Download 1 The northern pocket gopher (Thomomys talpoides) is a rodent

REQUEST FOR DEROGATION TO USE STRYCHNINE FOR THE CONTROL OF POCKET GOPHERS
The northern pocket gopher (Thomomys talpoides) is a rodent native to the northwestern United
States. Pocket gophers are fossorial animals, spending most of their time solitarily in
underground burrow systems (Engeman and Witmer 2000). Gophers feed on a variety of
vegetation, and can be considered keystone species in some systems due to their ability to
dramatically alter both soil and vegetation structure (Cantor and Whitham 1989, Reichman and
Seabloom 2002). Pocket gophers, like many rodents, have high population growth rates and are
able to quickly colonize and re-colonize suitable habitat even when their populations have been
reduced (Engeman and Campbell 1999, Sullivan et al. 2001).
Pocket gophers account for more damage to regenerating forests in the western U.S. than all
other animals combined (Crouch 1986, Borrecco and Black 1990). Repeated complete failures at
reforestation are not uncommon, sometimes resulting in non-forested conditions lasting for
decades (Barnes 1978). Pocket gophers damage and kill large percentages of conifer seedlings in
Idaho by feeding on their roots and stems (Graham and Kingery 1990, Ferguson and Adams
1994, Ferguson 1999). Regeneration failure is common in areas with high gopher densities.
The Forest Stewardship Council (FSC) mission is “...to promote environmentally appropriate,
socially beneficial, and economically viable management of the world’s forests.” Good forest
management should balance the three legs of FSC’s mission. This mission is further articulated
in FSC’s 10 Principles and Criteria, of which Criteria 6.6 defines chemicals that are banned by
FSC. Banned chemicals are those that are either: 1) WHO Type 1A or 1B (toxic to humans), 2)
chlorinated hydrocarbons, 3) have high persistence (half life >100 days), 4) high toxicity (LD50
up to 200mg/kg), 5) carcinogens, mutagens, or endocrine disruptors, or 6) banned by
international agreement. Although strychnine is banned by FSC due to its potential high toxicity
to humans and other vertebrates, this application for derogation shows that it is the only feasible
method for the control of pocket gophers in regenerating forests. Furthermore, we provide strong
evidence that the application method used for pocket gophers virtually eliminates the risk of
consumption by other vertebrates, and poses no adverse environmental impacts, and therefore its
use in controlling pocket gophers is in the best interest of promoting FSC’s goals.
A variety of methods are available to control pocket gopher damage in regenerating forests,
although all are not equally effective (Case and Jasch 1994, Engeman and Witmer 2000; see
Table 1). Cultural methods include modifying habitat through harvest prescription or site
preparation before regeneration, as well as planting unpalatable or genetically-resistant stock
(Engeman and Witmer 2000). Harvest prescriptions that result in early-seral vegetative
conditions also favor pocket gophers (Marsh and Steele 1992) by creating an abundance of
preferred food composed of grasses, forbs, and conifer seedlings. These prescriptions are often
necessary in order to restore the stand to the proper tree species composition. Harvest
prescriptions that result in natural regeneration often perform better than planted stands (Marsh
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and Steele 1992). However, successful natural regeneration is unpredictable, especially of
ecologically-appropriate tree species. In our area, pocket gopher sites will have natural
Table 1. Summary of relative cost-effectiveness of gopher damage management techniques.
Method
Cost
Effectiveness
Population Reduction
Strychnine Baiting
Zinc Phosphide Baiting
Fumigants
Trapping
Enhance Predation
Low
Low
Low
High (>6x baiting)
Mod
High (70-80% pop. reduction)
Mod
Low (20% pop. reduction)
Mod (45-60% pop. reduction)
Low
Habitat Manipulation
Herbicides
Burning
Sheep/cattle grazing
High
High
Low
Mod
Mod
Mod
Barriers
Plastic tubing
Repellents
High
High
High (some root deformities)
Low
Silvicultural
Natural Regeneration
Mod-High
Planting Resistant Stock
Low
Larger Seedling Size
High
Increase Stocking Rate
High
(Engeman and Witmer 2000, Brian Moser unpublished data)
Mod
Low
Low
Mod
regeneration overwhelmingly dominated by grand fir (Abies grandis), while our ecologicallyappropriate target species mix includes western white pine (Pinus monticola), western larch
(Larix occidentalis), ponderosa pine (Pinus ponderosa), and Douglas-fir (Pseudotsuga
menziesii).
Site preparation such as burning and herbicides can be used to reduce suitable gopher habitat
(Engeman and Witmer 2000) by reducing their food supply. We prefer to use herbicides
sparingly as they can be costly and also have the potential to reduce habitat for other wildlife. In
addition, site preparation methods only suppress vegetation for 2-5 years, after which gopher
habitat becomes suitable again. Currently, no “gopher resistant” tree varieties have been bred,
although some species appear to be relatively more resistant (Ferguson 1999).
Physical controls include both exclusionary devices and traps (Engeman and Witmer 2000).
Vexar tubing has been used to effectively protect seedlings and their roots from gopher damage
(Anthony et al. 1978, Engeman et al. 1997, Engeman et al. 1999, Pipas and Witmer 1999).
However, the extremely high cost of this method precludes its use in most situations. In
addition, the possibility of root deformation and constriction makes it unpractical (Ellis 1972).
Trapping has long been used to control pocket gophers with mixed success (Marsh 1998a,
Witmer et al. 1999). Potlatch research has shown that trapping is only 45-62% effective in
reducing gopher populations, and costs over 6 times that of baiting (Morrow 2004, Potlatch
Corp. unpublished data).
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Biological control through predation has often been hailed as a “natural” approach to pest
management (Moore et al. 1998). Although predator habitat can be enhanced (Moore et al.
1998), no studies have demonstrated effective control of gopher populations through predator
enhancement (Marsh 1998b).
Induced infertility is another method of biological control that is still in its infancy (Miller and
Fagerstone 2000). Although this approach shows promise, there are many biological and
regulatory hurdles before this becomes a cost-effective method to control gopher reproductive
rates (Miller and Fagerstone 2000).
Chemical controls include toxicants and repellents (Engeman and Witmer 2000). Repellents
would appear to be a humane way to reduce gopher damage, although no studies have found a
cost-effective method to do so (Witmer et al. 1998, Sterner et al. 1999), especially since gophers
are fossorial and for a repellent to protect tree roots it must be applied underground and persist
for many months including the spring snowmelt season when a significant flux of water moves
through the soil.
Toxicants are the most cost-effective chemical control, and strychnine alkaloid is the most costeffective gopher control method used in the western U.S. (Engeman and Witmer 2000).
Strychnine is an alkaloid derived from the seeds of Strychnos nux vomica, a native tree of India.
It is an acute toxicant that results in quick death when toxic amounts are consumed.
Strychnine is banned by FSC only due to its potential toxicity to vertebrates. It is not a
chlorinated hydrocarbon, has low persistence, is not a carcinogen, mutagen, or endocrine
disruptor, and is not banned by international agreement. Although strychnine is highly toxic, its
applicator exposure risk is low and it has a low hazard to non-target animals and for secondary
poisoning. Strychnine is applied to grain bait entirely below ground. Application rates range
from 0.25 to 1.0 lbs of 0.5% strychnine bait per acre. Once placed below ground the hole is
covered with soil. Because gophers vigorously defend their burrows, intrusion of the burrow and
consumption of bait by other species is unlikely. Furthermore, caches of bait degrade quickly.
Strychnine alkaloid has a half-life of <30 days, and 90% degradation after 40 days (Starr 1993).
Studies have shown that limited numbers of ground squirrels and other burrowing rodents may
accidentally be poisoned, but that the impact on those populations is minimal (Fagerstone et al.
1980, Anthony et al. 1984, El Hani et al. 2002).
Secondary poisoning can only occur if an animal consumes the digestive tract of a dead gopher,
because strychnine does not accumulate in tissues. In fact, a scavenger would have to consume a
lethal dose at one time, since strychnine does not accumulate over time. Scavenging of dead
gophers is highly unlikely as most gophers die underground (Barnes et al. 1985). The only
animals that might consume them are small mustelids, which are not usually scavengers. Birds
of prey and threatened species such as grizzly bears are unlikely to be affected by strychnine
baiting (Anthony et al. 1984, Barnes et al. 1985).
Strychnine does not persist long in the environment. Furthermore, strychnine alkaloid binds
strongly to soil particles and is not water soluble, thus there is limited movement of strychnine in
the environment (Smith 1982). Vegetation does not take up strychnine in detectable quantities,
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therefore the risk of an herbivore consuming strychnine is small (Smith 1982). Because
strychnine is applied underground and away from water sources, binds to soil particles, and is not
water soluble, the probability of water contamination is nearly zero. If small amounts did reach
water, they would be so diluted as to not be toxic. Since strychnine does not accumulate in
tissues, multiple sublethal doses would have no adverse effects on aquatic flora and fauna.
Both the Clearwater and Nez Perce National Forests have conducted thorough Environmental
Assessments for the use of strychnine baiting on National Forests, as required by the U.S.
National Environmental Protection Act (U.S. Forest Service 1993a,b). Both have concluded that
strychnine is not only the most cost-effective method of gopher abatement, but that it is also an
environmentally-safe alternative.
Without the use of strychnine, we have two alternatives. Either forgo the harvest on nearly 2000
acres each year, or accept failed regeneration on those acres. The latter is simply not possible in
Idaho where forest lands must be regenerated by state law. The former represents a substantial
amount of foregone revenue and over many years puts tens of thousands of acres on a trajectory
for significant forest health issues that may ultimately be manifested in catastrophic wildfire.
Because our acres subject to gopher damage tend to be geographically concentrated, the wildfire
concern is even further heightened. We believe that neither of these alternatives are in the best
interest of achieving FSC’s mission.
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