severe climatic conditions. Through periodic 
releases they could also bring established 
colonies to greater working strength at critical 
Stages in the weed cycle. Wilson (1964) cited 
progress made with the cochineal scale 
Dactylopius sp., which is systematically dis- 
tributed each year to control the cactus 
Opuntia aurantiaca Lindl. Through periodic 
releases we can maintain a continual pressure 
on the weedy plant and overcome any asynchro- 
nization between the phytoparasite and the 
host. Release of large numbers of phytopara- 
sites for immediate eradication of a weed has 
not been attempted. The practicality of such a 
technique is doubtful. It would depend on value 
of the land occupied by the weed, degree of 
control desired, and agents to be used, 
Success of a periodic release program 
depends on a suitable supply of the biotic 
agent. If field populations are not readily 
available, artificial propagation is necessary, 
which limits the technique primarily to multi- 
voltine species adapted to artificial culture 
conditions. From the standpoint of economy, 
both host and phytoparasite cycles should be 
reduced to aminimum. Plant diseases may also 
be amenable to artificial propagation tech- 
nique. 
Another possibility to overcome asynchro- 
nization of plant and parasite would be to 
artificially culture small plots of the weedy 
species to foster phytoparasite development, 
with subsequent spread to the natural weed 
population. 
An effective adjunct to biological control 
of weeds is a well-planned land-management 
followup program. The niche vacated by the 
weed will be filled by other plants, the species 
of which can be controlled somewhat by man. 
By planned grazing and other techniques, plus 
a reseeding program, we can bring pressure 
against the weed, often enhance the effective- 
ness of the phytoparasite, and help stabilize 
regrowth of the entire plant community. 
Biological control of weeds is still a new 
field, the full potentials of which we have not 
yet realized or tested. As already noted, when 
a weed problem arises, biological control 
81 
should be considered simultaneously with other 
control methods and not as a last resort. As a 
result of previous successes, the interest 
developed in the method has led to further 
study of the technique. 
Nonspecific agents are most effective in 
controlling weeds when they are confined to 
limited areas. Specific agents, however, can 
be used over wider areas; their use is limited 
only by their degree of host specificity, their 
lack of damage to plants of economic im- 
portance, and their climatic adaptations. 
Weed control through importation of insects 
has succeeded mainly with plant families 
containing few members of economic im- 
portance, The closer the relationship of a 
weed to an economic plant, the fewer the host- 
Specific agents available and the narrower the 
margin of safety. Since there isa"... .tendency 
towards weediness to run in certain families, 
e.g., Compositae, Cruciferae, Gramineae..." 
(Harper, 1960) and since these families also 
contain several economic plants, we must 
devise new systems to evaluate the safety of 
introduction rather than to reply primarily on 
exclusion or starvation testing methods, Pre- 
occupation with safety has caused us to focus 
attention on the feeding aspects of phytophagous 
parasite behavior. We have overlooked the fact 
that the host plant represents not only a source 
of food but the habitat of the insect, which, 
in turn, provides items other than nutrition. 
These factors should also be evaluated as to 
their role in determining host specific- 
ity. 
When we begin to understand how to conserve 
the biotic agents already present, as well as the 
effects of soil fauna on seed reservoir, we will 
be able to use them more effectively for bio- 
logical control of weeds. In addition, utilization 
of periodic release and colonization techniques 
will increase the effectiveness of a given biotic 
agent in a control program. Thus, making use 
of the full potential of biotic agents in weed con- 
trol will depend mainly on our ability to under- 
stand the forces of the ecosystem and our inge- 
nuity to manipulate them in the desired 
direction. 
