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Control of the Invasive Black Rat (Rattus rattus) and Pacific Rat (Rattus exulans) Using a Large
Scale Trapping Grid for Endangered Tree Snail and Plant Conservation in Hawaii
Stephen M. Mosher,1,2 Darren Peters,3 Lindsay Wilson4, Jobriath L. Rohrer1,2, and Aaron Shiels5
1Pacific
Cooperative Studies Unit, Botany Department, University of Hawaii at Manoa, Honolulu HI 96822 USA
2U.S. Army Garrison, Environmental Division, Directorate of Public Works, Schofield Barracks, HI 96857 USA
3Department of Conservation, National Office Predator Control, Wellington NZ
4Department of Conservation, Te Anau NZ. 5Botany Department, University of Hawaii at Manoa, Honolulu HI 96822 USA
INTRODUCTION
Achatinella mustelina
Introduced rats (Rattus sp.) in Hawaii are known predators of birds, tree snails, and plants. Since 1997, the Oahu Army
Natural Resources Program has been controlling rats through the use of diphacinone rodenticide in bait stations and snap
traps on a relatively small scale at multiple sites for the protection of endangered plants and animals. In May 2009, as an
alternative method of rodent control that did not require toxicants, a large scale trapping grid was initiated over a 26-ha
forested management unit with 440 snap traps on the island of Oahu. New Zealand Department of Conservation current
best practice rat trap technology was utilized for the first time in Hawaii with this trapping effort.
Goals of this study:
1) to reduce rat numbers to a level that significantly benefits endangered and other native species
2) to monitor endangered resources and forest health
3) to determine whether rats may be suppressing invasive slugs and predatory snails (Euglandina rosea)
Diospyros sp.
Cyanea superba subsp. superba
METHODS
Study Sites: Rodent control was conducted at the Kahanahaiki Management Unit in the northern
Waianae Mts. (Figure 1). Resources were monitored at Kahanahaiki, and at an adjacent site in the
Pahole Natural Area Reserve where there was no rat control (Table 1).
Rodent Control & Monitoring:
 440 Victor snap traps in wooden boxes (Traps: 234 perimeter (12.5m) & 206 interior (25m).
 Traps initially checked daily for 2 weeks then every 2 weeks thereafter.
 Traps baited with peanut butter or FeraFeed with a half of a Macadamia nut.
 Rat activity was measured monthly with 38 tracking tunnels divided among 3 zones at Kahanahaiki
and quarterly with 30 tracking tunnels along two drainages at Pahole (Figure 1).
Cyanea superba subsp. superba Fruit Predation Monitoring:
 Fruit predation monitored approximately every 2 days, December through January.
 Achatinella mustelina Monitoring:
 Annual census of live snails & quarterly ground shell plot monitoring for predated shells.
 Seed Rain Bucket Monitoring: (48 randomly placed & 10 under native Diospyros sp.)
 Buckets checked every 2 weeks.
 Seedling Plot Monitoring: (Kahanahaiki 80 plots & Pahole 65 plots)
 Plots read at 6 month intervals
Arthropod Composition & Abundance Monitoring: (16 pitfall traps & 32 vegetation beating samples)
 Sampling every 6 months.
 Slug & Euglandina rosea Monitoring: (Slug: 40 pitfall traps & Euglandina: 10 timed search plots)
 Sampling every 3 months.
Figure 1. Kahanahaiki Management Unit
Table 1. Management Actions
Kahanahaiki
Pahole
Rat Control
Yes
No
Rat Tracking Tunnel Monitoring
Yes
Yes
Oahu Tree Snail (Achatinella mustelina) Monitoring
Yes
No
Cyanea superba subsp. superba Fruit Predation Monitoring
Yes
Yes
Seed Rain Bucket Monitoring
Yes
No
Seedling Plot Monitoring
Yes
Yes
Euglandina rosea Monitoring
Yes
Yes
Slug Monitoring
Yes
Yes
RESULTS
Rodent Control & Monitoring:
 562 rats caught (May 2009 - January 2010).
Rat activity in tracking tunnels peaked in late fall at
Kahanahaiki (Figure 2).
There was no difference in tunnel activity during the
two sampling periods for Kahanahaiki and Pahole
(Figure 3).
Cyanea superba subsp. superba Fruit Predation
Monitoring
 30+ plants monitored per site.
 Fruit predation was statistically significant. More fruit
were predated at Pahole (no rat control) (Figure 4).
χ2 = 97.786, p = 0.000
Figure 4. Fruit predation at Kahanahaiki and Pahole.
Figure 2. Kahanahaiki rat captures & tracking tunnel activity May
2009 – January 2010
Figure 6. Pitfall trap data for slugs.
Seed Rain Bucket Monitoring
 Seed predation of introduced Psidium
cattleianum (Strawberry guava) generally reflected
rat activity (Figure 2, 5). No observed predation
on native Diospyros sp. (Lama) (Figure 5).
Invasive Psidium cattleianum (Strawberry Guava)
26
80
43
60
2
98
132
40
151
20
79
62
63
64 497 144 1003
492
110
245
9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 0
-0 -0 -0 -0 -0 -0 -0 -0 -0 -0 -0 -0 -0 -0 -0 -1
ay May Jun Jun -Jul -Jul Aug Aug Sep -Oct -Oct Nov Nov Dec Dec Jan
M
- - 1 14 1- 1- 2- 6 0 3- 7- 1- 5- 2- 2
1
1 1
2 3 2
14 28 10 17
Percentage of rodent-chewed seeds
Percentage of rodent-chewed seeds
100
A
0
B
80
60
Figure 7. Timed ground search survey data for Euglandia rosea
40
20
0
2
1
5
3
15
5
24
6
10
1
2
0
0
2
09 09 09 09 09 09 09 09 09 09 09 09 09 10
n- n- l- l- g- g- p- t- t- v- v- c- c- nJu -Ju 1-Ju 4-Ju -Au -Au -Se 6-Oc 0-Oc -No -No -De -De -Ja
3 17 1 15 12
1 21 31 22
2
10 17
Sampling date
Achatinella mustelina Monitoring
 ~11 ha were surveyed with 212 snails
counted in August 2009.
 Ground Shell Plots revealed no predated
shells.
P>0.05,
(P > 0.05)
T-Test
Native Diosyros sp. (Lama)
100
Figure 3. Rat activity in tracking tunnels during two quarters (August
& December) at Kahanahaiki and Pahole.
Slug & Euglandina rosea Monitoring
 Slug monitoring suggests that large scale rat control
does not increase slugs (Figure 6).
 Timed search survey data for Euglandina showed no
significant difference between sites (Figure 7).
Sampling date
Figure 5. Seed bucket results from Kahanahaiki for (A) Strawberry Guava (48 buckets) and
(B) Lama (10 buckets). Total number of seeds (chewed and unchewed) above each point.
Arthropod Composition & Abundance
Monitoring
 Arthropod samples are in the processing stage with 61
families in 17 orders identified at present (May-June).
Seedling Plot Monitoring
 313 seedlings were marked in 80 plots at Kahanahaiki and 140 seedlings marked in 65 plots at
Pahole. Plots will be read for the second time in February-March 2010.
CONCLUSIONS and FUTURE DIRECTIONS
 The rat control grid appeared to be effective in reducing the amount of predation on Cyanea superba subsp. superba fruits at Kahanahaiki (Year 1).
Rats were frequently captured, and tunnels tracked, in interior locations of the trapping grid when distances from edge to interior were less than 100m. Tunnels
appeared to track the natural cycle of rat activity outside of the grid. A more frequent trap check interval may be needed.
 Over time, data collection of annual tree snail counts, seedling plots, arthropods and Cyanea superba subsp. superba fruit predation will give us a better
understanding of what rat activity thresholds must be met to maintain and increase rare and common native species.
Data collected on slug and Euglandina rosea numbers suggests that rat removal does not cause increases in these highly invasive species.
 Bait consumption by invasive slugs poses a hurdle that still needs to be overcome. Alternative baits are currently being pursued (wax baits and scented lures).
ACKNOWLEDGMENTS
We would like to thank the many members of the Army Natural Resources Staff that have help with checking trap boxes, as well as, Stephanie Joe for slug and Euglandia rosea data collection, Richard Pender and Lalasia Bialic-Murphy for
Cyanea superba subsp. superba data collection, Vincent Costello for Achatinella mustelina data collection, and Paul Krushelnycky for arthropod data collection. We thank Talbert Takahama for permission to collect data in the Pahole Natural
Area Reserve.