Most Scolytidae fly and attack a new host as soon as they mature and some 
species congregate under the bark of the new host to hibernate or mature sexually. A 
few species feed on the twigs, buds, or bark of other trees before attacking a tree for 
breeding purposes. Generally speaking, the adults spend only enough time outside 
the bark to find new host material. Exceptions are species (Ips, Dendrocranulus, 
Trypodendron, etc.) that overwinter in litter on the forest floor. 
Beetles usually overwinter in the tree in the larval or adult stage, or in the forest 
litter as adults. In warm regions, eggs and pupae may also be present in winter. 
Activity is resumed with the advent of warm weather in the spring. Parent adults 
continue their egg laying in the extended portion of galleries started the year before 
or in newly constructed tunnels or freshly attacked trees. In the South, there usually 
are several generations per year. In the North and at high elevations, there may be 
only one or a partial generation per year. 
Bark Beetles 
Phloeophagus Scolytidae 
In the United States, bark beetles kill an average of 25.5 million cubic meters of 
sawtimber and pulpwood annually. This accounts for 90 percent of all insect-caused 
tree mortality and 60 percent of the annual loss of timber to all causes (/22/). The 
most destructive eastern species, the southern pine beetle, inflicted an annual 
economic loss of $225 million by killing 9 million cords of pulpwood and 3 billion 
board feet of sawtimber from 1960 to 1978 (969). 
Most tree mortality 1s caused by species in the genera Dendroctonus, Ips, and 
Scolytus. Scolytids in other genera occasionally may kill or damage trees or 
indirectly cause their death by transporting pathogens from diseased trees where 
they breed to healthy trees in which they feed. Rapid death of mass-attacked trees is 
actually caused by fungi that the beetles carry in mycetangia (fungal pockets). The 
fungal inoculum is able to penetrate the resinous (or other chemical) resistance of 
the host; then the fungi colonize (and usually stain) the sapwood and disrupt the 
flow of water to the tree crown. Even the tree-killing bark beetle species may most 
often be restricted to breeding in broken or moribund material because healthy trees 
are able to resist attacks of beetles and fungi in the vital phloem-cambial region. 
However, bark beetles can overcome host resistance by mass attacks orchestrated by 
aggregation pheromones produced when the pioneer beetles attack a tree (///). A 
sufficient population mass attack can kill any host, regardless of its vigor. When the 
attacking population is insufficient to exhaust the natural defense mechanisms of a 
tree, the host will survive and the attackers will be repelled or killed (/03). Success 
or failure of an attack can be visualized as a seesaw on which host resistance and 
beetle numbers are balanced on a fulcrum—time. The balance will be affected by 
an increase or decrease in either attack density or resistance, or a shift in the 
position of the fulcrum (duration of attack). This concept explains why bark beetle 
outbreaks generally follow a predisposing event (logging, flooding, drought, or 
fire) that reduces host resistance or allows for a dramatic increase in the bark beetle 
population. Once the population exceeds the threshold beyond which host resistance 
is a factor, tree killing will continue until the supply of hosts is exhausted or natural 
events (winterkill, disease, predation) reduce the population. 
Major outbreaks of bark beetles have probably never been controlled by human 
intervention. However, timber losses in localized eruptions have been curtailed by 
direct action such as logging infested trees or felling them for treatment with 
insecticides, solar heat, or fire. Synthetic pheromones have been able to disrupt 
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