Managing weeds with insects and pathogens - Ecological Management of Agricultural Weeds - page 379

experiments, however, exposure of P. oleracea to S. pilicornis and H. bertrandi
failed to limit the weed's negative impact on sugar beet yield in three of four
years, compared to treatments in which P. oleracea was protected from insect
attack by the systemic insecticide aldicarb. Insect attack of P. oleracea limited
the weed's competitive effect on sugar beet in only one of four years. In that
year, weed populations protected from herbivores by aldicarb reduced sugar
beet yield by up to 40%, whereas yield loss due to competition from unpro-
tected weeds did not exceed 15%. Norris (1997) concluded that S. pilicornis and
H. bertrandi could greatly damage P. oleracea without providing sufficient bio-
logical control to reduce the need for other weed management techniques.
Why do high levels of damage to weeds by indigenous herbivores and
pathogens fail to prevent yield loss? One explanation is that damage to the
weed may occur too late in the crop's development to prevent yield reduction
(Boyetchko, 1997). For this reason, it may be better to view indigenous herbi-
vores and phytopathogens as agents for reducing weed growth and reproduc-
tion, rather than as therapeutic tools for crop protection.
How can background levels of herbivory and disease be managed to consis-
tently subject weeds to stress? The first step is to identify systems in which res-
ident natural enemies sometimes suppress weeds effectively. Next,
experiments are needed to determine whether inoculation of fields with a
regionally native natural enemy can lead to a new local population.The extent
to which various taxa of weed natural enemies are dispersal-limited rather
than habitat-limited is largely unknown at present. Ultimately, methods for
increasing weed suppression by established natural enemy populations are
needed.Existing data suggest five approaches may be particularly worthwhile
in achieving the latter objective.
First, the use of pesticides that have detrimental effects on resident weed
biocontrol agents should be minimized or eliminated. If not, broad-spectrum
insecticides used to protect crops from insect attack may inadvertently
decrease endemic biological weed control and increase requirements for weed
management inputs (Norris, 1997). The impact of reducing pesticide use on
weed seed predators is suggested by farming systems comparisons conducted
in Switzerland. Pfiffner & Niggli (1996) reported that carabid beetle densities
were twice as high in organic and biodynamic systems than in a conventional
system, and attributed this effect to lower pesticide use, as well as greater
ground cover and greater use of compost and organic soil amendments.
Second,to maximize the percentage of weed seeds that are destroyed by seed
predators, post-harvest tillage should be delayed as long as possible (Cardina et
al ., 1996). This would leave weed seeds on the soil surface where they are most
vulnerable to attack by surface-searching insects, rodents, and birds.