Biological weed control
B. Frick, E. Johnson - Scott Research Farm
Problem
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?
Background
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鈥檚 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 traditional means.
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 and Food.
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 ... 鈥渃ommercial feasibility to mass-produce viable, infective and stable propagules of the pathogen is a major requirement for developing a bioherbicide鈥�.
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.
Conclusions
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.
Acknowledgements
Funding provided by the Canada-Saskatchewan Agri-Food Innovation Fund
Originally published in Research Report 2002, Canada-Saskatchewan Agri-Food Innovation Fund