4 / 
The bark beetle Pseudohylesinus nebulosus (4) occurs 
throughout the geographical range of Douglas fir and 
is often the most numerous scolytid beetle attacking 
a stand of these trees. Its abundance is caused by 
the plentifulness of its hosts — broken tops of live 
trees, upper trunks of fallen trees, and fallen 
branches as small as three centimeters in diameter. 
The species breeds in fresh inner bark that is always 
shaded, preferring areas that are rapidly losing 
moisture, through the bark itself or through the 
transpiration of live needles on adjacent branches. 
5 
Another bark beetle Scolytus unispinosus (5) has 
a well-known life cycle. It overwinters as third- 
and fourth-instar larvae, emerging as an adult 
between late spring and mid-July. The recently 
emerged adults feed in the phloem of healthy 
Douglas fir twigs before beginning their host 
tree selection flights. They breed in the 
fresh inner bark of the branches of dead 
trees that are always exposed to the sun. 
Spathius sequoiae (6) is the largest par- 
asitoid wasp that attacks the larvae of Scol- 
ytus beetles. Because of the relative short- 
ness of its ovipositor, this wasp can only 
attack those beetle larvae found near the 
surface of deadwood. The pinpointing of 
a host larva might be accomplished 
through heat perception. Ecphylus cali- 
fornicus (7) is a small parasitoid wasp 
that attacks bark beetle larvae too small 
to be adequate hosts for other para- 
sitoid wasps. This species is so small, 
and has such a proportionately short 
ovipositor, that it can only parasi- 
tize host larvae under very thin bark, 
thinner than is preferred by a bark 
beetle such as S. unispinosus. 
Alan Iselin 
The longhorn beetle Anoplodera cras- 
sipes (8) often breeds in wood that 
is old, partly rotten, and at least partly 
shaded. The beetles remain in the same 
material for successive generations as the 
wood decays. 
I 
87 
