Winter is usually spent in the tree in the egg, larval, or adult 
stage. Activity is resumed with the advent of warm weather in 
the spring. Old adults continue their egg laying in the extended 
portion of galleries started the year before or in newly con- 
structed tunnels in 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. 
The true bark beetles comprise one of the most destructive of 
all the groups of forest insects in the country. During an average 
year, they are credited with killing 414 billion board feet of 
sawtimber, an amount equal to at least 90 percent of all of the 
insect-caused mortality and 63 percent of the total growth impact 
in the forests of the country (729). Losses caused by these insects 
are not confined to damage resulting from their feeding activities 
alone; they are intensified and often greatly increased by disease- 
causing organisms which the beetles introduce into the affected 
trees. 
Many environmental conditions operate to control the abun- 
dance of bark beetles. When favorable conditions prevail, many 
species increase rapidly to epidemic proportions. At other 
times, their numbers may be low enough to escape notice. Some 
of the better known factors affecting abundance are: food supply, 
weather conditions, prevalence of natural enemies, and associated 
fungi (41). Many species increase most rapidly when there is an 
abundance of overmature or decadent trees, windfalls, slash, 
lightning-struck trees, or trees weakened by crowding, fire, 
drought, excessive moisture conditions, disease, or other causes. 
Factors tending to hold populations in check include natural 
enemies, low winter temperatures, and stands of healthy, vigorous 
trees. The most important natural enemies are insect parasites 
and predators and other closely related arthropods. Thatcher 
(707) presented a list of the parasites and predators of southern 
bark beetles, and Bushing (128) published a synoptic list of the 
parasites of Scolytidae in the United States and Canada. 
Key to Injuries by Scolytidae and Platypodidae 
1. Entrance gallery leading directly through the bark to the 
surface of the wood, where it is elaborated into one of 
VWarlous: types of bark DULTOWS 30k 2, 
Entrance gallery extending through bark into the wood or 
Sveneimtoutheypluly ere Ne. or ort les ere (eiece ie: 23 
2 linestoesmner bark Of Coniferous trees 22..2)...4.2)h ee. 3 
Inethnesnner bark of deciduous: trees 2. 20.060 20 
jp aelneexposedsroots or basal part of trumk =... 4 
In main trunk or thick barked, primary branches ................ 6 
Enemedimm-rtossmmall branchesied 42 2 ar eee il 
Hliineaeinryse See a ee ten ner cee he), hh Bont giaey eet Thee OMe elem 16 
TRC OME Sete er ee hehe eS) eh ieyecel or sty aoe Mele ee vee LS 
4. In fresh stumps or the bases of living pines and spruces; 
the entrance hole often marked by large pitch tubes; 
229 
