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BIOLOGICAL CONTROL OF WEEDS 



William J. Lord, Department of Plant X Soil Sciences 



In recent years attention of weed scientists has been directed to 

 biological control of weeds using insects, pathogens or allelopathy. 

 Successful examples of weed control with insects or pathogens can be cited 

 and include control of cactus in Australia by a moth borer imported from 

 Argentina, control of St. Johnswort in western United States by leaf-eating 

 beetles, and use of pathogens to control northern jointvetch in rice fields 

 and strangler vine in citrus orchards. 



Plants may release chemicals ( al lei opathics) which are harmful or bene- 

 ficial to other plants and to pests that affect the plant. The release of 

 these chemicals by either living plants or their residues is called allelo- 

 pathy. Studies have been directed toward the use of allelopathy. A.R. 

 Putnam, Department of Horticulture, Michigan State University, is 

 researching the use of cover crops or companion crops whose residues can 

 provide mulch that chemically suppresses the germination and/or growth of 

 weeds under fruit trees ( 12t h Ann. Rpt . , Michigan State Hort. Soc . 1982, pp. 

 193-196). His studies show that residues from fall -pi anted cover crops of 

 Tecumseh vvheat (spring or fall -killed with herbicides) and Balboa rye 

 ( fal 1 -ki 1 led) reduced both weed density and total weed biomass during the 

 following growing season. In contrast, Garry oat (fall-killed by cold 

 weather) residue appeared to stimulate weed germination and Balboa rye resi- 

 due (spring-killed with herbicide) was not toxic to weeds. Other experi- 

 ments indicated that Bird-a-Boo sorghum or Monarch sudangrass mulch provided 

 weed biomass reduction of approximatl ey 90% and 85%, respectively. 



The weed species encountered in both the cover crop and mulching 

 experiments were primarily annual s--large crabgrass, common ragweed, common 

 1 ambsquarters, ladysthumb, green foxtail, and mousear chickweed. Perennial 

 fescue species also were partially controlled. 



Putnam concluded that selected cover crop residues or mulches can pro- 

 vide excellent suppression of a number of annual weed species but that long- 

 term studies must be conducted to determine effects of these cover crops or 

 mulches on fruit trees. A major disadvantage of mulch is that it provides a 

 favorable habitat for mice. 



