control have been insects, with their greatest 
use and success involving perennial host 
plants in undisturbed areas, Some examples 
of these successes include the work on cactus 
(Opuntia inermis DC,) and other Opuntia spp. 
with the moth Cactoblastis cactorum (Berg), 
the cochineal scale Dactylopius spp., plus 
other insects. Klamath weed (Hypericum 
perforatum L.) had infested approximately 
2,333,000 acres in California, but was later 
reduced to 1 percent of this area after the 
introduction of beetles of Chrysolina spp. 
(Huffaker, 1959). 
Although the perennial plant introduced into 
a noncultivated area offers what appears 
to be the ideal situation for. the obligate 
phytophagous parasite, this kind of parasite 
is now finding use in controlling annuals and 
biennial weeds, both native and alien species, 
plus aquatic and parasitic plants. One ad- 
vantage of the specific agent is thatit searches 
out the weed pest, and thus its value is en- 
hanced in both widespread and inaccessible 
areas, 
According to Beirne (1963), there are three 
ways of applying biological control: "...by 
preserving, and facilitating the increase of, 
existing natural enemies; by adding new kinds 
of natural enemies, and by adding natural 
enemies, existing kinds or new, in quantities." 
Each of these methods--conservation, intro- 
duction, and redistribution of control agents-- 
will be discussed with regard to weed con- 
trol. 
BIOLOGICAL CONTROL BY 
CONSERVATION 
The practice of biological control by con- 
servation requires a knowledge of the biotic 
elements present in the plant community. It 
also requires a thorough knowledge of their 
biologies and their overall ecological relation- 
ship to the community, with particular em- 
phasis on their role in relation to the weed 
problem, Ideally this knowledge should include 
a familiarity with all the biotic factors men- 
tioned previously. 
In the past, we often considered that when a 
weed is a problem, the biotic agents present, 
whatever they may be, are obviously not exer- 
77 
cising control and we should search for new 
species to introduce. However accurate this 
statement may be, if we are to attempt bio- 
logical control, we should note and evaluate the 
role of the biotic factors already pres- 
ent. 
The parasitic fauna associated with intro- 
duced weeds is often limited, depending on the 
methods and the time the weeds were intro- 
duced and how closely related the exotic 
species may be tonative plants. Insects feeding 
on closely related native plants may, in all 
likelihood, attack the new species. These 
insects tend toward polyphagy (Frick, 1964); 
however, their lack of specificity seldom 
warrants this development for control pur- 
poses, 
For native weeds, the associated fauna may 
be extensive and already capable of exerting 
control under favorable conditions, One ex- 
ample of such a weed is the native sagebrush 
(Artemisia tridentata Nutt.) and its insect 
parasite, Aroga websteri Clarke. This insect 
is capable of denuding its host plant, and in 
some years destroys thousands of acres of 
sagebrush, sometimes permitting return of the 
native grasses, which, in turn, enhance the 
range feeding value. With some native plants, 
however, certain faunal elements have de- 
veloped a dependency on the weedy species, 
which dependency leads to a conflict of in- 
terests when destruction of the plant is pro- 
moted. Because sagebrush is a major plant in 
the diet of antelope and deer, the Fish and 
Wildlife Service personnel decry any efforts to 
spread or conserve A, websteri (Ritcher and 
Dickason, 1964). 
Another instance of insect action on anative 
plant is the attack on shadscale, (Atriplex 
confertifolia (Torr. & Frem.) S. Wats.) by the 
scale insect Orthezia annae Cockerell on the 
roots and a moth, Eumysia idahoensis Mackie, 
in the crown. Their combined action may lead 
to the death of this plant in central Idaho, 
with subsequent invasion of the area by 
Halogeton glomeratus C, A, Mey., a weed pest 
(Mackie, 1957). 
Biological control by conservation is one 
aspect of weed control that has received little 
attention. Contingent upon evaluation of those 
agents already present in the problem area, 
preservation and increase of these agents may 
develop into a useful weed-control tool. 
