Managing weeds with insects and pathogens - Ecological Management of Agricultural Weeds - page 400

livestock on desirable pasture species cripples their ability to compete against
a target weed, biocontrol agents may be of little value. Conversely, if livestock
grazing is regulated through timing and stocking densities to maintain a high
degree of plant competition against the target weed, biocontrol agents may
make valuable contributions toward weed suppression. Further increases in
the strength of competition against weeds subjected to biocontrol efforts in
rangelands and pastures might be gained by intentionally sowing desired
plant species to increase their densities (Muller-Scharer & Schroeder, 1993;
Jacobs, Sheley & Maxwell, 1996).
In some cases, the weed-suppressive effects of microorganisms used as
inundative control agents may not be evident unless the target weed species
grows in competition with other plants.DiTommaso,Watson & Hallett (1996)
found, for example, that the fungus Colletotrichum coccodes had little impact on
seed production by Abutilon theophrasti when the weed grew in pure stand.
After an initial period of stunting due to disease, height growth and leaf pro-
duction resumed rapidly.In contrast,when A.theophrasti grew in mixture with
soybean, early-season growth suppression caused by the fungus allowed
soybean to dominate the canopy, shade the weed, and prevent it from recover-
ing later in the season. In two of three years, spraying weed-crop mixtures
with C. coccodes reduced the weed's height 20-30 cm and diminished its seed
production by an average of 60% compared with uninoculated plants.
DiTommaso,Watson & Hallett (1996) noted that protocols for screening
potential agents for weed biocontrol may overlook useful organisms if they do
not include competition from other plant species within the set of experimen-
tal conditions.
Moving ahead with weed biocontrol
In this chapter we have seen that insect herbivores and phytopatho-
gens can strongly affect the survival,growth,and competitive ability of weeds.
Weed-attacking organisms can be conserved in fields where they occur
already, added inoculatively where they are not yet present but can be estab-
lished,or introduced in an inundative manner after mass-production in insec-
taries and microbiology laboratories. The conservation approach seems well
suited to weed seed predators and might be extended, after more research, to
better exploit the impacts of foliage and flower feeders and pathogens.
Inoculative releases of biocontrol agents are currently best suited to intro-
duced weeds of pasture and rangeland, whereas inundative releases are best
suited to annual crops. Due to the need for fast action and cheap production,
most inundative biocontrol of weeds is with microorganisms.