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376 W. G. H. Ives and J. A. Muldrew
Sorex cinereus
cinereus Kerr
(lnsedivora: Sorlddae)
Evaluation of Control Attempts
The masked shrew, introduced into Newfoundland in 1958, has continued to spread.
Survey reports for 1969 and 1970 (Anon. 1970-79) mentioned the concern of local
residents as high initial numbers were noted in each area: by 1971 the shrew had spread
over 95% of the island. Populations on the island, excluding the first 2 or 3 years' data,
averaged about eight shrews per hectare, which seems to be unusually high. However,
the 18-year average populations of the masked shrew in the Rennie life-table plot (Ives
1981) was 5.2/ha. If one includes populations of the closely related arctic shrew, Sora
arcticus Kerr, which averaged 1.8/ha, the total of 7.01ba is comparable. Populations of
the masked shrew in Newfoundland therefore seem to have stabilized, and can be
expected to fluctuate in the future in response to environmental conditions.
O. benefactor and larch sawfly populations have probably not yet reached an equilibrium,
even at the original release point near Pine Falls. As shown in Fig. 19, the level of O.
benefactor parasitism at or near the release point reached 90% in 1967. It remained at or
near this level until 1972, but larch.sawfly populations collapsed in 1973, presumably as a
result of the continued high rates of parasitism (rves 1976). No larvae or cocoons were
obtained near the plot in 1973 or 1974, although intensive sampling was conducted
(Muldrew in press). Detailed studies were discontinued after 1974, and further trends
are based on limited sampling. In 1977, a collection of 72 fourth- and fifth-instar larvae
was made near the original release site at Pine Falls. None was parasitized by O.
benefactor. In 1978, collections of larvae from the same area yielded 1730 cocoons.
Parasitism by O. benefactor was 1.5%, hyperparasitism by Mesochorus dimidiatus was
86%. A total of 237 third- to fifth-instar larvae collected 8 km north of the release point in
1978 contained no O. benefactor, but 19% of 424 fourth- and fifth-instar larvae from an
area 8 km south of the release point were parasitized. NinetY7three per cent of these O.
benefactor larvae were parasitized by M. dimidiatus. In 1980, a total of 535 cocoons
was obtained from the same location; 15% of these contained O. benefactor, of which
75% were parasitized by M. dimidiatus. No O. benefactor were present in 150 cocoons
obtained by rearing late-instar larvae collected in 1980 near the Seddon's Comer lifetable
plot, about 70 km south of the Pine Falls release point.
Although the above data are rather fragmented, there is some evidence that O.
benefactor is in danger of local extinction over a fairly wide area. Should this happen, it
is possible that the larch sawfly may increase in abundance if environmental conditions
are favourable. No population sampling of the larch sawfly was done in the Pine Falls
release area in 1980, but observations by Ives indicated that this population increase
may already be taking place. It remains to be seen whether or not O. benefactor will be
able to prevent an outbreak in spite of the adverse influence of M. dimidiatus. O.
benefactor is host specific, but M. dimidiatus is not (Pschorn-Walcher & Zinnert 1971).
Because of this, one would expect M. dimidiatus to be able to persist on alternate hosts,
thus obtaining a "head start" on any re-invading O. benefactor. Under these circumstances,
M. dimidiatus would probably be able to control O. benefactor, thus permitting larch
sawfly populations to reach outbreak proportions. However, a crude model developed
by Ives (1976) indicated that the two parasitoids should reach an equilibrium with each
other and with the larch sawfly. According to the model, the larch sawfly populations
would fluctuate in response to density-independent environmental factors, but should not
reach outbreak proportions in the presence of O. benefactor, even when M. dimidiatus is
present. It is too early to tell which hypothesis is correct.
The impact of the masked shrew on larch sawfly populations in Newfoundland has not
been assessed in detail (Warren 1971). The fact that larch sawfly infestations reached