Final Draft
Christopher Thompson
ENGL 102 sec. 004
Wednesday, September 24, 2008
A Plan for Combating the Hemlock Woolly Adelgid
The hemlock woolly adelgid is threatening the population of eastern hemlocks in the
Eastern United States (Tingley et al. 2002). This Japanese foreign invader (Eschtruth et al. 2006)
is advancing into uninfected forests at a rate of 20-30 km per year (McClure et al. 2001, ctd. in
Ward et al. 2004). Current research shows that the eastern hemlock is an integral component of
the forest ecosystem and that its death has the potential to cause many negative trickle-down
effects on other species and the surrounding environment (Ross et al. 2003). Examples of such
effects include negative effects on stream flow (Ford and Vose 2007), avian populations (Tingley
et al. 2002), fish populations (Ross et al. 2003), and changes to the composition of the forest
canopy (Daley et al. 2007). The mortality of the eastern hemlock will also have adverse effects
on the economies of New England towns that really upon forest resources (Ward et al. 2004).
Due to the far-reaching negative effects of the hemlock woolly adelgid, action needs to be taken
immediately to combat this pest. Based upon current research and the current status of hemlock
woolly adelgid control, several different steps need to be taken. These include further research
on biological control species and increased cultivation of the proposed biological controls
(Preston 2007). Also, individuals and states need to take steps to limit the spread of the hemlock
woolly adelgid (Ward et al. 2004). Applying insecticides may also be used as a method of
combating the hemlock woolly adelgid in a limited number of special situations (Webb et al.
2003; Ward et al. 2004). A plan for reintroducing the eastern hemlock into previously infected
areas will be necessary for fully overcoming the hemlock woolly adelgid’s effects.
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Due to the complex nature of this problem, additional explanation is necessary to fully
understand the far-reaching ramifications of hemlock woolly adelgid infestation. The eastern
hemlock is an important component of the forest ecosystem, and biodiversity is greatly increased
in forest stands where the eastern hemlock is established (Orwig et al. 2002). It is also believed
that species may have evolved alongside the eastern hemlock and are therefore dependent upon
the species for their survival (Ross et al. 2003). Research by Lishawa and colleagues (2007) has
shown that white-tailed deer take shelter in the microclimate induced by eastern hemlocks during
the winter. The mortality of eastern hemlocks would have adverse effects on the white-tailed
deer populations that take shelter under the tree during winter months (Lishawa et al. 2007). The
effects of hemlock woolly adelgid infestation also extend into nearby bodies of water. Scientists
have found that fish populations become more diverse in close proximity to eastern hemlocks
(Ross et al. 2003). The death of this species would also negatively affect stream flow (Ford and
Vose 2007). The eastern hemlock requires relatively little water for survival (Ford and Vose
2007); however, its replacement, black birch (Daley et al. 2007), requires more water for survival
and will cause decreases in water flow in small streams (Ford and Vose 2007). Although eastern
hemlock is not a highly valuable wood (Orwig et al. 2002), many towns dependent upon forest
resources may feel negative impacts from the species’ mortality (Ward et al. 2004). In parts of
New England, eastern hemlock stands are being logged in an effort to harvest the timber before it
becomes infested or more severely infested (Orwig et al. 2002). Such efforts increase the spread
of the hemlock woolly adelgid (Orwig et al. 2002). Scientists believe that if hemlock woolly
adelgid infestation is not controlled, “unprecedented hemlock loss” could occur (Orwig et al.
2002, p. 1476). The unprecedented loss of eastern hemlocks will correlate to many of the
aforementioned effects on forests in the Eastern United States.
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Currently, biological controls show the most promise in combating the hemlock woolly
adelgid (Orwig et al. 2002); therefore, additional resources need to be focused on researching
possible predators. Scientists are primarily focusing on three different foreign lady beetle
species for control: Sasajiscymnus tsugae (formerly known as Pseudoscymnus tsugae (Cheah
2004)), Scymnus sinuanodulus, and Laricobius nigrinus (Ward et al. 2004). The hemlock woolly
adelgid has no natural predator in the Eastern United States (McClure 1995a, ctd. in Orwig et al.
2002); therefore, scientists have recommended that a diverse population of predators be
established to successfully control the pest (Reardon and Onken 2004, ctd. in Ward et al. 2004;
Asaro et al. 2005). Reports have shown that some species of the beetles better establish
themselves in different environments (Preston 2007). For example, near Banner Elk, NC L.
nigrinus has shown promising signs of establishment (Preston 2007). However, research on the
British Columbia-native L. nigrinus and the Chinese S. sinuanodulus has only recently begun,
and these species are not as well understood as S. tsugae (Ward et al. 2004).
S. tsugae is currently the most widely used and studied of the three species, with over one
million beetles released along the East Coast (Ward et al. 2004). Currently, researchers studying
hemlock woolly adelgid predators have reached a consensus that S. tsugae will not harm native
species (Cheah 2004). The beetle will only consume the hemlock woolly adelgid, the balsam
woolly adelgid, the pine bark adelgid, and the Cooley spruce gall adelgid (Cheah 2004).
However, additional research will be necessary to better understand how S. tsugae and other
predators interact with their surrounding ecosystem.
Research on S. tsugae establishment is mixed but optimistic (Cheah 2004). Localized
studies have found the beetle to be very effective in reducing hemlock woolly adelgid infestation;
however, reductions are dependent upon a number of factors such as climate, severity of
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infestation, and soil composition (Cheah 2004). Despite its possible pitfalls, S. tsugae has
“shown good potential for producing localized reductions in pest levels” (Cheah 2004, par. 8).
Research conducted in Connecticut has found that the hemlock woolly adelgid and S. tsugae
have closely synchronized life cycles (Cheah 2004). This correlation in life cycles will result in
increased efficacy in controlling the hemlock woolly adeglid (Cheah 2004). Research has shown
that the severity of hemlock woolly adelgid infestation correlates to latitude (Orwig et al. 2002).
Therefore, the efficacy of S. tsugae may differ in other infected areas where infestation is more
severe or the climate differs. As evident in the Banner Elk, NC study, more than one biological
control agent may be necessary for differing environments and climates.
In addition to lady beetles, preliminary research is being conducted on a native fungus
(Lecanicillium muscarium) that kills the hemlock woolly adelgid (Boxley 2007). Lab research
and small-scale field tests have shown that L. muscarium has the potential to effectively combat
the pest at low costs and without hampering lady beetle biological controls (Boxley 2007). It
also has a high rate of reproduction which will allow it to spread to areas inaccessible by humans
(Boxley 2007). Lead researcher Scott Costa at the University of Vermont has said that this will
not be a cure-all solution; instead this will be one approach in a multi-approach assault against
the hemlock woolly adelgid (Boxley 2007).
Scientists currently believe that biological controls show the most promise for combating
the hemlock woolly adelgid. Therefore, a sufficient amount of resources should be devoted to
advancing the study of potential biological controls. Currently, scientific laboratories are
producing the lady beetles that are released for control (Cheah 2004). There are indications that
adequate numbers of the beetles cannot be bred (Preston 2007). Therefore, additional resources
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for breeding the lady beetles will be a necessary step in effectively controlling the hemlock
woolly adelgid.
While scientists will play an important role in combating the hemlock woolly adelgid, the
average person and state governments can also take action to prevent the invader’s spread (Ward
et al. 2004). Currently, the states of Michigan, Vermont, New Hampshire, and Maine have
enacted quarantines on wood and nursery products that may contain the hemlock woolly adelgid
(Ward et al. 2004). It will be important for these states to strictly enforce these quarantines to
prevent the continued advance of the hemlock woolly adelgid. It may also be necessary for some
states without hemlock woolly adelgid infestation to enact similar quarantines. There are also
several steps the average homeowner can take to slow the spread of the pest. Birds are a primary
means of transporting the hemlock woolly adelgid; therefore, birdfeeders should not be placed
near eastern hemlocks (Ward et al. 2004). Also, an eastern hemlock’s soil moisture should be
maintained and nitrogen fertilizer should be avoided (Ward et al. 2004). The removal of isolated
infected eastern hemlocks may also be necessary to slow the spread of the hemlock woolly
adelgid (Ward et al. 2004). There will likely be a correlation between the public’s awareness of
this invader and its successful control. Therefore, an informed public will increase the chances
of successfully controlling the hemlock woolly adelgid. These actions will help slow the
infection of eastern hemlocks and should therefore be implemented.
Insecticides will only play a limited role in combating the hemlock woolly adelgid due to
high cost and likelihood of adverse environmental effects. The use of insecticides, especially
Imidacloprid, has proven to be very effective in reversing the effects of the hemlock woolly
adelgid (Webb et al. 2003). However, the use of insecticides is prohibitively expensive and
could harm biological control efforts (Ward et al. 2004). Also, the use of insecticides raises
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concerns of cross-contamination into the surrounding environment (Preston 2007). Due to the
high cost and negative effects of insecticides, their use should be limited to trees of significance
or historical importance.
Preparations are already being made to reintroduce eastern hemlocks into affected forests.
The Camcore program, based at North Carolina State University, is currently creating a seed
collection from different eastern hemlock stands throughout the affected area (Jettson 2008).
The program will create seed producing eastern hemlock populations in Arkansas, Brazil, and
Chile from the collection (Jettson 2008). If the hemlock woolly adelgid is successfully
controlled, the program will reintroduce the eastern hemlock into affected areas (Jettson 2008).
If the pest cannot be controlled, the program is prepared to engineer a hemlock woolly adelgid
resistant version of the eastern hemlock for reintroduction (Jettson 2008). This program will be
an important component in fully overcoming the hemlock woolly adelgid’s negative effects.
Any plan to combat the hemlock woolly adelgid should include methods for slowing the
invader’s spread while simultaneously shrinking the infected area. Orwig et al. (2003) propose
that the most effective way of controlling the hemlock woolly adelgid will be to constantly
monitor forests for infestation. At the first sign of infestation, biological controls should be
deployed to quickly squelch the hemlock wooly adelgid’s establishment and spread (Orwig et al.
2003). These measures appear to be the most promising based upon scientists’ current
understanding of hemlock woolly adelgid infestation. Funding for additional research and
reintroduction plans will also be an important component of any plan to combat the hemlock
woolly adelgid.
The eastern hemlock, and indirectly the forest ecosystem, is being threatened by the
hemlock woolly adelgid. As an integral part of the forest ecosystem, many trickle-down effects
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will result from the eastern hemlock’s mortality such as negative changes in stream flow and
reductions in forest diversity. It will be necessary to control the hemlock woolly adelgid’s
spread and ultimately shrink the infected area. Effective large-scale controls have not been fully
tested and it will be necessary to conduct further research on how to control the hemlock woolly
adelgid. Programs such as Camcore will also be important in reversing the damage caused by
this pest. However, without some type of control put in place “continued [hemlock woolly
adelgid] infestation will lead to unprecedented hemlock loss” (Orwig et al. 2002, p. 1476).
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References
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