Managing weeds with insects and pathogens - Ecological Management of Agricultural Weeds - page 380

Third, because higher levels of biological activity occur when crop residue
is maintained near the soil surface, the use of minimum tillage techniques
should be considered. Pitty, Staniforth & Tiffany (1987) studied fungal attack
of Setaria viridis and S. faberi seeds under field conditions in Iowa and found
that more seeds were killed by pathogens when maize and soybean plots were
disturbed with shallow sweep cultivation rather than a sequence of mold-
board plowing, disking, harrowing, and sweep cultivation. Derksen,
Blackshaw & Boyetchko (1996) suggested that in semiarid farming regions of
Canada, minimum tillage practices may enhance weed suppression by micro-
organisms favored by the cooler, moister conditions created by residues
retained on the soil surface.Brust & House (1988) found carabid beetle density
and weed seed removal were two to three times higher in a no-tillage soybean
production system that maintained wheat straw on the soil surface compared
with a conventionally tilled system without residue cover (Figure 8.1). The
effects of minimum tillage practices on the soil environment,weeds,and their
natural enemies are examined in more detail in Chapter 5.
Fourth, the impacts of native pathogens attacking weed seeds and other
organs may be increased by using complementary weed management tactics,
including inoculative or inundative releases of other biocontrol agents.
Kremer & Spencer (1989) found that infection of Abutilon theophrasti seeds by
fungi in the genera Alternaria , Fusarium , Cladosporium , and bacteria in the
genera Pseudomonas , Erwinia , Flavobacterium increased greatly when the seed-
piercing bug Niesthrea lousianica was released at experiment sites. Viability of
Abutilon theophrasti seeds declined linearly as fungal infection increased. An
average of 92% of A. theophrasti seeds were viable at sites where the insect was
not released, whereas only 16% were viable where releases were made. More
research needs to be directed at this type of synergistic interaction between
indigenous and introduced biocontrol agents.
Finally, farm landscapes might be altered to provided better habitat for
weed biocontrol agents. Carabid densities, for example, can be greatly
increased by planting narrow strips of perennial grasses at 200-m intervals
within arable fields (Wratten & van Emden,1995).Research is needed to deter-
mine how grass strips and other forms of non-crop vegetation affect weed
seed predation and weed dynamics.
Inoculative releases of control agents
Many weed species have been introduced into new regions by human
activities (see Chapter 10). During the immigration process, introduced weed
species may leave behind herbivores and pathogens that suppressed them to