the biotic influences are the effects of the 
plant itself on the environment; the indirect 
effects of shade, soil composition, and struc- 
ture; the direct effects of parasitism, pollina- 
tion, and symbiosis; and the effects of 
competition among plant species, The relative 
importance of these factors in regulating plant 
numbers and distribution has been poorly 
understood; this misunderstanding has created 
doubt as to the usefulness of each within the 
biological control program. However, on the 
basis of recent successes through the intro- 
duction of new plant parasitic organisms, 
scientists have begun to take a greater interest 
in the role of biotic agents and to reevaluate 
their use. Huffaker (1959) and also Wilson 
(1964) summed up the role and importance of 
the different agents. I will attempt to 
summarize the role of certain biotic factors 
in weed control, including a few notes on 
recent developments and ideas. 
In considering the use of biotic agents, we 
should keep in mind that weed control is 
basically a problem in applied ecology and that 
the biotic elements of the plant community 
constitute a set of tools whose usage warrants 
consideration along with other methods of 
control. Beirne (1963) pointed out the ad- 
vantages and disadvantages of biological control 
for insects, but they are equally valid for weeds. 
At its best, the method produces "...perma- 
nent, self-perpetuating, self-increasing control 
without human intervention other than to initiate 
this process."' However, it is "...slower in 
reducing the pest population, often less effective 
and commonly less certain than chemical 
pesticides." 
Man, in his exploitation of natural resources, 
has created a variety of situations that offer 
special opportunities for development of weeds. 
Planting widespread areas with crops not 
adapted to exploit the full resources of the 
habitat and poor land management are but 
two examples of such unwise exploitation 
(Harper, 1960), Because of the variety of 
situations that man has created, the biological 
approach to control of weeds is not always 
applicable and will depend on the stability and 
complexity of the plant community, the degree 
of weed control desired, the botanical position 
of the weed, the biotic agents available with 
which to work, and the imagination and inge- 
nuity of the research worker. 
76 
The arsenal of available biotic agents ranges 
from those organisms with a facultative de- 
pendence on the weed plant to the strict, 
obligate phytophagous parasite. The nonspe- 
cific herbivores often exhibit varying degrees 
of polyphagy and as such can be utilized only 
in limited situations. Their attack on the pest 
plant does not depend on its stage of develop- 
ment, although certain periods of growth may 
be preferred, but is merely coincidental with 
their general search for food. 
Examples of weed control by facultative 
agents include the use of chickens and geese 
to control nutgrass (Cyperus rotundus L.) in 
cultivated areas (Mayton et al., 1945), or 
geese alone for weed control in cotton (Miller, 
1961), The manatee from the rivers of British 
Guiana have been used for water weed control 
(Bertram and Bertram, 1962), Israeli carp 
are being tested in California against water 
weeds (Berry, 1960), also the aquatic snail 
(Marisa cornuarietis (L.)) (Seaman and Porter- 
field, 1964), 
To be effective, the feeding must occur within 
a restricted or fenced area enclosing the 
weedy species and the control agent, with the 
concentration of the agent controlled by man. 
The method involves management problems 
similar to those encountered in handling graz- 
ing animals, The snail or similar type of 
agent will often clean an area of allextraneous 
vegetation, regardless of the botanical position 
of the plants, and thus finds use where clean 
cultivation is desired. As noted, this method 
could be used in crop areas, aquatic situa- 
tions, or even in limited range areas where 
the weed has not spread to any great extent. 
Control by this means is often not self- 
perpetuating, and because of its denuding 
effect does not add to the stability of the 
community. It is also important that the 
weeds thus grazed upon are not toxic to the 
biotic agent. 
The obligatory phytophagous parasite, as 
opposed to the facultative agent, is limited in 
development to a particular plant or narrow 
group of plants, and its life cycle is closely 
tied to the developmental cycle of the host. 
This close tie to the host hampers its use 
in cultivated areas, especially where the 
catastrophic effects of harvesting and culti- 
vation periodically interrupt the plant-parasite 
cycle. Most obligate parasites used for weed 

