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CephaJogl)pta
murinanae Bauer
(Hymenoptera:
Ichneumonidae)
D. Testing of parasitoids 269
success varying from 17 to 60% as indicated by dissection of samples. Substantial host
larval mortality during early diapause was attributed partly to ovipositional wounding. A
problem was to optimize survival of parasitized hosts by exposing candidate larvae long
enough to achieve a high percentage of attacked hosts. but short enough to avoid a high
percentage of superparasitism and fatal wounding. The exposure patterns usually resulted
in a single egg per host; the egg hatched within a few weeks and the parasitoid overwintered
as a first-instar larva.
In rearing thousands of prospectively parasitized spruce budworrn larvae, only 70 C.
laritis pre-pupae emerged from their hosts. Of these, 27 successfully pupated in a
cocoon, a few adults emerged, and only one female oviposited. Efforts to rear its
progeny were unsuccessful. Between autumn and late spring in all years, the percentage
parasitism of sampled host stock declined sharply, almost to zero. Hypotheses to
account for the decline included ovipositional injury to hosts, lack of cold hardiness in
the parasitoid, host diet unfavourable to the parasitoid, encapsulation reaction, and
other biochemical defence mechanisms.
Mortality of hosts in hibernacula was 20-62%, higher than the usual 9-22% recorded
from populations in their natural environment. Some of the additional mortality was
attributed to ovipositional wounding, as C. laricis has a large ovipositor compared with its
native homologue, Glypla fumiferanae (Vier.). However, mortality of hosts was not an
important factor in reduction of percentage parasitism. It is unlikely that low winter
temperature discriminated against parasitoids within their spruce budworm hosts. In a
test for cold hardiness, overwintering larvae of C. fumiferana, G. fumiferanae, and C.
larieis were rapidly supercooled down to -60°C. All three species showed ice-crystal
formation in the -40 to -43°C range, which is below the coldest natural environment in
New Brunswick. Thus the parasitoid stock of Hokkaido provenance appeared to be as
hardy as the native stocks of parasitoids and hosts, just as one would expect from a
comparison of the monthly mean temperatures of the two regions.
When a stock of second-instar spruce budworrn larvae was held in storage from
October to May at a constant temperature of 7°C, the rate of decline of parasitism was
unusually rapid compared with stock held out-of-doors at much lower temperatures.
Evidently the "cold room" was a selectively hostile environment to the parasitoid, but
not to the host. This suggested that the decline in parasitism was temperature-related
rather than time-related.
Most of the loss of parasitism occurred in post-diapause stocks; however, no evidence
implicating host diet appeared, as rearings with either artificial diet or with natural
foliage produced healthy hosts and moribund or vanished parasitoids. Dissections
revealed very few encapsulated parasitoid larvae, not enough to indicate a mechanism
for decline in percentage parasitism. The expected parasitoids seemed to disappear
without trace, but occasionally parasitoid head capsules, faintly yellowed and very
membranous, were discovered within the body cavity of healthy maturing hosts. Thus
the most plausible hypothesis is that the spruce budworrn is able to kill and absorb larvae
of C. larieis, leaving the host unimpaired.
A small shipment (Table 74) of pupae was air-freighted from Czechoslovakia to New
Brunswick in July 1972. Some 1300 spruce budworrn larvae in hibemacula were exposed,
and females oviposited vigorously with behaviour similar to C. larieis. A January
sample of hosts wintered out-of-doors indicated 25% parasitism of survivors and 18%
host mortality. Overwintered hosts were successfully reared in May on artificial diet, or
on diet followed by foliage, but only four parasitoid pre-pupae matured, two dying
within pupal hosts and two emerging from larval hosts but failing to pupate. The
disappearance of the expected population of parasitoids is unexplained, but circumstances
were similar to those of C. larieis.