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THE ROLE OF BIOLOGICAL AGENTS IN THE CONTROL OF WEEDS 
L. A. Andres, Entomology Research Division, Agricultural Research Service, 
U.S. Department of Agriculture, Albany, Calif. 
Plant abundance is primarily determined by 
the climatic, edaphic, and biotic factors of the 
ecosystem. For many years man has been a 
part of this ecosystem; however, it is only 
recently that he has developed an appreciation 
of the dynamic equilibrium existing among the 
elements of his environment and of the role he 
is able to have in shifting the balance. By 
75 
understanding why a weed occurs where it does 
and the factors that have led to its present 
abundance, man can sometimes effect subtle 
changes that permit temporary or permanent 
economic benefits. 
Of the three prime factors determining plant 
distribution and abundance, the easiest to 
manipulate is often the biotic. Included among 
