S Sullivan* Daniel

632.6 Use of

3232 diphacinone—

A3t r87 — 3 "treated bait

blocks for control
of the northern
pocket gopher

MONTANA department OF AGRICULTURE
Environmental Management Division
Technical Services Bureau
Helena, Montana 59620-0205

MONTANA STATE LIBRARY

S 632.63232 A3tr87-3 c.1 Sullivan

Use of diphacinone<treated bait blocks f

3 0864 00061499 3

COLLECTION

D^-CG 1988

MONTANA STATE LIBKaky
E. 6th AVE.

HELENA. MONTANA 59620

Use of Diphacinone-treated Bait Blocks for Control
of the Northern Pocket Gopher

By

Daniel Sullivan and Monty Sullins

ABSTRACT

The efficacy of a paraffinized diphacinone (50 ppm) grain bait
formed in 113 g blocks and a strychnine (500 ppm) hulled oat bait
was tested on the northern pocket gopher. Bait was applied under
field conditions by hand methods. The diphacinone bait blocks
averaged 70.7 percent reduction in gopher activity 28 days after
application. The strychnine bait averaged 78.4 percent reduction
in gopher activity during the same period.

Technical Report 87-3
November 1987

INTRODUCTION

The use of semipermanent baits for control of pocket gophers was
explored by Marsh and Pleese (1960) and more recently by Tumberg,
et al (1984). Their investigations showed initial control of the
resident gopher population and potential for control of
immigrating gophers that fed on bait remaining from the original
treatment. A commercially available semipermanent bait for
gopher control has been registered under an intrastate
registration in California for some time (Marsh 1987). Recently
an attempt has been made to introduce this control method to a
wider market (J.T. Eaton & Co.). A field study was conducted to
evaluate the efficacy of this technique on the northern pocket
gopher (Thomomys talpoides ) in Montana.

OBJECTIVE

The objective of this study was to test the efficacy of a
paraffinized diphacinone (50 ppm) bait block for control of the
northern pocket gopher under field conditions in Montana.

STUDY AREA

Five study plots (4 treatment, 1 control) were selected from
alfalfa and grass hayfields occupied by pocket gophers near
Lewistown, Fergus County, Montana. The study period began
September 20, 1987 and ended October 30, 1987. Below normal
precipitation fell on the study area in August and September and
only trace amounts fell during the study period. As a result,
the soil was hard and compacted and soil moisture was well below
normal. Mound building activity by the gophers occurred in early
September after light rains but virtually ceased duging Octgber.
Temperatures during the study period ranged from 25 F to 75 F.

METHODS

A paraffinized diphacinone (50 ppm) mixed-grain bait formed into
113 g blocks (12x4x2 cm) was tested. A commerial pocket gopher
bait (500 ppm strychnine on whole, hulled oats) was used as an
efficacy standard. Each test was replicated twice.

The diphacinone bait blocks were applied to pocket gopher systems
at sites of most recent mound building activity. One application
of two bait blocks was made per burrow system by excavating a
lateral tunnel back to the main tunnel and inserting one block in
each direction of the tunnel. The opening excavated for bait
block placement was closed with a clod of soil or plug of sod in
a manner that left the tunnel open under the seal. Where the
high density of pocket gophers made it difficult to determine
indiviual systems the maximum distance between bait placement was
30 feet.

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The strychnine bait was applied by hand using a probe to locate a
pocket gopher tunnel. Approximately 5 g of bait was placed in
the tunnel through the probe hole. Two to three bait placements
were made per burrow system.

A 75 foot buffer zone was established around each plot to retard
immigration of pocket gophers onto the study plots. The buffer
area was treated with the same bait as the plot it bordered.

Efficacy was measured using active burrow counts to determine
reduction in pocket gopher activity. A minimum of 20 active
gopher burrows were located per study plot. Pocket gopher
activity separated by a distance of 40 or more feet was
considered a separate burrow system. Burrow systems were opened
and any burrow not closed by gopher activity after 48 hours was
considered inactive.

At 14 and 28 days posttreatment the burrow systems marked active
during the pretreatment census on each study plot were opened.
Each burrow was checked at 24 and 48 hours after opening. Any
opened burrow that remained open was considered inactive as a
result of the bait treatment.

A control plot was monitored for pocket gopher activity in the
same manner as the treatment plots. No bait was applied to the
control plot.

Percent reduction in activity was calculated using the following
formula (Henderson and Tilton 1955) .

Holes Closed Holes Closed

Percent Posttreatment X Pretreatment

Reduction (Treatment Plot) (Control Plot)

in Active = 1 - ^

Burrows Holes Closed Holes Closed

Pretreatment X Posttreatment

(Treatment Plot) (Control Plot)

Time required for bait application and amount of bait applied per
acre were recorded. Application labor and bait cost for the
strychnine and bait block treatments were compared.

Following the 28 day posttreatment census, 20 bait placement
sites on each study plot treated with bait blocks were selected
randomly and excavated. The number of bait blocks remaining at
each site was recorded.

For the purpose of testing methodology, three gophers on one of
the bait block treatment plots were live trapped, fitted with
radio transmitter collars and released at the site of capture.

In addition, radio transmitters were embedded in four bait
blocks. One unmarked and one radio-tagged bait block was placed

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in the burrow system of each of the radio-tagged gophers. The
fourth radio-tagged bait block and an unmarked bait block was
placed in the burrow system of an unmarked gopher. Movements of
the radio-tagged gophers and bait blocks were recorded and
subsequently recovered to determine their fate.

RESULTS

After excavating the bait block placement sites we found that
both blocks had been removed from 35 (87. 5Z) of the sites
sampled. Of the remaining 5 (12. 5Z) sites sampled, one of the
two blocks had been removed. Two of the five remaining bait
blocks were partially consumed.

Reduction in pocket gopher activity on the plots treated with
diphacinone bait blocks averaged 70.7 percent 28 days after
treatment. This compares with an average 78.4 percent reduction
in activity on the plots treated with strychnine baits (Table 1).

We baited an average of 1.5 acres per manhour with strychnine
bait using the hand probe method. The application rate was
1.33 pounds of bait per acre at a bait cost of about $1.65 per
acre. We treated 0.5 acres per manhour with bait blocks. Each
acre averaged 7.5 pounds of blocks (15 sets/acre) at a bait cost
of $7.58 per acre based on suggested retail price (M. Godfrey,
pers comm) .

Each of the four radio-tagged bait blocks were moved from their
initial placement location. Three of the blocks were recovered
14 days after application. The fourth block was not recovered
because of transmitter failure. Two transmitter blocks were
found alone in gopher tunnels and the third was found in a food
cache of alfalfa roots along with an unmarked bait block. Blocks
were moved from 7 to 27.5 feet from their original placement
locations. They were found at depths from 16 to 26 _ inches. _
Recovered blocks had been fed on by gophers and their individual
weights had decreased 30 g (26.5%), 41 g (36.3%) and 48 g
(42.5%). A surface mold covered each of the bait blocks
recovered. The blocks had become softer and crumbly compared to
their original hard, brittle composition and could be crushed
rather easily by hand.

Two of the radio-tagged gophers were recovered dead 14 days after
application. One gopher was found in a nest of dry grass at a
depth of 31 inches. The second gopher was found in a food cache
of alfalfa roots at a depth of 20 inches. Necropsy of both
animals revealed extensive subdermal hematomas and bloody fluid
in the abdominal and thorasic cavities. The third radio-tagged
gopher was kill-trapped 28 days after bait application. The
carcass showed no visual signs of anticoagulant poisoning. ^The
radio-tagged and unmarked bait blocks placed in this gopher s
system had been moved but transmitter failure prevented recovery.

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TABLE 1. Percent Reduction in Pocket Gopher Activity Using
Diphacinone Bait Blocks and Strychnine Grain Bait.

Date

Date

Bait Blocks

Plot No.
1

10/15/^

80.0

Average

10/29/87+

82.4

Average

56.7

70.7

Bait Blocks

2

33.3

59.0

Strychnine

3

55.2

57.6

80.3

78.4

Strychnine

4

60.8

76.5

Control

5

46.4

39.3

* 10/15/87 and 10/29/87 are 14 and 28 days after treatment,

respectively .

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DISCUSSION AND RECOMMENDATIONS

The management strategy for semipermanent baits is to control a
resident gopher and one or more immigrant gophers with one bait
application. This study showed that diphacinone bait blocks and
strychnine baits gave similar efficacy over a short term, but we
did not determine if bait blocks continued control beyond the
resident population. If agricultural producers are to accept
semipermanent baits they must feel confident that the additional
cost and labor results in control of immigrant gophers and gives
a longer period of control. We recommeded that further
investigations be conducted to determine 1) if control of
gophers extends beyond the resident population, 2) how long
residual control lasts, 3) the longevity of the bait blocks
within the environment of a burrow system, 4) the acceptance and
palatability of the bait blocks over time, and 5) the cost and
efficacy of bait blocks compared to currently available control
methods. This information is needed before vertebrate pest
professionals can confidently recommend this method to growers as
an effective and economical control option.

ACKNOWLEDGEMENTS

Financial support was provided by J.T. Eaton and Company and the
Montana Department of Agriculture. The test bait was provided by
J.T. Eaton and Company. George Algard and John Larson reviewed
the manuscript. Ward Burleigh and George Hamilton allowed access
to their properties to conduct the study.

LITERATURE CITED

Henderson, C.F. and W. Tilton 1955. Tests with acaricides
against the brown mite. J Econ Entom. 48 (2) : 157-161 .

Marsh, R.E. 1987. The role of anticoagulant rodenticides in
pocket gopher control. pp . 87-92 Animal Damage
Management in Pacific Northwest Forests, Spokane, WA.

Marsh, R.E. and L.F. Plesse 1960. Semipermanent anticoagulant
baits. The Bulletin 49 ( 3 ) : 195 -197 . Calif. Dept, of Agric.
Sacramento, CA.

Tunberg, A.D. , W.E. Howard and R.E. Marsh 1984. A new concept
in pocket gopher control. Proc. Vertebrate Pest Conf.
11:7-16. Univ. of California, Davis, CA

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