Biological weed control
B. Frick, E. Johnson - Scott Research Farm
Weeds are often introduced species in agricultural habitats. In their
original habitats, they may have been less abundant, in part because their
natural predators and diseases reduced their vigour and they were in competition
with other species. How can we use these biological methods of weed control?
Biocontrol of weeds is using living organisms to destroy weeds, or to
inhibit their growth and ability to compete with crops. Biocontrol is
often separated into two categories: introducing classical biocontrol
agents, often insects, and the increase and inundative use of organisms,
often disease agents.
Classical biocontrol introduces natural predators to weed populations.
Classical biological weed control with insects involves introducing host-specific
natural enemies from the target weed’s native range. Classical biocontrol
has had good long-term success in some instances, particularly in rangeland.
It less commonly used in cropped land. Generally, the target weed is a
long-lived perennial species that is especially difficult to control by
Nodding thistle is attacked by a weevil, Rhinocyllus conicus introduced
to Saskatchewan in 1968. Weevils may be gathered by collecting about 500
infected nodding thistle seed heads in mid-August, and placing them in
new stands. Several years are required for the weevil population to become
established so that it can effectively control the thistle.
Two insects, the black dot spurge beetle Aphthona nigriscutis , and the
copper spurge beetle Aphthona flava can be used as biocontrol agents in
leafy spurge. Larvae feed on spurge roots. The black dot spurge beetle
is more effective on high, dry and exposed sites, on coarse soils. In
Alberta, redistribution of beetles is about 65% successful. Redistribution
is accomplished by collecting and releasing adult beetles.
Toadflax seed predators Brachypterolus pulicarius and Gymnaetron antirrhini
can be spread by placing infected toadflax stems among flower stems at
the new site. Additional agents being tested for toadflax control include
the stem boring weevil Mecinus janthinus , the root boring moth Eteobalea
serratella and the root galling weevil Gymnetron linariae. However, initial
releases of Mecinus janthinus and Eteobalea serratella did not establish
successfully in Saskatchewan.
The seed weevil Apion hookeri has successfully established on scentless
chamomile after initial releases in 1992. The population is increasing
over time and the weevil can reduce up to 40% of seed the plant produces.
Other biocontrol agents released on scentless chamomile include a stem
weevil Microplontus edentulus and a gall midge. Releases are currently
being monitored for establishment success. Growers who want to obtain
scentless chamomile seed weevils may contact Saskatchewan Agriculture
Inundative biocontrol involves applying large quantities of a control
agent (such as a fungal pathogen) to weeds in much the same manner as
a chemical herbicide. Once an inundative biocontrol agent is identified,
its propagules (eg. spores, mycelium) can be produced in large quantities
through fermentation techniques. The ... “commercial feasibility to mass-produce
viable, infective and stable propagules of the pathogen is a major requirement
for developing a bioherbicide”.
BioMal is a bioherbicide that contains viable spores of a fungus,Colletrotichum
gloeosporoides f. sp. malvae, that infects round-leaved mallow. Tests
indicate that it can have a significant effect on the weed population.
It currently is not available on the market.
The Saskatoon Research Center is a leader in research and developing inundative
biocontrol agents. Several promising agents have been identified and are
being developed to control weeds such as wild oat, green foxtail, dandelion,
and scentless chamomile. Developing a bioherbicide is challenging since
it is a living organism that must remain viable after application in order
to be effective. Researchers at the Saskatoon Research Center are developing
novel delivery systems for applying biocontrol agents.
Classical biocontrol agents are available for a few perennial weed species.
Agents can be collected at previous release sites where they are already
established. These agents offer the possibility of long-term, lowcost
control. Inundative biocontrol agents have not yet reached the farm gate,
but they have good potential.
Provided by the Canada-Saskatchewan Agri-Food Innovation
Brenda Frick, Ph.D., P.Ag.
Organic Agriculture Centre of Canada
c/o Department of Plant Sciences
University of Saskatchewan
51 Campus Drive, Saskatoon
Saskatchewan, Canada S7N 5A8
Tel: (306) 966-4975
Fax: (306) 966-5015