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Background and Objectives:
Trapping is the only method that effectively controls northern pocket gopher (Thomomys
talpoides) populations (Proulx 1997, 1998). Proulx (1997) recommended that trapping be
carried out in spring, and completed before June, in order to remove breeders and
eliminate the expansion of pocket gopher populations over summer. However, trapping is
time-consuming and labor-intensive, and the use of an attractant would be useful to
quickly remove breeders of the year.
Pheromones are groups of biologically active substances that are secreted to the
outside by an individual, and received by another individual in which they trigger a
specific reaction (Birch 1974). Although the specific functions of such chemical
communication are not well understood, pheromones may play a role in activities such as
foraging, territory maintenance, and reproduction (see Pawlina and Proulx 1999). Proulx
and Cole (1996) found that northern pocket gophers reacted to the presence of conspecific
odors during the reproduction season. A significantly greater number of animals were
captured in adult female-scented traps than in male- and juvenile-scented traps, and
unscented traps. However, Proulx and Cole’s (1996) work was conducted during one
growing season only.
The objective of this study was to assess the ability of female odors to improve the
trappability of northern pocket gophers during the reproduction season.
Procedure and Project Activities:
The study was carried out from April 18 to May 25, 2001 in Sherwood Park, Alberta. One
male and 3 female gophers were live-trapped and placed individually in 66 x 34 x 37 cm
cages in a shed. The animals were subjected to a normal day-night cycle, and fed a daily
ration of vegetables and alfalfa (Medicago spp.) roots. Knowing that male scent may
incite females to produce sexual pheromones (see Pawlina and Proulx 1999), a male was kept
in proximity to females. However, male pheromones are not as effective as female ones to
attract pocket gophers into traps (Proulx and Cole 1996) and, for this reason, male scents
were not collected. The bottom of the cages of the females was lined with absorbing pads.
Urine, feces, and feeding material were in contact with the pads for at least 48 hours.
Scented pads were used immediately after removal from the cages.
Scented pads were evaluated in alfalfa fields with killing box traps from April 27 to
May 25. During each of the April 27 and May 2 tests, only 1 dozen traps with scented pads
and 1 dozen traps with unscented pads were used. During this time of year, it was
difficult to live-capture female pocket gophers, and tests were carried out with pads
scented by one pregnant female only. The limited amount of scented pads explains the
smaller number of traps used during each test. In all subsequent tests, tests involved two
groups of at least 16 traps with scented (from 2 pregnant and 1 non-pregnant females) and
unscented pads. The scenting of traps was done by placing a 3x 6 cm piece of scented pad
behind the trap trigger, on the floor of the pocket gopher’s tunnel. In control
traps, a piece of clean pad was used. Traps were set early in the morning, and visited and
removed the following morning. Traps were therefore set for 1 trapnight (TN) only. The sex
of the captured animals was determined in the field. When no capture occurred, information
on the state of the trap (i.e. plugged, disturbed, etc.) was recorded. Traps were
thoroughly washed between test series.
Scents impact on the trappability of pocket gophers only during a short period of the
reproduction season (Proulx and Cole 1996). In this study, this impact was monitored after
each trapping session using the ratio of captures in scented traps to captures
in unscented traps. A ratio of 1 was obtained when an equal number of captures was
recorded in scented and unscented traps. When the ratio < 1, the scented traps captured
less animals than the unscented ones. A ratio > 1 indicated that scented traps were
more efficient than unscented ones, and therefore had a positive impact on capture
success. Chi-square statistics (Siegel 1956) were used to analyze trapping results and
determine whether there were any differential responses of pocket gophers to scented and
unscented traps.
Results and Discussion:
Figure 1. Ratio of captures of pocket
gophers in scented traps to captures in unscented traps,
April 27 to May 25, 2001 Sherwood Park, Alberta.
During three trapping series carried out from April 27 to May 10, scented traps failed
to outperform unscented traps (Fig. 1). Altogether, these trapping series amounted to 48
TN, with 18 pocket gophers captured in scented traps (0.38 animals/TN) and 23 in unscented
traps (0.48/TN) (Table 1). There was no significant difference (c
2 = 0.39, df:1, P > 0.05) between the number of captures in
scented and unscented traps. Also, both populations had an even sex ratio (c 2 £ 2.78, df:1, P
> 0.05). A total of 8 unscented traps were found plugged compared to 2 scented ones
(Table 1).
From May 15 to May 24, scented traps continuously outperformed unscented ones (Fig. 1).
After 82 TN, there was a significantly greater (c 2
= 5.15, df:1, P < 0.025) number of captures in scented traps (45 animals or
0.55/TN) than in unscented ones (25 animals or 0.30/TN) (Table 1). The sex ratio of both
populations was even (c 2 < 2.56, df:1, P
> 0.05). A total of 5 traps were plugged in each group (Table 1).
From the April 27 - May 10 period to the May 15 - 24 period, the overall number of
captures/TN increased by 45% in scented traps, but decreased by 38% in unscented traps.
Rowe (1970) was among the first to suggest that susceptibility of
rodents to being trapped might be influenced by trap odors, noting that wild male house
mice (Mus musculus) were caught more often in traps marked with odors of
conspecific than in clean, unmarked traps. An increase of capture success due to
conspecific scents was also reported for voles (Boonstra and Krebs 1976, Stoddart 1982,
Heske 1987), cotton rats (Sigmodon hispidus) (Summerlin and Wolfe 1973), California
ground squirrels (Spermophilus beecheyi) (Salmon and Marsh 1989), and other rodents
(Ritter et al. 1982, Stoddart and Smith 1986).
 In this study, increased capture success was due to pocket gopher female scents. This
finding ascertains Proulx and Cole’s (1996) conclusion that female scents attract
pocket gophers and render trapping more efficient during the reproduction season. Proulx
and Cole (1996) found that scented traps removed a significantly greater number of females
than males. In this study, the sex ratio of the captured population was even.
The pocket gopher reproduction season lasts from mid-April to late May (Proulx 2001).
This study showed that the influence of female scents on the trappability of pocket
gophers occurs towards the end of the reproduction season, i.e. after May 10. Proulx and
Cole (1996) also reported that scented traps became more efficient starting May 10. This
difference in the response of animals to scented traps from the beginning to the end of
the reproduction season is not easily explained. During the first weeks of the
reproduction season, most females are pregnant and may not yet be concerned with the
safety of their young. In mid-May, however, most females have given birth (Proulx 2001)
and their maternal protection instinct may result in a thorough investigation of scented
traps, and a greater number of captures. Since most females produce only one litter per
year, males’ breeding activities have likely decreased by mid-May (Proulx 2001).
Males that are still sexually active may be eager to find the origin of an invasive female
scent, and readily enter scented traps. It is also possible that towards the end of the
reproduction season, territorial males become more aggressive and readily enter scented
traps to eliminate an intruder. Detailed behavioral work on pocket gophers’
territoriality and response to conspecifics will be required to explain differences in the
attraction of female scents from the beginning to the end of the reproduction season.
This study showed that northern pocket gophers reacted to the presence of female scents
during the reproduction season, thus resulting in increased trapping success. In light of
this result, it is recommended that this work be repeated next year and, upon a
significant increase in the trappability of pocket gophers, female scents be analyzed for
specific components, e.g. hormones. Such components should then be field-tested to
determine if they have an impact similar to that of female scents. The identification of a
component that can be commercially produced and distributed would significantly improve
pocket gopher control programs.
Acknowledgments:
This study was funded by the Governments of Manitoba and Canada through the
Canada-Manitoba Agri-Food Research and Development Initiative (ARDI), Saskatchewan
Agriculture & Food, and Alpha Wildlife Research & Management Ltd. I am grateful to
George Bonnefoy, Manitoba Forage Council, for inciting me to develop this project; Sid
Zdrill, Saskatchewan Provincial Council of ADD Boards, and Scott Hartley, Saskatchewan
Agriculture & Food, for obtaining Saskatchewan funding for this study; Pauline
Feldstein, Alpha Wildlife Research & Management Ltd., for reviewing an earlier version
of this manuscript; and Daniel G. Proulx and Benjamin P. Proulx for technical help.
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