Weed management: a need for ecological approaches - Ecological Management of Agricultural Weeds - page 25

ness in managing insects, mites, and plant pathogens calls attention to its
potential for weeds.
As described by Bottrell (1979), IPM involves the concerted use of multiple
tactics to suppress and kill pests and reduce crop damage to economically
acceptable levels. Emphasis is placed on modifying habitat characteristics to
reduce pest densities and promote crop health, conserving and releasing ben-
eficial organisms that attack pests, and planting pest-resistant cultivars.
Pesticides are used in IPM systems as therapeutic tools only when preventive
practices fail to provide adequate control.If pesticide applications are deemed
necessary,selective materials are applied in a manner that poses minimal risks
to human health and the environment. A key component of IPM systems is
timely farmer decision-making based on knowledge of (i) crop, pest, and
natural enemy biology; (ii) pest abundance and distribution; (iii) impacts of
environmental factors and farming practices on crop-pest-natural-enemy
interactions; (iv) cost and income implications of different management
options; and (v) human health and environmental impacts of different man-
agement options.
Multitactic, ecologically based, information-intensive pest management
strategies are desirable for several reasons (Bottrell & Weil, 1995; Lewis et al .,
1997). First, effective pest control can result from tactics whose individual
impacts are weak, but whose cumulative impacts are strong. Second, risks of
crop failure or serious loss can be reduced when the burden of crop protection
is distributed across many tactics, and when information is available to allow
rapid adjustments in management strategies. Third, the rate at which pests
adapt or evolve resistance to a given management tactic can be decreased
when the frequency of their exposure to that tactic is reduced. Fourth, envi-
ronmental disruptions and threats to human health can be minimized as pes-
ticide inputs are reduced. Finally, reductions in operating costs and increases
in profitability can result from lowering the need for purchased inputs
through better use of locally generated materials and site-specific knowledge.
How can transitions be made from conventional weed management
systems toward more sustainable,ecologically based systems? Bird et al .(1990)
and MacRae et al . (1990) have described a general model that we find relevant
for weed management in industrialized countries. It involves passage from
heavy reliance on conventional herbicides through stages of improved efficiency
of herbicide use, substitution of more benign inputs and practices for conven-
tional herbicides, and finally system-level redesign to manipulate multiple eco-
logical interactions, facilitate decision-making, and minimize reliance on
purchased, nonrenewable inputs. Alternatively, for farmers in developing