1. Bark Beetle Life Cycle and Effect on Forest Health

2. Bark Beetles in California…
Are all native inhabitants of the forest
Are causing widespread mortality in the forests and forested communities
The principal species: mountain pine beetle, fir engraver beetle, western pine beetle, Jeffrey pine beetle, and pine engraver beetles.

4. Average size of an adult bark beetle compared to a penny.
What are Bark Beetles…
Small insects – as small as a grain of rice
Feed and reproduce in the inner bark of trees (phloem)
Bark beetles are small insects — not much bigger than a grain of rice. They feed and reproduce in the inner bark layer, called the phloem. The phloem is found between the outer bark and the wood of the tree. For some species, like the Jeffrey pine beetle, when an adult female attacks a tree, she bores through the bark and begins to excavate a tunnel, called a “gallery”, into the phloem and on the wood
surface. She then chews pockets in the sides of the gallery, laying one egg in each pocket, until she’s laid
several dozen eggs.
Average size of an adult bark beetle compared to a penny.

5. Attack only the Jeffrey pine
Many different kinds of bark beetles.
Some beetles attack only one species of tree (i.e. Jeffrey pine beetle)
Others attack several species of related trees (i.e. Fir engraver beetle)
Attack all Fir trees
Fir engraver beetle
“Bark beetle” is a general term that is used to describe any one of around 6,000 species of beetles found in the U.S. Some beetles attack only one species of tree, such as the Jeffrey pine beetle, which attacks only Jeffrey pine. Others attack several species of related trees. For example, the fir engraver attacks several species of firs, but doesn’t attack pines.
Attack only the Jeffrey pine
Jeffrey pine beetle

6. Bark Beetle Identification
Each species of bark beetle carves a distinct shape for their gallery, so the beetle species can often be identified based on the host plant and the gallery shape alone.

7. Regardless of the type of bark beetle…
They all attack trees in a similar way
They all go through a complete metamorphosis from egg to larva to pupa to adult.

8. A Bark Beetle’s Life Cycle….
Spend most of their lives under the bark of trees ingesting wood and leaving behind tunnels
Produce different kinds of tunnels as they go through different stages in their life cycle

9. Breeding
Some bark beetles mate first in mating chambers; others mate outside the tree and don’t use chambers
When breeding, adults make one long tunnel and lay eggs along the side
When breeding, adults make one long tunnel and lay eggs along the side. Sometimes these egg tunnels have one notch or hole per egg; in other species, the eggs are deposited in the center of the tunnel.

10. Larval Stage
Larva hatch from eggs and eat their way through tunnels that radiate from the center tunnel.
When they first hatch the tunnels are thin, but grow wider as the Larva grow
Larva hatch from eggs and eat their way through tunnels that radiate out from the center tunnel. In some species, these larval galleries are straight, while others twist and curve. When they first hatch, their tunnels may be so thin as to be hard to even see, but as they grow, their tunnels often gradually become wider. The tunnels they leave behind are evidence of their growing body size.

11. Larva eating through the tree and leaving tunnels
Larva eating through the tree and leaving tunnels. Some species of bark beetles make straight larval galleries, others twist and turn.

12. As beetles bore, they create “frass”, a sawdust-like dust that is a by-product of boring combined with beetle waste.
As beetles bore, they create “frass,” a sawdust-like dust that is a by-product of boring combined with beetle waste. Some adult beetles clear frass out of tunnels, larvae don’t clear frass out of larval galleries (those messy kids!).

13. Pupa to Adults The larva will pupate and eventually become adults
Adults will bore out of the tree and fly to another tree … where the cycle starts again.
Boring Holes

15. Most beetle activity takes place in the spring and summer months
Most of this activity occurs during the spring and summer months from May through September. Beetles will fly during warm and windless days anywhere from 500 feet to two miles to find new trees. During the winter months, most species hibernate as larvae under the bark of trees, although some may overwinter as adults in the forest litter above the soil. A full life cycle usually takes up to one year to complete. However, some Ips (pine engraver) species may go through several life cycles in one season.
Most beetle activity takes place in the spring and summer months
In the winter, most species hibernate as larva under the bark

16. Life Stages
The time it takes to go through the life cycle varies in length from about 2 months in warm weather to 10 months in cool weather.
In the North and higher elevations = 2 generations a year
In the South and lower elevations = 3-4 generations a year
In the northern part of their range and at higher elevations, the beetles produce two generations each year, with attacks in early June and late August. In the South and at lower elevations, the beetles produce three and sometimes four generations each year, with attacks as early as March and as late as November.

17. Bark Beetle Effects on Trees
When adult beetles enter a tree through bark, they send out pheromones (chemical signals) attracting other beetles
When a tree is attacked, it emits a chemical-filled sap that kills or immobilizes the insects.
Typically, a few adult beetles enter a tree through the bark, then send out pheromones (chemical signals) to attract other beetles to the area. When a tree is attacked by even a few bark beetles it will emit chemical-filled sap that kills or immobilizes the insects. Early signs of bark beetle infestations are pitch tubes, cylindrical formations of resin that appear outside tree bark; this means a few individuals are beginning to occupy the tree. Sometimes a healthy tree can emit enough sap to fend off a beetle infestation, but often, as more and more beetles bore holes into the bark, their tunneling causes a disturbance in the trees’ fluid and nutrient transport system. If the tree can’t transport water and nutrients, it starves and dies. A later sign of bark beetle infestation in conifers is their needles changing from green to orange.
When the female beetles successfully attack a tree, they release minute amounts of behavioral chemicals into the air. These odors (pheromones) attract males and other females to the tree, causing a mass attack that tends to overcome the tree's natural resistance. If numerous beetles are flying and fair weather persists, adjacent trees may be attacked, resulting in a group of infested trees. Usually, an insufficient number of beetles are flying, or bad weather delays the arrival of more beetles to the vicinity of the newly attacked tree. Either of these conditions will usually result in the typically isolated dead tree.
Besides attracting western pine beetles themselves, the pheromones also attract their natural enemies, such as predaceous checkered and ostomid beetles. The ability of these beetles to sense the pheromones makes them effective predators during the critical attack phase.
Pitch Tubes

18. If the tree can’t transport water and nutrients, it starves and dies.
As more and more beetles bore holes into the bark, their tunneling causes a disturbance in the trees’ fluid and nutrient transport system.
If the tree can’t transport water and nutrients, it starves and dies.
A later sign of bark beetle infestation in conifers is their needles changing from green to orange.
When the female beetles successfully attack a tree, they release minute amounts of behavioral chemicals into the air. These odors (pheromones) attract males and other females to the tree, causing a mass attack that tends to overcome the tree's natural resistance. If numerous beetles are flying and fair weather persists, adjacent trees may be attacked, resulting in a group of infested trees. Usually, an insufficient number of beetles are flying, or bad weather delays the arrival of more beetles to the vicinity of the newly attacked tree. Either of these conditions will usually result in the typically isolated dead tree.
Besides attracting western pine beetles themselves, the pheromones also attract their natural enemies, such as predaceous checkered and ostomid beetles. The ability of these beetles to sense the pheromones makes them effective predators during the critical attack phase.

19. Some beetles carry a fungus that can be injected into the trees
The fungus inhibits the tree’s water conducting ability
The fungus causes a blue/gray staining of the wood

20. Healthy trees can typically withstand an attack from a few beetles
Stressed trees cannot produce enough sap to ward off an attack
Mass infestations = large rates of tree mortality

21. Bark Beetles and Forest Health
Bark beetles are a natural part of the forest ecosystem
Usually present in small numbers and occasionally killing weakened trees
When beetle populations are at a low level, the beetles play a useful role

22. Beetles Help… remove clusters of trees
provide dead trees for wildlife use
help to recycle valuable nutrients from diseased and dying trees.
Bark beetles also provide food for woodpeckers and other insects.
Mass Infestations Lead to High Rates of Tree Mortality
Low Bark Beetle Populations Promote Forest Health

23. Why the Outbreak in California?
Trees weakened due to competition for resources, especially water.
Warmer, longer summers and shorter winters have made it possible for bark beetles to have multiple generations of young in one breeding season – leading to massive beetle population explosion
Research suggests that the trees in these forests are not sick, but are thought to be weakened due to competition for resources—especially water. This makes them less able to defend themselves by emitting sap, and more susceptible to being overwhelmed by beetle attacks. Additionally, due to global warming, the warmer, longer summers and shorter winters have made it possible for bark beetles to have many more generations of young in one breeding season than in the past, which has led to massive population explosions of beetles.

24. Conditions Affecting Outbreaks
Food Supply - availability of suitable host material - phloem and inner bark
Moisture-stressed trees cannot produce sufficient pitch and their nutritious food supply helps beetles reproduce many times
The availability of suitable host material - phloem and inner bark - is a key condition influencing western pine beetle outbreaks. Most trees are either too healthy or too weak to provide material in which beetle numbers can increase. Healthy trees can withstand many attacks before the beetles are successful, the brood is established, and new adult beetles are produced. Weak trees, such as those that have been smog damaged, diseased, or suppressed by competition, although easily killed, also produce relatively few beetles.

25. Food Supply (cont.)
Healthy trees can withstand many attacks before the beetles are successful…producing lots of young
Weakened trees are easily attacked but succumb quickly so not as many adults are produced
The thick, nutritious phloem and inner bark of healthy trees become host material for attacking western pine beetles when these trees undergo sudden and severe moisture stress. Healthy trees ordinarily produce abundant amounts of resin, which pitch out or eject attacking beetles. But, when suddenly deprived of moisture, stressed trees cannot produce sufficient resin flow to resist attack, and their nutritious food supply becomes available to beetles. In these trees, almost all attacking beetles can succeed and reproduce many times their number of offspring, increasing the beetle population to outbreak levels.
Moisture stress results when the water balance between the foliage and the roots changes dramatically. An imbalance may result from increased water loss from the needles (transpiration), decreased water uptake by the roots (physiological drought), or from a combination of the two.
Any condition that results in excessive demand for moisture, such as tree crowding, competing vegetation, or sudden exposure to severe sunlight; or any condition that reduces the ability of the roots to supply water to the tree, such as mechanical root damage, root disease, soil compaction, or drought, can cause moisture stress and increase susceptibility to attack by the western pine beetle.

26. Tree Resistance
Healthy pines…
Pitch the beetles out
Soak the phloem tissue surrounding egg galleries with resin inhibiting larval and fungal development

27. Woodpeckers – feed on the larva
Natural Enemies
Predators include…
Woodpeckers – feed on the larva
Other species of beetles (i.e. checkered beetle)
parasites
Predators stabilize conditions at low beetle population levels.
Figure - Adult, black-bellied clerids feed on western pine beetles attacking the tree. Two times the natural size.
However, their action alone cannot control outbreaks.

28. Cold Temperatures
-20° F persisting for approximately 5 days can cause heavy brood (young) mortality
Winter temperatures below - 20°F (-27°C) and persisting for several days can cause heavy brood mortality in those portions of the tree above the snow-line. Effects are only temporary, however, and after a few generations, the population usually recovers.