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41'cic-,-
Mammalian Predators of the Larch Sawfly
in Eastern Manitoba'''
By C. H. BUCKNER
Forest Biology Laboratory,
Winnipeg, Man.
ABSTRACT
The small mammal species frequenting sawfiy-infested tamarack stands are listed.
It was found that 12 species feed upon cocoons of this insect in a confined universe.
Of these, Sorex cinereus, the cinereous shrew, S. arcticus, the saddle-backed shrew,
Clethrionomys gapperi, the red-backed vole, and Microtus pennsylvanicus, the meadow
vole, are probably the most important mammalian predators because these species
maintain substantial populations in tamarack bogs. Laboratory studies indicate that
these four species reject dead cocoons and cocoons attacked by a fungus. Also, S. cinereut:
and C. gapp,:ri reject cocoons parasitized by the tachinid, Bessa harveyi, to some extent.
Predation under field conditions was estimated b y using the "cocoon planting" technique.
Although the insect spends a large portion of the year within a cocoon in the soil, most
of the predation is exerted in September and October. This is coincident with the small
mammal population peak. Cocoons formed in hummocks of the hog are heavily preyed
upon and those formed near the roots of trees are especially vulnerable to small mammal
attack. Evidence suggests that the foraging of small mammals for larch sawfly cocoons is
exerted uniformly with depth, except where the water table prohibits small mammal
activity.
The total cocoon predation ranged from 37 to 98 per cent and was usually over
75 per cent. Shrews destroyed more cocoons than mice, although they were usually
outnumbered by mice.
INTRODUCTION
Small mammals, especially rodents, are usually considered as vermin, an ' man's
endeavours have been historically directed toward discouragement or elimination of
small mammal plagues (Elton, 1942). Both mice and shrews have been shown to cause
damage to forests by consuming large quantities of tree seeds and destroying seedlings
and saplings. Recently however, it has been suggested by Graham (1928) and Hamilton
and Cook (1940) that these animals are not entirely detrimental to our forest economy
and that they may be doing considerable service for us by destroying noxious insects.
It has been mentioned (Buckner, 1955b) that predation of forest insects by small
mammals is greatest where the insect spends a portion of its life cycle in the forest
floor. Most sawflies meet this condition and it has been suggested that the larch sawfly,
Pristiphora erichsonii (Htg.), is particularly vulnerable to small mammal predation
because it remains within its cocoon in the ground for a large portion of the year.
Small mammal predation of larch sawfly cocoons is a major controlling factor in
some outbreaks, having almost exterminated the insect from isolated stands (Graham,
1928). Graham suggests that these animals usually account for about 50 per cent of
the total cocoon mortality, and occasionally 100 per cent. Despite their importance, little
research has been carried out to determine the full role of mammalian predators of
insects.
This paper concerns the effect of mammalian predators on the population of the
larch sawfly. Field research was conducted in the Whiteshell Forest Reserve in eastern
Manitoba. The study may be subdivided into two phases as follows:
studies on small mammal populations
studies on the coactions of mammals and larch sawfly.
The first phase has been presented (Buckner, 1957b) and this report considers
only that phase of the study concerning the coaction of small mammals and their
insect prey. The investigation included determination of species inhabiting larch sawfly
Contribution No. 364, Forest Biology Division, Science Service, Department of Agriculture, Ottawa, Ontario, Canada.
2 Based in part upon a thesis presented to the University of Manitoba, 1954, in partial fulfilment of the degree of M.Sc.
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PROCEEDINGS TENTH INTERNATIONAL CONGRESS OF ENTOMOLOGY-VOL. 4, 1956 (1958)
infested stands and the abundance of the various species; species that could be induced
to feed upon larch sawfly cocoons; the feeding reaction to various types of cocoons;
and predation on the sawfly under natural conditions.
Although experiments involving mammals in small feeding cages provide valuable
clues to natural behaviour, they should be treated with some caution. The author
has attempted to corroborate information derived from this approach with natural
field conditions.
SMALL MAMMAL SPECIES OF TAMARACK BOGS
The following annotated list of small mammals was made from collections taken
in larch sawfly infested tamarack bogs. Subspecific identifications are based on the
ranges recorded by Anderson (1946), and those species followed by an asterisk (*)
were verified by Dr. Austin Cameron (Curator of Mammalogy, National Museum,
Ottawa) from collections made in the study area.
ORDER INSECTIVORA
Family Soricidae
Sorex cinereus cinereus Kerr.,* the common cinereous shrew. This is the most
common and widespread insectivore of the Canadian boreal forest. It has been taken
in low to moderate numbers on all plots studied and reached its highest populations
in relatively dense stands with wet floors (Buckner, 1957b). This species has been
observed feeding upon larch sawfly cocoons in observation cages. It attacks the cocoons
by placing a forefoot on the cocoon and biting off the end and consuming the larval
inhabitant. The voracity of the species is illustrated by the following observations.
The cinereous shrew has been observed to capture and quickly devour adult tabanids,
dragon flies, and with some difficulty to open cocoons and pupae of such large species
as Actias luna Linn., Antheraea polyphemus (Cram.), and unidentified sphingids.
On one occasion it was seen by the author to attack and kill an adult wood frog.
S. arcticus laricorum Jackson,* the southern saddle-hacked shrew. This species
is the Liccond most common insectivore of the study region. It has been taken in low
numbers on all plots studied, and in moderate populations . a dense stand with a
drying floor. It has been observed feeding upon larch sawfly cocoons and the other prey
mentioned for the previous species.
S. palustris palustris Richardson,* the American water shrew. The water shrew
was taken only once in tamarack bogs during four years of intensive trapping. It fed
upon larch sawfly cocoons in cages, but cannot he considered important because of
its low occurrence in the bogs under investigation.
Microsorex hoyi hoyi (Baird), the American pigmy shrew. This species has been
identified on several occasions from collections in the study region. It resembles
S. cinereus so closely that it is impossible to differentiate the two without resorting to
skull characters. No feeding observations have been made on this species, but presumably
it reacts in a manner similar to that of the cinereous shrew.
Blarina brevicauda manitobensis Anderson, the Manitoba short-tailed shrew.
The short-tailed shrew was recorded only as a single specimen taken in 1952, until nine
specimens were taken on each of two plots in 1955. It readily attacks cocoons but can
he considered only as a minor predator in the study region because of its low population.
ORDER CARNIVORA
Family Mustelidae
Mustela erminea richardsonii Bonaparte, the Richardson's ermine. This small
carnivore has been recorded in moderate numbers on all plots trapped. There is no
evidence that this animal feeds upon larch sawfly cocoons but it is important as a
predator of small mammals that do feed upon cocoons.
ORDER LAGOMORPHA
Family Leporidae
Lepus americanus americanus Erxleben., the American snowshoe hare. The
snowshoe hare occurs in low numbers on one plot. There is no evidence to suggest
that it is a cocoon predator.
FOREST ENTOMOLOGY: General Papers
355
ORDER RODENTIA
Family 8. citaidae
Marmota monax canadensis (Erxleben), the groundhog. A juvenile groundhog
was observed once on one plot. This animal was probably exploring, and there is no
evidence that this species is a normal resident of tamarack bogs.
Citellus tridecemlineatus tridecemlineatus (Mitchell), the thirteen-striped ground
squirrel. This species was observed in tamarack bogs during 1952 in very low numbers.
The animal has virtually disappeared from the study region in the succeeding three years.
Citellus franklinii (Sabine), Franklin's ground squirrel. Franklin's ground squirrels
were observed on one plot in very low numbers. It is probably not a normal resident
of tamarack bogs. One animal that was caged with larch sawfly cocoons was observed
to consume them whole.
Tamias striatus griseus Mearns, the grey eastern chipmunk. This animal occurs
in very low numbers throughout the study region, and is found occasionally in the
bogs. For feeding behaviour and importance see account of next species.
Eutamias minimus borealis (Allen), the northern interior chipmunk. This small
chipmunk is common throughout the area and occurs in low numbers in tamarack bogs.
It has fed upon larch sawfly cocoons in cages and presumably the previously mentioned
species will also open cocoons. Because of the low populations of this and the previous
species in tamarack bogs, they are probably not important cocoon predators.
Tamiasciurus hudsonicus hudsonicus (Erxleben), the Hudson Bay red squirrel.
The red squirrel is common in the study region and occurs in low numbers in the bogs.
Presumably this species will feed upon larch sawfly cocoons, but the high level of the
water table in sawfly infested bogs is possibly a deterrent to predation.
Family Cricetidae
Peromyscus maniculatus bairdii (Hoy and Kennicot), Baird's white-footed mouse.
This species occurs in moderate numbers on the fringes of tamarack bogs. It feeds
readily on larch sawfly cocoons, but since it does not penetrate deeply into the bogs,
it cannot be considered as an important predator of this insect in the study region. On dry,
sites this species would undoubtedly be a valuable predator of the larch sawfly.
Synaptomys cooperi cooperi Baird, Cooper's lemming mouse. The lemming,
mouse has been captured several times in the bogs of this region (Buckner, 1957a).
Presumably it would feed upon larch sawfly cocoons but its failure to maintain high
populations suggests that it is not an important predator in this area.
Clethrionomys gapperi loringi (Bailey),* the plains red-backed vole. This is a
common species usually occurring in the drier bogs in this area, where it has been
found in moderate populations. It feeds readily on larch sawfly cocoons and hence
is an important cocoon predator in the study region. Both forefeet are used to grasp
and raise cocoons to the mouth. The subspecific status of this population is questionable
in the Whiteshell region since the area is the hub of the dines of possibly three
recognized subspecies.
Microtus pennsylvanicus drummondii (Audubon and Bachman),* Drummond's
meadow vole. The meadow vole is found in all the bogs studied, and reaches its highest
population in open sites. It feeds on larch sawfly cocoons and is an important predator
in the study region. Its mode of feeding is similar to that of the red-backed vole.
Family Zapodidae
Zapus hudsonius hudsonius (Zimmermann), the Hudson Bay jumping mouse. This
species occurs in low numbers throughout the area. The general remarks accorded
P. maniculatus may also be applied to this species, except that the population of
Z. hudsonius does not reach the high levels of its cricetid counterpart.
PREDATION UNDER LABORATORY CONDITIONS
Hardy (1939), using field collections, has shown that 46 per cent of the cocoons
of Diprion similis (Htg.) in Poland were opened by small mammals during 1936.
He points out that there is often considerable overlapping of control factors, since
small mammals will prey upon parasitized and diseased cocoons. He suggests that this
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PROCEEDINGS TENTH INTERNATIONAL CONGRESS OF ENTOMOLOGY—VOL. 4. 1956 (1958)
overlapping occurs randomly. Morris (1949), on the other hand, disagrees with this
assumption since he has shown that mammals, depending upon their insectivorous
nature, can differentiate between sound and unsound cocoons of the European spruce
sawfly to varying degrees.
Tests were made using caged mammals to determine to what extent these concepts
arc applicable to small mammal predation on larch sawfly cocoons. The supply of
known cocoon types was obtained by collecting large quantities of fifth instar larvae
and rearing them to the cocoon stage. Those larvae harbouring eggs of the tachinid
parasite, Bessa harveyi (Tnsd.), were reared separately, and constituted the supply of
parasitized cocoons. Cocoons produced by unparasitized larvae were divided into two
classes, determined by pressing them lightly between the fingers. Those in which the
contents felt "hard" were classed as "attacked by fungus". Cocoons that felt pliable
were classed as healthy. Frequent checks by dissection indicated that the separation
techniques were at least 95 per cent accurate. In preparation for the feeding experiment,
cocoons of each type were marked with coloured pigments chosen at random.
Preliminary experiments with the red-backed vole, using cages with about 2 1/4 square
feet of floor space, indicated that the normal feeding behaviour of this rodent was
restricted by the size of the cage. All additional tests were made in cages with about
36 square feet of floor space. The bottom of the cage was covered with about four inches
of moist sphagnum moss and an excess of natural food was supplied so that the
experimental animals would not be driven by hunger to feed excessively on the sawfly
cocoons. The test cocoons were scattered under a layer of moss to simulate natural
conditions.
During these experiments the meadow vole opened 72 per cent of the sound
cocoons, 67 per cent from which B. harveyi had emerged and 19 per cent from which
the sawfly had emerged. The red-hacked vole opened 73 per cent of the sound cocoons,
79 per cent of the cocoons from which B. harveyi had emerged, and 21 per cent of the
cocoons from which the sawfly had emerged. The cinereous shrew opened 91 per cent
of the sound cocoons, 21 per cent of the cocoons from which B. harveyi had emerged,
and no cocoons from which the sawfly had emerged. The saddle-backed shrew opened
97 per cent of the sound cocoons, 10 per cent of the cocoons from which B. harveyi
had emerged, and no cocoons from which the sawfly had emerged. All species unerringly
rejected cocoons attacked by fungus. Shrews opened very few cocoons with B. harveyi
emergence holes, and this was significant at the one per cent level when tested by
means of individual 2 x 2 Chi-square tests. These experiments agree with those conducted by Morris (1949) and provide additional evidence that the degree of rejection of
unsound cocoons is related to the proportion of insect food in the diet of each species.
Morris suggested that living parasites would probably be as acceptable to small
mammal predators as the sawfly itself, but unfortunately he had no source of parasitized
material to test this hypothesis. Parasitized cocoons obtained by the author provided
a means of testing the validity of Morris' suggestion. Table I lists the results of
experiments to determine whether or not the various mammalian species reject cocoons
parasitized by B. harveyi.
TABLE I — Results of Feeding Experiments to Test the Ability of Small Mammals to
Reject Larch Sawfly Cocoons Containing Living•Parasites of Bessa harveyi.
Species
M. pennsvlvanicus
(2)
C. gapperi
(10)
S. cinereus
(20)
S. arcticus
(20)
Type of Cocoon
Sound
Parasitized
Sound
Parasitized
Sound
Parasitized
Sound
Parasitized
*Difference significant at the 3% level
**Difference significant at the 1% level
Bracketed numbers indicate number of animals tested
No. Opened
56
9
591
51
1393
110
1496
349
Total
78
19
780
190
1560
380
1560
380
% Opened
72*
47
77**
27
89**
29
96
92
FOREST ENTOMOLOGY: General Papers
357
This experiment suggests that the red-backed vole and the cinereous shrew reject
cocoons containing living B. harveyi larvae. The discriminating powers of the meadow
vole are doubtful in this respect, and are negligible for the saddle-backed shrew.
The selectivity of the red-backed vole and the masked shrew was tested using various
percentages of parasitized cocoons. Above 50 per cent parasitism, the percentage of
parasitized cocoons opened by the red-backed vole increased, while rejection of parasitized
cocoons by the masked shrew remained relatively constant at various levels of parasitism.
An attempt was made to determine the mechanism governing rejection. Two
hypotheses were tested, namely that rejection is dependent on the size of the parasite,
and that the degree of super-parasitism influences rejection. Accordingly, a series from
the parasitized cocoon stock was dissected, the length of the parasite measured, and
the degree of super-parasitism recorded. Parasitized cocoons rejected by the red-backed
vole and the cinereous shrew were also dissected and the results compared. If one of
these factors had been influencing rejection, its frequency would have risen in the
rejected cocoons. No significant differences could he found in the sample and rejected
cocoons indicated that these factors may not be held responsible for rejection behaviour.
Honing (1955) has shown that the size of a parasite of the European pine sawfly
influenced the degree of selectivity by small mammals. However, the extremely small
size of the parasites used in the present experiment probably preclude degrees of
detection.
In attempting rejection experiments, the duration of the trial is important. If the
experiment is to yield valid results, the trial should not be extended beyond a period in
which starvation causes the experimental animals to feed indiscriminately on the
cocoons. During the author's experiments it was concluded that 24 hours in the case of
rodents, and 8 hours in the case of the insectivores constitutes a fair trial. This will give
an approximate indication of the capacity of each species'in destroying cocoons.
It appears that the rodents could open about 200 cocoons in a 24-hour period and
that the cinereous and saddle-backed shrews could open about 500 and 800 respectively
over the same period.
The ability to distinguish cocoons opened by mice from those opened by shrews
is important if any degree of accuracy is to be obtained in assessing control of the larch
sawfly by small mammals using cocoon analysis. Graham (1929) has suggested that
it is possible to determine whether a cocoon of the larch sawfly has been opened by a
mouse or by a shrew by examining the opening. Morris (1949) however, has claimed
that this is not possible with cocoons of the European spruce sawfly. In order to test
this possibility for the larch sawfly, 20 cocoons known to be opened by mice and 10
cocoons known to be opened by shrews from the preceding experiment were examined
by 25 independent observers using the criteria that cocoons opened by shrews have
serrated edges and often have a cap or "lid" over the opened end. Cocoons opened by
mice have scalloped edges. Shrews usually make only one opening in the cocoon, but
may make a second opening. Mice however may open a cocoon in three or more places.
Fig. 1 illustrates these differences.
Cocoons of the larch sawfly showing differences in openings made by small mammals.
From left to right, a sound cocoon, a cocoon with scalloped edges characteristic of mouse
predation, a cocoon with serrated edges characteristic of shrew predation, and a cocoon illustrating
the "cap" sometimes left by shrews.
Fig. 1.
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PROCEEDINGS TENTH INTERNATIONAL CONGRESS OF ENTOMOLOGY—VOL.
4, 1936 (1958)
It was found that untrained persons could distinguish cocoons opened by mice
with 85 per cent accuracy (with a confidence interval of the mean at the 95 per cent
level from 84.54 to 85.46 per cent) and cocoons opened by shrews with 83.6 accuracy
(confidence interval of mean 82.81 to 84.39 per cent). Evidence indicates that proficiency
in determining cocoons opened by these two mammalian orders may be increased by
experience.
PREDATION UNDER FIELD CONDITIONS
Small mammal predation on larch sawfly cocoons under field conditions was
estimated for four years on three plots using a modification of the "cocoon planting
technique" employed by Graham (1928). He planted five cocoons per set, all enclosed
in a small cotton bag. These were examined at intervals and the percentage of
predation recorded.
The present author wired two cocoons about two inches apart on tree tags and
these were buried to a depth of about two inches with the top inch projecting from
the soil in order to locate the tags during subsequent examinations. One hundred sets
were planted on each plot. These were examined at intervals and the individual
cocoons recorded as being untouched, attacked by fungus, parasite emerged, opened
by a mouse, opened by a shrew, or removed (presumably by a small mammal).
Cocoons that were missing or opened were renewed after each examination. The plots
studied have been described in some detail (Buckner, 1957b), but brief descriptions
follow:
Plot 1.—This plot was situated in a mixed stand of tamarack and black spruce
about 30 to 40 feet high with a relatively dry floor. The dominant small mammal
species was the red-backed vole except in 1954 when the meadow vole was more
common. The cinereous,shrew was present in low numbers.
Plot 2.—This plot was located in a mixed stand of tamarack and black spruce
about 12 to 18 feet high. The water table was closer to the surface than that on Plot 1.
The meadow vole was the common small mammal species and the cinereous shrew
was present in low numbers.
Plot 3.—This plot was located in a pure tamarack stand. The trees were about
40 feet in height. The water table was very close to the surface in 1952 and 1953 but
fell considerably thereafter because of a drainage operation (Buckner, 1955a).
The cinereous shrew was the dominant mammal during the period of high water levels,
but appears to have been replaced by the saddle-hacked shrew and low numbers of the
red-hacked vole in the subsequent period.
By using the cocoon planting technique, it was found that small mammals prey
upon larch sawfly cocoons during a limited season. It was formerly supposed that this
insect was particularly vulnerable to mammalian predation because it remained in the
ground for some 10 months of the year, thus affording an extended period in which
small mammals could prey upon it. However, it is apparent that light predation begins
in August, (or perhaps earlier in some seasons) reaches its greatest intensity during
September, and then slowly declines until November (Buckner, 1953). After this,
little or no predation occurs on this generation of cocoons. Thus small mammals prey
upon the larch sawfly to a limited extent in late August, reach a climax in September,
and then slowly relax their predation pressure until the ground is frozen. This constitutes
the entire foraging period until the following August when mammalian predators again
begin to destroy cocoons. Observations suggest that small mammals prey upon cached
cocoons during the winter months. However these have already been removed from
the field population, and once cached are potentially destroyed.
The planting technique is also useful in determining the extent of predation in
various field situations. It was found that cocoons planted beside the roots of trees and
in hummocks in the bogs were heavily preyed upon by small mammals. On the other
hand, cocoons in the low lying places between the hummocks were frequently missed
by mammalian predators especially when these locations were at or near the water
table. These observations have been substantiated by persons collecting cocoons in
these situations.
FOREST ENTOMOLOGY: General Papers
359
The effect of cocoon depth on predation may also be tested by means of the
cocoon planting technique. Accordingly, cocoons were planted at various depths in
the moss and the predation assessed. One hundred sets (200 cocoons) were planted
at each depth and the results are recorded in Table II. The water table was relatively
TABLE II — Small Mammal Predation at Various Depths as Assessed by
the Cocoons Planting Technique.
Depth in
Inches
% Predation—Plot 1
M
S
T
2
60
51
69
9
5
14
4
6
8
10
12
M— % opened by mice
S — % opened by shrews
T — Total % predation
Heavy line - 77
58
81
9
6
14
17
7
12
0
1
0
% Predation—Plot 3
T
M
S
% Predation—Plot 2
T
M
S
6
9
88
77
59
22
0
0
4
2
0
0
5
11
9
8
12
9
94
86
63
24
0
0
93
90
90
81
86
88
88
79
81
73
74
79
approximate depth of water table
close to the ground on Plots 1 and 2 and the extent of predation diminished as the
water table was approached. On Plot 3, the water table was considerably deeper, and
predation was not influenced by depth. This experiment suggests that small mammals
operate uniformly with depth except where the water table prohibits their activities.
However, the insect is probably prevented from spinning a cocoon under these con,
ditions, unless the water rises after the cocoons have been forme3.
The most important feature of the cocoon planting technique is that it provides
a measure of small mammal predation on natural populations of larch sawfly cocoons.
Most small mammals leave the opened cocoon on the tag, so it is possible to determine
the order of the predator that has opened the cocoon. The results of four years of
predation studies are listed in Table HI. These data illustrate the importance of small
TABLE III — Percentage of Cocoons Opened by Small Mammals as
Determined by the Planting Technique.
Plot
Date
% Opened by Mice % Opened by Shrews Total
1
1952
1953
1954
1955
43
60
14
11
14
17
23
43
57
77
37
54
2
1952
1953
1954
1955
28
6
36
22
41
88
43
54
69
94
79
76
3
1952
1953
1954
1955
17
9
8
6
32
89
78
86
49
98
86
92
mammals as predators of the larch sawfly in the study region and emphasize the
proportional effect of shrews in reducing the population. The greater intensity of
predation by shrews is further emphasized by pointing out that in most cases (with the
notable exception of Plot 3), mice outnumber the shrews by at least two to one
(Buckner, 19571)).
GENERAL DISCUSSION
The results of this study indicate that small mammals are important natural control
agents operating against the larch sawfly in the study region. It has been shown that
shrews are especially beneficial in destroying larch sawfly cocoons because of their
voracious appetites. In addition, at least one species of shrew and one species of mouse
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4,
1956 (1958)
reject some cocoons containing parasitized larvae. Also, cocoons attacked by fungus
organisms were rejected unerringly by all mammalian species tested. Hardy (1939)
suggested that small mammal predation encroached randomly onto other natural control
factors. This eliminates some of the overlap of various control factors operating against
the host insect. The results of this experiment however, coupled with data presented
by Morris (1949), indicate that the interaction of other biological control factors
and predation by small mammals is considerably less than that suggested by the
random concept.
From these experiments it is possible to rate the various mammalian species
according to their relative effectiveness as predators of the larch sawfly. The field vole
and the red-hacked vole have a daily cocoon capacity of about 200. The red-backed
vole, however, rejects almost twice as many parasitized cocoons as the meadow vole.
Considering a hypothetical case in which parasitism is 50 per cent, the red-backed
vole would destroy daily about 140 sound and 60 parasitized cocoons. The field vole
in the same situation would destroy about 120 sound and 80 parasitized. The cinereous
shrew would destroy 380 sound and 120 parasitized cocoons while the saddle-backed
shrew would account for about 400 each of the parasitized and unparasitized cocoons.
This reasoning of course makes no attempt to relate searching ability of the various
predators, or to account for differences due to varying cocoon or predator densities.
It does, however, illustrate the probable superiority of the insectivores as predators,
the cinereous shrew from its powers of discrimination and the saddle-hacked shrew
from its potential cocoon capacity. Since each plot studied has a predominant small
mammal species, the effect of each predator can he observed under natural conditions.
The effects due to the feeding capacities have already been noted. However, no concrete
evidence has been brought forward to show a changing parasite complex in the field
due to the rejection behstviour.
This paper has been concerned only with small mammal predation on the cocoons
of the larch sawfly. The insect is also vulnerable to predation when it falls from the
host tree as a mature larva but before it spins a cocoon. Inadequate data prohibit a
discussion of this phase of the study at the present time.
The full importance of mammalian predation upon larch sawfly cocoons will not
be realized until the relationship between predator and prey populations and the
resultant utilization of the prey insects by the predator is established. Preliminary
studies indicate that the rodents do not exhibit any obvious relationship in this respect.
This might he expected since rodents are presumably chiefly vegetarian, and their
utilization of insect food under these circumstances would he somewhat haphazard.
Shrews, being chiefly insectivorous, should show a greater density relationship. This
of course would be influenced by the abundance of alternative food supplies, and the
minimum sawfly cocoon populations which would support active shrew predation.
Investigations on this phase of the problem have been delayed because of inadequacies
in the existing cocoon sampling technique. Gross estimates however intimate that there
is a clearer relationship between shrew populations, sawfly cocoon populations, and
shrew predation than there is with the comparable rodent series.
REFERENCES
Anderson, R. M. 1946. Catalogue of Canadian recent mammals. Can. Dept. of Mines and Res.,
Nat. Mus. Can. Bull. No. 102.
Buckner, C. H. 1953. Small mammals as predators of the larch sawfly. Can. Dept. Agr. For. Biol
Div. Bi-Monthly Prog. Rept. 9(6) :2.
Buckner, C. H. 1955a. Small mammal populations on a changing site. Ibid. 11(3) : 2.
Buckner, C. H. 1955b. Small mammals as predators of sawflies. Canadian Ent. 87:121-123.
Buckner, C. H. 1957a. Home range for a specimen of Synaptomys cooperi cooperi (Baird).
J. Mammal. 38(1):132.
Buckner, C. H. 1957b. Population studies on some of the small mammals of southeastern Manitoba.
Ibid. 38(1):87-97.
Elton, C. S. 1942. Voles, mice and lemmings. Oxford Press.
Graham, S. A. 1928. The influence of small mammals and other factors upon the larch sawfly
survival. J. Econ. Ent. 21:301-310.
Graham, S. A. 1929. The larch sawfly as an indicator of mouse abundance. J. Mammal. 10:189-196.
Hamilton, W. J., Jr., and David B. Cook. 1940. Small mammals and the forest. J. For. 38:468-473.
FOREST ENTOMOLOGY: General Papers 361
Hardy, J. E. 1939. Natural control of Diprion similis Htg. in Poland during 1936. Bull. Ent. Res.
30:237-246.
Holling, C. S. 1955. The selection by certain small mammals of dead, parasitized, and healthy
prepupae of the European pine sawfly, Neodiprion sertifer (Geoff.). Can. J. Zool. 33:404-419.
Morris, R. F. 1949. Differentiation by small mammal predators between sound and empty cocoons
of the European spruce sawfly. Canadian Ent. 81:114-120.
DISCUSSION
Has the speaker any evidence that the small mammals feed upon
the falling full-fed larvae before spinning of cocoons?
C. H. BUCKNER. Small mammals have been demonstrated to feed upon larvae that
have not yet spun cocoons by staking out larvae in a similar manner to cocoon "planting".
A small percentage of these were taken. Animals in cages readily fed upon uncocooned
larvae, but sawfly remains in stomachs examined during the larval drop period were rare.
C. M. BAETA NEVES. Je veux savoir si dans la determination des numeros qui font
part du tableau III l'auteur a employe des methodes des statistiques.
C. H. BUCKNER. In the preparation of Table III statistics were not employedol
but the data are amenable to statistical analysis and have been analysed by statistical
methods in earlier reports.
M. L. PREBBLE.