in soggy, rotting root crowns of small 
dead trees. 
An important factor to dead-tree 
insects is the portion of the tree avail- 
able. Root, trunk, branch, twig, bark, 
sapwood, and heartwood are all grist 
to the insect mill, but the individual 
millers are specialists. Scolytus, oc- 
cupying the crown and branches of 
an exposed dead tree, gives way in 
the lower bole to the metallic wood 
borer Melanophila and the longhorn 
beetles Monochamus and Semanotus, 
which in turn are replaced in the roots 
by the bark beetle Hylastes and the 
weevil Steremnius. In a cross section 
of a shaded tree, the bark beetle Den- 
droctonus and the weevil Pissodes 
consume the inner bark, the wood 
wasp Urocerus and the false darkling 
beetle Serropalpus bore in the sap- 
wood, and the longhorns Opsimus, 
Leptura, and Anoplodera eventually 
pulverize the heartwood. 
Another factor of great importance 
is the amount of exposure to direct 
sunlight. A study published by S.A. 
Graham in 1925 showed that tempera- 
tures beneath exposed and shaded pine 
bark can differ by as much as 35 °C. 
The great temperature extremes in ex- 
posed bark combine with accelerated 
drying and chemical decomposition to 
make exposed bark and wood a special 
habitat that must be exploited by spe- 
cialized insects. 
The time elapsed after the death 
of the tree or branch also determines 
the composition of its insect commu- 
nity. The bark feeders — rapidly nour- 
ished by the abundant and relatively 
easily digested nutrients of the inner 
bark — arrive first and leave first. Sap- 
wood feeders begin to arrive at almost 
the same time as the bark feeders, 
and in fact many sapwood feeders en- 
rich their diet with inner bark while 
deeply engraving the surface of the 
sapwood. Sapwood feeders often re- 
quire a year or more for development. 
Insects that feed in rotting sapwood 
and in heartwood persist for many 
years, often producing several genera- 
tions over a period of twenty years 
or more. Several years after a tree’s 
death there is little left but cellulose 
and lignin, both materials resistant to 
digestion. Accumulated resins may 
make the heartwood still more intrac- 
table. Many insects that feed in wood 
of dead trees are actually unable to 
digest much wood and derive their 
nourishment from fungi that permeate 
the wood. 
The fungal flora of the dead tree 
is one of the most important deter- 
minants of the insect fauna, and the 
insects are often vectors of the fungi. 
There are many examples of symbi- 
osis, as in the case of the wood wasps, 
which actually inject fungal spores 
into the wood along with the egg, or 
the ambrosia beetles, which cultivate 
special fungi along the galleries and 
chambers excavated in the wood. In 
many other cases insects seem to be 
associated with wood invaded by cer- 
tain types of fungi, but the exact re- 
lationship between insect and fungus 
is not known. The fruiting bodies of 
shelf fungi have distinctive insect fau- 
nas separate from the group of insects 
that feed in wood filled with the 
hyphae of these fungi. 
The insect community of a dead 
tree is also determined by the season 
when the tree became available for 
colonization. When I cut a series of 
Douglas fir trees at regular intervals, 
starting in early spring and ending 
in late fall, I found that trees cut 
during spring and most of the summer 
were regularly invaded by the bark 
beetles Dendroctonus and Pseudo- 
hylesinus and by the weevil Pissodes. 
Trees cut in August and September 
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missed the flight season of these bee- 
tles, and by spring the bark had 
fermented so extensively that it was 
no longer suitable for insects that feed 
on fresh bark. These fermenting trees 
became enormously attractive to the 
ambrosia beetles Gnathotrichus and 
Trypodendron , which are attracted by 
alcohol. The sapwood of fall-killed 
trees is quickly riddled with the gal- 
leries of three species of ambrosia bee- 
tles, which build long, branching gal- 
leries lined with a black fungus. The 
larvae of ambrosia beetles live in small 
niches, called cradles, and feed on the 
fungus growing on the walls of the 
cradle or on a special pellet of fungus 
supplied by the adults. Wood that has 
been attacked by ambrosia beetles is 
filled with pinholes, each surrounded 
by a black stain associated with the 
fungal hyphae. Such wood has lost 
much of its economic value. 
Regional factors, too, may deter- 
mine which insects invade a dead tree. 
The geographical ranges of many 
Douglas fir insects are determined by 
climatic and historical factors, so that 
insects present in a dead Douglas fir 
in the mountains of New Mexico are 
often different from those that are 
found in western Washington. The in- 
sects found in dead trees of one species 
may be influenced by the regional 
abundance of hosts of another species. 
For example, the bark beetle Ortho- 
tomicus caelatus attacks Douglas fir 
but is more common in pines; in areas 
of western Washington where lodge- 
pole pine is abundant, Orthotomicus 
regularly seems to spill over into Doug- 
las fir; but where lodgepole pine is 
absent, Orthotomicus is quite rare. 
Some insects attack one host in one 
region and another host in another 
region: over most of its range the bark 
beetle Pityokteines elegans breeds in 
pine and true fir, but in northern Idaho 
P. elegans abruptly begins to attack 
Douglas fir. 
All organisms tend toward ecologi- 
cal specialization as they evolve ad- 
aptations to enable them to exploit 
specific resources. This tendency to- 
ward specialization is counteracted by 
the necessity of maintaining a large 
enough resource base to insure sur- 
vival. The insect community of dead 
trees demonstrates the compromises 
organisms must make between the ef- 
ficiency of specialization and the bene- 
fits of a large, diverse resource base. 
As described above, there are many 
environmental and nutritional factors 
that can be bases of specialization. 
90 
