Report 16
Control of Kangaroo Rats
With Poison Baits
Range Improvement Task Force
Agricultural Experiment Station
Cooperative Extension Service
ABSTRACT
Contents
Description of study area . . . . . . . . . . . . . . .
Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Results and discussion . . . . . . . . . . . . . . . . .
Kangaroo rat control . . . . . . . . . . . . . . . . .
Non-target animals killed . . . . . . . . . . . . . .
Other observations . . . . . . . . . . . . . . . . . .
Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Literature cited . . . . . . . . . . . . . . . . . . . . . . .
1
2
2
2
2
2
4
5
Kangaroo rats (Dipodomys spp.) were treated
with three poison baits in tobosa (Hilaria sp.) grama (Bouteloua spp.) grasslands in southern
Hidalgo County during 1981 and 1982. Two
strychnine-oat baits (0.50% A1 and 0.16% AI)
and zinc-phosphide (2% AI) pelleted bait were
used in each of four treatment periods.
Treatments were hand applied to 100 active
kangaroo rat mounds for each bait in September
1981, and January, April and June, 1982, to
determine seasonal control levels and differences
between baits during each treatment period. The
best overall period of control occurred in June
when considering all three baits (74.9%). The
highest rate of control (88.3%)was with 0.50%
strychnine oats in June. When considering all
baits, January produced the second highest control percentage of (72.8%).April produced the
lowest levels of control for all baits. Overall,
0.50% strychnine oats gave the best control (avg.
80.4%) followed by zinc-phosphide pelleted bait
(55.5%) and 0.16% strychnine oats (53.5%).
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January 1984
Las Cruces, New Mexico
Control of Kangaroo Rats
With Poison Baits
V. W. Howard, Jr. and Michael Bodenchuk*
Kangaroo rats are, in many areas of the
Southwest, serious range pests. Wood (1969)
found that Ord's kangaroo rats (Dipodomys o r
dii) ate about 1300 lbs air-dry of plant
materiallsectionlyear based on average population densities on the Jornada del Muerto. He also
found that banner-tailed kangaroo rats (D.spectablis), at average densities in the same area, ate
an additional 336 lbs air-dry of plant
materiallsectionlyear. Additional to these
amounts are plant materials stored underground
(2.9 tonslsectionlyear for D. spectablis alone) and
forage production lost by rodent mounds (10.6%
of the excellent condition rangeland).
Poison baits are an efficient mechanism to control rodents on rangelands. In 1964 the Leopold
Committee recommended compound 1080 be
banned as a rodenticide on rangelands because
of secondary hazards. This left strychnine as the
only rodenticide labelled by the Environmental
Protection Agency (EPA)for range rodents control. Personnel a t the Denver Wildlife Research
Center (DWRC) subsequently found that
strychnine poses hazards to humans and nontarget wildlife. The EPA is now reviewing all
strychnine labels under the Rebuttable Presumption Against Registration (RPAR)process. Suggested modifications would cancel the current
strychnine (0.50%) label and require a new
(0.16%)label registration. This concentration is
untested for rangeland rodent control. Furthermore, chemical data indicate better toxicants
(zinc phosphide) may be available.
Wood (1965) investigated control of desert
rangeland rodents by various techniques using
*Professor, Department of Fishery and Wildlife Sciences and former
Range Specialist, N.M. Department of Agriculture. respectively.
compound 1080 and 0.50% strychnine treated
oats. He determined that den baiting was the
most effective technique for kangaroo rats with
both toxicants. The efficacy of zinc phosphide
and 0.16% strychnine oats in field applications
on kangaroo rats is unknown.
The objective of this study was to determine
the efficacy of zinc phosphide rodent bait and
two concentrations of strychnine treated oats on
kangaroo rats during different seasons of
treatment.
DESCRIPTION OF STUDY AREA
Study plots were located on the Gray Ranch
approximately 20 miles south of Animas,
Hidalgo County in southwestern New Mexico.
The area is a broad, gently sloping valley with
mountain ranges to the east and west. Maker et
al. (1970) describes the climate for the area as
continental, arid to semi-arid, characterized by
light (10to 20 in) precipitation and large dirunal
and annual temperature changes, distinct
seasons, plentiful sunshine and low humidities.
Half the annual rainfall occurs from July to
September as brief, but often heavy, showers and
thunderstorms. Average annual snowfall is less
than 10 in., but more than 16 in. may fall in the
surrounding mountains. Average annual
temperatures are between 58 and 62 degrees F
with daily ranges in excess of 30 degrees. Mean
maximum and minimum temperatures are 73
and 42 degrees F, respectively.
Soils in the area are the Eba-Cloverdale-Eicks
association which support a good' cover of
. vegetation used by livestock and wildlife. Principal grasses include tobosa (Hilaria mutica),
'
fluff-grass (Erioneuronpulchellum),black grarna
(Bouteloua eriopoda), blue grama (B. gracilis),
sideoats grarna (B, curtipendula), three awns
(An'stida spp.),vine mesquite (Panicurnobtusum)
and ring muhly (Muhlenbergia torreyziz. Common
shrubs are mesquite (Prosopsis juliflora), fourwing saltbush (Atriplex canescens), white thorn
(Accacia sp.) and mormon tea (Ephedra sp.).
Thin, scattered stands of cholla cactus (Opuntia
sp.) and sachuista (Nolina sp.) occur on the upper
slopes near the outer fringes of this association.
METHODS
The field research was conducted during
September 1981, and January, April and June,
1982. Plots for each of the three treatments were
randomly assigned during each treatment period.
Each plot was large enough to include 100 active
rat mounds. The plots were adjacent to each
other to include similar terrain and vegetation,
but were separated by a 200-ft buffer strip to
reduce possible crossing-over by kangaroo rats
while gathering food. Plot corners were marked
with a 3-ft wooden stake painted blaze orange
for ease of location. Each mound within the plots
received approximately 2 teaspoons of bait. The
three baits included 0.50% active ingredients
(AI)and 0.16% A1 strychnine treated oats, and
2% A1 zinc phosphide rat bait. Each bait was
hand distributed in two areas on opposite sides
of 100 mounds per treatment. Each treated
mound was marked with surveyor's wire and
flagging. A different color was used for each
treatment to facilitate relocation of mounds and
to separate data. All treatments remained in
place for a minimum of 7 days before the mounds
were checked for signs of kangaroo rat activity.
These signs included fresh tracks, dropping and
mounding activity. Presence of spider webs in
burrow entrances also indicated no recent activity by kangaroo rats. Whenever no recent signs
of activity were noted and bait was missing, the
rat(s) occupying the mound were classified as
dead. I t was assumed the difference in death
rates among mounds treated with the three baits
was the result of difference in baits. Death losses
from other causes were assumed to be similar
among the three treatments. Additional data
were recorded for each treatment. These included incidence of precipitation between treatment
and checking for rates of control, presence of
dead rats above ground, presence of dead nontarget species, and digging in mounds by
scavenging carnivores.
RESULTS AND DISCUSSION
Kangaroo Rat Control
The results of the four seasonal treatments are
shown in table 1. Data in this table indicate
0.50% strychnine treated oats had the highest
average control rate and the highest control rate
during each of the four treatment periods. Zinc
phosphide rodent bait gave slightly better
results than did 0.16% strychnine oats, but
results were variable with season. Neither of
these treatments was as effective as the 0.50%
(AI) strychnine oats.
Overall, June was, the best period to treat
kangaroo rats. Both 0.50% strychnine oats and
zinc phosphide rodent bait had their highest control percentages during this period (88 and 70%
control, respectively).The 0.16% strychnine oats
were most effective in January, the second most
effective treatment period of this study. April
was the least effective treatment period as all
three baits were least effective during this treatment period.
Non-target Animals Killed
Relatively few non-target species were observed dead on the study plots while checking
mounds to determine the efficacy of each treatment. During the September treatment, one
grasshopper mouse (Onychomys sp.) was found
on the plot treated with 0.16% strychnine oats.
During the January treatment, one male horned lark (Eremophila alpestris) was found on the
plot treated with 0.16% strychnine oats and one
silky pocketmouse (Perognathus flavus) was
found on the plot treated with 0.50% strychnine
oats. No dead non-target animals were found on
any of the treated plots in April or June. The oats
were apparently more attractive to some nontarget rodents and seed-eating birds than was
the zinc phosphide treated rat pellets.
Other Observations
In September and January, light to moderate
rain fell on the plots between application of the
baits and subsequent readings for efficacy. No
rain was recorded during either April or June
sampling periods. Other observations such as
numbers of mounds dug into by scavenging
mammals, numbers of mounds with'bait remaining and numbers of dead kangaroo rats are
shown in table 2. Most scavenging occurred dur-
Table 1. Results of experiments to control kangaroo rats in southern Hidalgo County, 1981-82
Treatment
Treat.
September 1981
Read
Actv.
Contr.
Treat.
January 1982
Adv.
Read
Contr.
Treat.
April 1982
Read
Actv.
Contr.
Treat.
June 1982
Read
Actv.
Contr.
Mean
Contr.
no.
no.
m.
%
no.
no.
no.
%
no.
m.
no.
%
no.
no.
no.
%
Zinc-Phoshide
Rodent Bait
100
96
51
46.9
100
99
42
57.6
100
87
47
46.0
100
98
29
70.4
55.2
0.16%
Strychnine Oats
100
95
45
52.6
100
92
23
75.0
100
90
74
17.8
100
99
33
66.7
53.0
0.50%
Strychnine Oats
100
94
17
81.9
100
99
14
8'5.9
100
90
32
64.4
100
94
11
88.3
80.1
Average
60.5
72.8
42.7
%
75.1
Table 2. Incidental observations during experiments to control kangaroo rats in southwestern Hidalgo County, 1981-82
September 1981
'
Dug
Into
Treatment
Rat
Carcasses
January 1982
With
Bait
Dug
Into
Rat
Carcasses
April 1982
With
Bait
Dug
Into
......................................................................
June 1982
Totals
Rat
With
Dug
Rat
With
Dug
Rat
Carcasses
Bait
Into Carcasses
Bait
Into Carcasses
number----------------------------------------------------------------------
With
Bait
Zinc Phosphide
Rodent Bait
0
0
0
0
1
7
0
0
20
22
12*
0
22
13
27
0.16%
Strychnine Oats
0
2
0
2
0
18
1
0
1
18
1
0
21
3
19
0.50%
Strychnine Oats
0
2
0
2
2
4
2
0
2
8
0
9
12
4
15
Total
0
4
0
4
3
29
3
0
23
48
13
9
*Partial carcasses only.
ing the June treatment and was done by coyotes
(Canis latrans).This activity coincides with the
time adult coyotes would require large amounts
of prey to feed rapidly growing pups. No dead
coyotes were found on or around the study plots.
A high incidence of zinc phosphide bait being left
outside the burrows coincided with the low control percentage in April. However, this was not
true for 0.16% strychnine oats in January when
18mounds had bait remaining (table2), but there
was a 75% control rate recorded for the study
plot (table 1).This control rate was the highest
recorded for any season for this bait. I t appears
that some mounds believed to be active when
baits were applied were either inactive or were
receiving some use by kangaroo rats from
neighboring mounds on that particular study
plot. Similarly, there were nine mounds with
0.50% strychnine oats remaining in June (table
2) and simultaneously the highest percentage
(88.390)of control (table 1)was recorded for any
season with that bait.
SUMMARY AND CONCLUSIONS
Wood (1965) investigated control of desert
rangeland rodents in southern New Mexico by
various techniques using compound 1080 and
strychnine treated oats. He determined that den
baiting was the most effective technique for control of kangaroo rats with both toxicants. Wood
(1969)also determined that 12 species of range
rodents in southern New Mexico were removing
almost 4 tons of vegetation per section per year.
The banner-tailed kangaroo rats alone, in the
climax grassland areas, kept 10% of the area out
of vegetative production by denuding the areas
of their mounds. Two kangaroo rat; species and
two woodrat species were found to have similar
ranges b u t different food and shelter
requirements and, as a result, were able to more
completely utilize the food resource with little
competition for any specific food item. Food
items stored in banner-tailed kangaroo rat dens
included perennial grasses (18%),annual grasses
(23%),snakeweed (Gutierreziasp.) (43%)and pepper grass (Lepidium sp.) (7%).
Athough rodents and livestock are not in
direct competition for all their food items, any
competition for perennial grasses or seasonal
forbs would be expected to decrease stocking
rates or prevent recovery of areas where previous
overgrazing had caused a decline in successional
stages. This would be particularly critical where
the carrying capacity for livestock is already low,
or- in areas where rest can enhance pasture
recovery. Recent chemical data indicate zinc
phosphide treated grain or pelleted bait could
prove to be more effective and environmentally
safe. However, zinc phosphide was not registered
for control of kangaroo rats. I t was registered
for orchard mice (primarilyMicrotus sp.), prairie
dogs (Cynomys spp.), nutria (Myocastorcoypus),
pocket gophers (Geomyidae)and rats (Sigmodon
spp. and Rattus spp.) with some in-crop use permitted. Registration for range application required tolerances be set and LD50's be investigated. Personnel at the DWRC reported the
approximate lethal dose (ALD)for banner-tailed
kangaroo rats as 8.0 mglkg body weight. The
currently marketed 2% A1 formulation was
lethal to prairie dogs whose LD50 is 18.0 mglkg.
Therefore, we could reasonably assume the 2%
A1 formulation would be lethal to large portions
of the baited populations of all Dipodomys spp.
if applied at the proper season. Furthermore, the
economy of the control with this product should
depend largely upon the economy of the method
of treatment.
Results of this study indicate 0.50%
strychnine oats is the best of the three baits
tested. Neither the zinc phosphide bait nor the
0.16 % strychnine oats performed well during all
seasons. Average percentages of control were
69% for zinc phosphide rodent bait and 66% for
0.16% strychnine oats, when compared to 0.50%
strychnine oats. Mortality from these baits was
31 to 34% less than 0.5090 strychnine oats, making these alternative baits less desirable for controlling kangaroo rats on western rangelands.
The added cost of a second (follow-up)treatment
would probably add considerably to the cost of
control which could be achieved with a single application of 0.50% strychnine oats.
Because no measurable difference in hazard to
non-target species was observed between the
three baits tested, it would appear registration
of 0.50% strychnine oats can be maintained and
not replaced with one for 0.16% strychnine oats.
In addition, we recommend the label for 2% zinc
phosphide rodent bait be expanded to include
Dipodomys sp. on rangelands.
The EPA strychnine RPAR team requested
our results from the first three seasonal
treatments on 24 May 1982. The Draft Decision
Document from EPA states the data from New
Mexico indicated a less-than-desirable efficacy
with the recommended 0.16% A1 concentrations
and, because the agency had no data indicating
adverse environmental impacts, the present label
(0.50% AI) would be allowed.
/-
LITERATURE CITED
Maker, H. J., D. N. Cox, and J. U. Anderson. 1970.
Soil associations and land classification for irrigation,
Hidalgo County. N. Mex. Agri. Exp. Sta. Res. Rep.
177. 29 pp.
Wood, J. E. 1965. Response of rodent populations
to controls. J. Wildl. Manage. 29(3):425-438.
Wood, J. E. 1969. Rodent populations and their impact on desert rangelands. N. Mex. Agri. Exp. Sta.
Bul. 555. 17 pp.