not deter the parasitoid. The inves- 
tigators finally found a minute dif- 
ference between the temperature of 
the bark above the larva and the tem- 
perature of the surrounding bark. An 
artificial warm spot on the underside 
of a piece of bark elicits initial stages 
of oviposition behavior by the para- 
sitoid. The sensory receptors involved 
seem to be a series of microscopic 
platelets located on the wasp’s anten- 
nae. 
The parasitoids of S. unispinosus 
must know not only the position but 
also the size of the prospective host 
larva. A host that is too small produces 
a miniature parasitoid or none at all. 
Within the range of host larvae of 
acceptable size, the smaller hosts still 
produce parasitoids of subnormal size. 
Undersized male parasitoids produced 
from a small host may be at a slight 
disadvantage when competing for 
mates and may have a reduced life 
span due to their small food reserves. 
Undersized female parasitoids are at 
a much greater disadvantage because 
they must use stored food reserves 
to disperse to a new tree site and pro- 
duce eggs. Worse yet, the ovipositor 
length is proportional to the size of 
the body, so a small female has a 
short ovipositor that is unable to reach 
hosts under thicker bark. It would be 
advantageous for the ovipositing fe- 
male to lay eggs destined to become 
males on smaller host larvae and eggs 
destined to become females on larger 
host larvae. 
My studies of several distantly re- 
lated parasitoids of bark beetles sug- 
gest that the females tend to lay male 
eggs on small hosts and female eggs 
on large hosts. The mechanism that 
determines sex is very simple: all male 
wasps are haploid, produced from un- 
fertilized eggs, while all female wasps 
are diploid, produced from fertilized 
eggs. The mother wasp, like the queen 
honeybee, controls the sex of her off- 
spring by controlling the release of 
sperm from a special storage sack that 
was filled during copulation. 
Given the similarities of their life 
cycles, how can six species of para- 
sitoids coexist on one species of bark 
beetle? That question cannot be an- 
swered completely, but by making 
notes on the location of parasitoid lar- 
vae and then rearing them to adults 
(the larvae are difficult to identify), 
I have found several instances of spe- 
cialization. One parasitoid, Spathius 
sequoiae, and possibly another, Eury- 
toma tomici. usually attack bark bee- 
tles living in shaded material. Cheiro- 
pachus brunneri and Dinotiscus thom- 
soni attack larvae in exposed bark. 
Ecphylus californicus and Eurytoma 
phloeotribi attack young larvae that 
are too small to be adequate hosts 
for the other four species. This seems 
to give E. californicus and E. phloeo- 
tribi a great advantage since they can 
preempt hosts not yet available to the 
other parasitoids, but E. californicus 
and E. phloeotribi are so small and 
have such short ovipositors that they 
can only parasitize hosts under very 
thin bark, thinner than is actually pre- 
ferred by Scolytus unispinosus. 
Predators of Scolytus unispinosus 
are known to include two species of 
beetles: Enoclerus lecontei and Las- 
conotus subcostulatus. Both species 
are most often found in galleries that 
the mother bark beetle failed to block 
with her body. The larva of E. lecontei 
is a large-mandibled, voracious pred- 
ator that tunnels through the inner 
bark devouring all bark beetle larvae 
in its path and probably consuming 
parasitoid larvae as well. Several 
Scolytus larvae are needed to produce 
one Enoclerus. The larva of L. 
subcostulatus has a different ap- 
proach, feeding on fungi for its first 
two larval instars before attacking a 
Scolytus larva. Lasconotus behaves al- 
most like a parasitoid, for it slowly 
consumes a host larva, having first 
immobilized its prey by stinging it 
with a pair of caudal organs called 
urogomphi. 
The remaining common associates 
of Scolytus unispinosus include two 
beetles, four flies, and one wasp. One 
of the two beetles, Rhizophagus mi- 
nutus, is probably a predator, but it 
is not known whether this species eats 
eggs and larvae of Scolytus or whether 
it attacks one of the other associates. 
The other beetle, Renardia jubilaea, 
and two of the flies, Sciara sp. and 
Asynapta sp., feed on a fungus that 
regularly appears in the galleries of 
S. unispinosus. It is possible that this 
fungus is a symbiote of the bark bee- 
tle, as it has been shown that some 
other species of Scolytus depend on 
fungi to modify their subcortical en- 
vironment; these species of Scolytus , 
like many bark beetles, have special 
cavities, or pits, in which spores of 
the fungus are carried and nourished 
with special secretions. One of the fun- 
gus-eating flies, an undescribed spe- 
cies of Asynapta, is parasitized by an 
unidentified platygasterid wasp. The 
remaining species of flies, Lonchaea 
corticis and Palloptera terminalis, are 
primarily scavengers that feed on dead 
insects such as bark beetle larvae that 
have died from unfavorable environ- 
mental conditions. These flies may ac- 
tually benefit from the defense of the 
gallery entrance by the mother bark 
beetle, for the young maggots often 
seem to get their start by consuming 
the internal organs of the dead guard- 
ian. 
This subcommunity of insects as- 
sociated with Scolytus unispinosus is 
by no means the most complex of the 
constellation of subcommunities oc- 
curring in dead Douglas fir. S. uni- 
spinosus has an ecological twin, 
Pseudohylesinus nebulosus, which 
breeds in the same sort of material 
but is only found in heavily shaded 
bark. I have found thirty-three species 
of insects associated with P. nebulosus ; 
most of these do not occur in galleries 
of S. unispinosus. Dendroctonus pseu- 
dotsugae, the most intensively studied 
of Douglas fir bark beetles, is known 
to have forty-three insect associates. 
Each of the insects that feeds on 
bark and wood of Douglas fir has its 
predators, parasites, and scavengers. 
The relationship between the associate 
and its host may be casual, for ex- 
ample, the little Leptolhorax ants set 
up their colonies in the abandoned 
galleries of bark and longhorn beetles. 
Other relationships are intimate: the 
wasp Eubadizon crassigaster waits 
with quivering antennae, while the fe- 
male of the weevil species Pissodes 
fasciatus deposits and carefully covers 
her eggs in a small excavation in the 
bark of dead Douglas fir; when the 
Pissodes moves away, the wasp de- 
posits one of her own eggs inside each 
Pissodes egg. 
The multiplicity and complexity of 
relationships between the phyto- 
phagous insects and their associates 
are easily matched by the number and 
variety of niches that the phyto- 
phagous insects use in dead Douglas 
fir. Specialized habitats have been 
found by fifty species of bark beetles, 
twenty-two metallic wood-boring bee- 
tles, fifty-two longhorn beetles, six 
weevils, eight wood wasps, and a dozen 
or so miscellaneous bark and wood 
eaters. Careful observation and col- 
lecting make it possible to obtain de- 
scriptions of the habitats of many spe- 
cies. The longhorn beetle Phymatodes 
maculicollis, for example, usually oc- 
curs in recently killed, dry, shaded 
branches; the false blister beetle 
Calopus angustus may be expected 
89 
