predators, and pathogens.  

Parasites -

Several parasitic wasps (eg: ichneumon wasp), ants and other insects contribute as parasitoids. The larvae of these insects live as parasites and eventually kills the hosts (bark beetles).

They play a significant role in maintaining and re-establishing the natural balance.

Predators -

Mites are the common guests found in every breeding system of bark beetles. They survive eating the cocoons, larvae, and eggs of the bark beetles.

Checkered beetles are known as an important predator for bark beetles. They hunt both the adults and larvae.

Snakeflies are also known to have bark beetles in their food chain. They are the important predators for regulating the bark beetle population.

Several woodpecker species arbitrate in a regulatory manner in the beetle population.

Pathogens -

Fungi and bacteria have a significant influence on the bark beetle population. This is the reason why the bark beetle population under control in hot and humid conditions.

Nematode worms are also known to be found in the bark beetle breeding system. They suck on eggs and larvae.