Managing weeds with insects and pathogens - Ecological Management of Agricultural Weeds - page 398

conditions. One suggested avenue for addressing this problem is improve-
ments in delivery systems,including the use of clay,peat,and alginate carriers
and encapsulations (Boyetchko, 1996; Kremer & Kennedy, 1996). DRB can
rapidly colonize crop residues, such as barley straw (Stroo, Elliot & Papendick,
1988), and such materials may serve as appropriate media for improved DRB
application and management.
The integration of multiple stress factors
As discussed previously, the joint use of several insect or microbial
biocontrol agents can result in better weed suppression than that obtained
with a single organism.The integration of biocontrol agents with mechanical,
chemical, and other biotic stress factors can also provide advantages over reli-
ance on a single agent alone. For example, in three out of five experiments
conducted by Klein & Auld (1996), wounding Xanthium spinosum by dragging
steel mesh over it before applying Colletotrichum orbiculare spores caused
greater mortality than did spores alone; mowing X. spinosum before applying
spores of the fungus increased weed mortality in two out of three experi-
ments.Application of the fungus Puccinia canaliculata in mixture with the her-
bicide paraquat provided 99% control of Cyperus esculentus , compared with
60% control with P. canaliculata alone and 10% control with paraquat alone
(Phatak, Callaway & Vavrina, 1987). Similarly, application of the growth regu-
lator thidiazuron in concert with the fungus Colletotrichum coccodes provided
greater control of Abutilon theophrasti than did use of either stress factor separ-
ately (Hodgson et al ., 1988).
Plant competition can add greatly to or synergize the effects of pathogens
and insects on weeds. Groves & Williams (1975) conducted a pot experiment
to determine how infection by Puccinia chondrillina and competition from sub-
terranean clover affected growth of the weed Chondrilla juncea. At 146 days
after planting, weed biomass was reduced 51% by the pathogen alone, 69% by
clover competition alone, and 94% by the combination of P. chondrillina infec-
tion and competition (Figure 8.2).
Cumulative effects of plant competition and insect attack on Senecio jacobaea
were studied experimentally by McEvoy et al . (1993) in an Oregon pasture. At
the start of the experiment, the pasture community was dominated by the
perennial grasses Holcus lanatus , Dactylis glomerata , Anthoxanthum odoratum ,and
Festuca arundinacea; S. jacobaea represented only 0.1% of the above-ground dry
mass of the community . Plots were then tilled to stimulate germination of S.
jacobaea. To manipulate interspecific competition against the emerged S. jaco-
baea plants, other plants colonizing each plot were either manually removed,