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BIOLOGICAL CONTROLS
What is biological control of weeds?
Biological control of weeds is the use of
parasites, predators, and pathogens to reduce the population of a weed. While it can refer to the use of
sheep or cows to graze weeds to reduce seed production and plant health, we
will refer to that as cultural control.
For the purposes of this project we will use biological control to
refer to invertebrate parasites and predators (usually insects, mites, and
nematodes) and plant pathogens.
In addition, while biocontrol can refer to the use
of native insects and pathogens, we are primarily concerned with the
use of nonnative insects and pathogens to control nonnative
invasive plants. To date, the
use of biological control on weeds in Forest Service Region 6 has been
exclusively the use of nonnative parasites, predators, and pathogens. While we are not ignoring the
possibility of using natives, this discussion primarily applies to the use of
nonnative agents including nematodes, mites, insects, and rust fungi.
Biological control background
Biological control is based on the idea that one
of the reasons plants from other countries become invasive pests in this
country is that when they arrived here their natural enemies were left
behind. Many of the nonnative
plants that become invasive in this country are not invasive in their native
lands and are only minor components of their native plant communities. By introducing predators and
parasites from a weed’s country of origin, we hope to bring the population of
the weed down to an acceptable level.
Biological control does not attempt to eradicate any given weed, but
simply to make it a minor component of it’s newly adopted community. Successful biological control
programs result in permanent reductions in weed density and the accompanying
permanent existence of the biocontrol agent.
Examples of successful biocontrol projects include
flea beetles from Italy on tansy ragwort (see photos) and flea beetles from
Europe on leafy spurge.
Frequently Asked Questions
What is the protocol for
biological control agent screening?
All potential agents must be approved by the
United States Department of Agriculture’s Animal and Plant Health Inspection
Service (APHIS). Once a target
weed and biological control agent are identified, the agent goes through
extensive host-specificity testing.
This testing is designed to ensure that introduced biological weed
control agents are limited in host range and do not threaten endangered,
native, or crop plants.
The plant species tested are chosen from three
groups of plants. The first
group identified includes those native North America plants in the same
family, genus, species, or type as the target weed. The next group is threatened and endangered species in the
same family, genus, or species as the target weed. Finally, species in other orders or families that are
similar in form or shape or have historical or chemical similiarities to the
target weed are tested. This
last group of plants would include any economically or environmentally important
plants.
The development of a list of host plants for
host-specificity testing is aided by the involvement of an interagency group,
the Technical Advisory Group. The Technical Advisory Group
for Biological Control Agents of Weeds (TAG) is a voluntary committee first
formed in 1957 to provide advice to researchers. Today, TAG members review
petitions for biological control of weeds and provide an exchange of views,
information and advice to researchers and those in APHIS responsible for
issuing permits for importation, testing, and field release of biological
control agents of weeds. Members
in TAG include weed managers from the Bureau of Land Management, National
Park Service, Bureau of Reclamation, National Plant Board, Agricultural
Research Service, U.S. Army Corps of Engineers, National Biological Control
Institute, U.S. Geological Survey, U.S. Forest Service, EPA, APHIS, U.S. Fish
& Wildlife Service, Citrus Research and Education Center, Bureau of
Indian Affairs, and the Weed Science Society.
Precautions are also taken to ensure that the
introduced agents are not parasitized nor diseased so that when an
introduction is made, only one organism is introduced. This requires that several
generations of the proposed agent are reared in the lab.
For more information on permits to release
biological control agents, go to:
www.aphis.usda.gov/ppq/permits/biological/weedbio.html
For more information on developing a test plant
list and the role of the Technical Advisory Group in biological control, go
to:
www.aphis.usda.gov/ppq/permits/tag/
Can these biological control agents become
pests on other plants?
There are several documented examples of
introduced biological control agents becoming pests on plants other than the
weeds originally targeted (Louda et al. 1997, Howarth 2001). Perhaps the most well-known is the
flowerhead weevil, Rhinocyllus conicus, introduced into North America
for the control of Eurasian thistles in the genus Carduus, primarily
musk thistle, C. nutans.
The original releases were made in Canada in 1968 and releases in both
the U.S. and Canada continue today.
Approval for the release of this insect was granted knowing that the
weevil’s host range included three native North American thistle genera. Female egg-laying behavior was
expected to restrict the weevil’s host range. Current evidence shows this weevil continues to expand
it’s geographic and host range, which now includes a close relative of the
federally listed threatened Pitcher’s thistle (Cirsium pitcheri)
(Louda et al. 1997).
However, Rhinocyllus conicus would not be
approved for release by the current standards used by USDA APHIS. APHIS modified the testing process in
the mid-1980’s to include more potential hosts in host specificity testing. They also modified the approval
process to permit the introduction of the safest biological control agent
first. APHIS continues to work
on refining regulations and procedures for introducing biological control
agents (L.A. Andres et al. 2000).
Studies of non-target effects of agents released under current
protocols are not yet complete.
How do we know if the biological controls work or
are becoming a problem?
There is an historical lack of post-release
monitoring of biocontrols.
Biocontrol is usually deemed successful when weeds no longer dominate
an area of agent release. The usual method in field releases is to put
hundreds or thousands of individuals in one area and return 2-3 years later
and look for reductions in the target weed. There are almost no post-release studies on the effects of
the introductions on nontarget plants or other insects (D. Simberloff and P.
Stiling 1996). Most examples of
agents attacking non-target plants come from studies originally focused on
other questions. The work by
Svata Louda (1977) that illuminated the wide host range of the seedhead
weevil, R. conicus, on native thistles was designed to look at
interactions of native thistles and their native predators (Stolzenburg
1999). Some interpret this lack
of monitoring information to mean there are no negative effects (J.H. Frank 1998). The USDA now requires post-release
studies on the impacts of biocontrol agents approved for new releases
(Randall & Tu 2001).
Links to biocontrol sites:
www.invasive.org/biocontrol.cfm photos of biological control agents
http://dlp.cs.berkeley.edu/calflora/ good site for links and lots of info
on most weeds
www.cdfa.ca.gov/ California Dept of Food and Ag., good
weed information
Link to Featured
Weed, yellow starthistle:
Create a live link with the word “insects” in the
middle of the 5th line in the last paragraph of the yellow
starthistle page.
Approved biocontrol agents of yellow starthistle:
Six natural enemies of yellow starthistle have
been imported from Greece and are established as biological control agents:
three weevils, Bangasternus orientalis, (brought into the U.S. 1985), Eustenopus
villosus (1990), and Larinus curtus (1992); one gall fly, Urophora
sirunaeseva (1984); the peacock fly Chaetorellia australis (1988),
and the false peacock fly, Chaetorellia succinea (1991?) (Turner et al. 1994,
Turner et al. 1995).
All six insects attack the flower/seed head, and
directly or indirectly reduce seed production, the only means of
reproduction. They lay their eggs in or near the flower/seed heads and
complete their development within them. They are all specific to yellow
starthistle and knapweeds and do not attack commercially valuable or native
plants. It should be noted
however, that the false peacock fly was accidentally introduced with an
introduction of the peacock fly.
Host specificity testing was conducted after the accidental
introduction was discovered (E.S. Delfosse & K.J. Hackett 2001).
Photos
1)
Yellow starthistle bud weevil
(starthistle_bud_weevils.jpg)
photo by USDA ARS European Biological Control Laboratory
2)
Yellow starthistle flower weevil (starthistle_flower_weevil.jpg) photo
by G.R. Johnson, USDA ARS
3)
Yellow starthistle gall fly (starthistle_gall_fly.jpg) photo by Eric
Coombs, Oregon Department of Agriculture
4)
Yellow starthistle hairy weevil (starthistle_hairy_weevil.jpg) photo
by Eric Coombs, Oregon Department of Agriculture
5)
Yellow starthistle peacock fly (starthistle_peacock_fly.jpg) photo by
Charles Turner, USDA ARS
References:
Andres, L.A., J.R. Coulson, T.D. Center, C.E.
Turner, and C.J. DeLoach.
2000. Biological control
of weeds: 1. Invertebrate natural enemies of
weeds. p.75-76 and see epilogue,
part A.. In Coulson,
J.R., P.V. Vail, M.E. Dix, D.A. Nodlund, and W.C. Kauffman, eds. 2001. 110 Years of Biological Control Research and Development
in the United States Department of Agriculture: 1883-1993.
U.S. Department of Agriculture, Agricultural Research Service.
Delfosse, E.S. and K.J. Hackett. 2001. ARS Research to Combat Invasive Species. www.invasivespecies.gov/toolkit/arsisresearch.doc.
Frank, J.H.
1998. How risky is
biological control?
Comment. Ecology, 79:1829-1834.
Howarth, F.G. 2001. Environmental issues concerning the importation of non-indigenous
biological control agents. In
J.A. Lockwood, F.G. Howarth, and M.F. Purcell, eds. 2001. Balancing Nature: Assessing the impact of importing
non-native biological control agents (an international perspective). Entomological Society of America,
Lanham, Maryland.
Louda, S.M., D. Kendall, J. Connor, and D.
Simberloff. 1997. Ecological
effects of an insect introduced for the biological control of weeds. Science 277:1088-1090.
Randall, J.M. and M. Tu. 2001. Chapter
4: Biological Control. In Weed Control Methods
Handbook, The Nature Conservancy.
Simberloff, D. and P. Stiling. 1996. How risky is biological control? Ecology 77:1965-1974.
Stolzenburg, W. 1999. Double
agents. Nature Conservancy
July/August 1999: 18-24.
Turner, C. J., R. Sobhian, D.B. Jolley, E.M.
Coombs, and G.L. Piper. 1994. Establishment of Urophora sirunaseva
(Diptera: Tephritidae) for biological control of yellow starthistle in the
western United States. Pan-Pac. Entomol. 70:206-211.
Turner, C.E., G.L. Piper, and E.M. Coombs. 1995. Knapweeds:
Yellow Starthistle pages.
In N.E. Rees, P.C. Quimby, Jr., G.L. Piper, E.M. Coombs, C.E.
Turner, N.R. Spencer, and L.V. Knutson, eds. 1996.
Biological Control of Weeds in the West. Western Society of Weed Science, Bozeman, Montana.
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