that begin their own galleries at suit- 
able intervals up and down the tree 
or branch. The male helps the female 
excavate a long gallery parallel to the 
grain of the wood, using a special con- 
cave area on the front of his head 
to scoop out the boring dust. After 
mating, the male may leave the gallery 
and go in search of females that have 
not yet attracted mates. The female 
remains to lay eggs in small niches 
along the sides of the gallery. Her 
supply of forty to one hundred eggs 
exhausted, the female wedges herself 
into the gallery entrance where she 
serves, even after her death, as a bar- 
rier to prevent the entrance of young 
larvae of predators that would eat her 
young. The beetle larvae feed in gal- 
leries extending outward from the egg 
gallery, each larva doing its best to 
maintain a thin partition of uneaten 
bark between itself and its neighbors. 
In western Washington the larvae 
overwinter under the bark, pupate in 
spring, and emerge in early summer 
in time to start their own galleries 
in trees and branches downed in winter 
storms, as well as trees that are show- 
ing signs of water stress at the onset 
of the dry season. 
This bark beetle is a common insect 
and is able to find and colonize almost 
all suitable Douglas firs. Lifting the 
bark from a small section of dead 
Douglas fir, one is almost certain to 
find a tableau of mother bark beetle 
guarding the entrance of her gallery 
and plump white grubs at the end of 
gradually widening, frass-packed gal- 
leries. The tranquillity of this scene is, 
however, undermined by a series of in- 
terlopers. In western Washington I 
have found twenty-nine insect species 
in the galleries of S', unispinosus. Some 
of these insects are uncommon in Sco- 
lytus galleries; these rare insects may 
be primarily associated with another 
species of bark beetle or they may be 
at the edge of their geographical ranges 
or perhaps their ecological require- 
ments are so restricted that they are 
seldom abundant. The fourteen spe- 
cies of insects more dependably found 
in galleries of S. unispinosus can be 
divided into three groups: parasitoids, 
predators, and associates; the last are 
composed of scavengers and species 
whose roles are still unknown. 
Parasitoid insects are those whose 
larvae gradually devour and kill an- 
other insect, either working from the 
inside or the outside. The six common 
parasitoids of S. unispinosus are all 
tiny wasps, and they all have similar 
life histories. The female wasp finds 
a beetle-infested tree, probably locat- 
ing the site by the odors of resins 
or alcohol emanating from the gal- 
leries of the bark beetles. These spe- 
cies of wasps use their ovipositor to 
drill through the bark, sting the beetle 
larva in its gallery, and lay an egg 
on or near it. The parasitoid wasp 
larva slowly consumes the host until 
nothing is left but the hard head cap- 
sule and a pellet of crumpled skin. 
The parasitoid then pupates, trans- 
forms to an adult, and gnaws its way 
out through the bark. 
How these parasitoids manage to 
pinpoint the location of a host larva 
through the intervening outer bark of 
the host tree is not known. A much 
larger species of wasp parasitizing a 
large bark beetle has been studied by 
J.V. Richerson and J.H. Borden, and 
the results of their studies may have 
some general application. Entomolo- 
gists originally thought that the female 
wasp could pick up the vibrations of 
gnawing larvae, but wasps were shown 
to be able to locate quiescent larvae. 
Odor was ruled out by covering a larva 
with a thin, airtight shield that did 
The plumelike tunnels engraved by a 
brood of Scolytus unispinosus bark bee- 
tles eating their way outward from the 
vertical egg gallery is a model of ef- 
ficient use of a rich, but limited, 
resource. The egg gallery is ini- 
tiated by the female, which is soon 
joined by the male. He aids in 
construction of the gallery by 
pushing excavated material 
out the entrance hole. Be- 
tween 40 and 100 eggs 
are deposited along the 
sides of the gallery. 
88 
