Agriculture Reference
In-Depth Information
10.3.2 Diversity and insect pest control
Mechanisms
Several surveys (Vandermeer, 1989; Andow, 1991) indicate that intercropping can
be beneficial for insect pest control. The mechanisms affecting insect infestation in
diverse systems are more complex than for pathogen-plant interactions because, in
addition to the host plant and the insect, predators and parasites of the insects have
to be considered (hyperparasites of pathogens occur but they are not considered,
generally, to be important). Some hypotheses for the mechanisms of interaction have
been proposed (Altieri and Letourneau, 1982; Andow, 1991; Letourneau, 1997):
The
natural enemies hypothesis
states that predators and parasites are more
effective in complex environments because diversity of prey, alternative food
sources and microhabitats are greater, allowing for the persistence of generalized
natural enemies. Specialized natural enemies, moreover, are less likely to fluctuate
because their prey can more easily escape extinction and should therefore provide a
more continuous food source even if less abundant (Root, 1973; Risch, 1981;
Andow, 1991).
The
resource concentration hypothesis
(Root, 1973) was formulated because
insects are more likely to find and remain on their hosts in uniform and concentrated
patches than in mixed stands, confirmed in experimental studies (Bach, 1980; Risch,
1981; Risch
et al.
, 1983). In general, it appears that lack of resource concentration is
more important than are natural enemies in polycultures (Andow, 1991). However,
there are cases where the mechanisms work in opposite directions (Andow, 1991).
In addition, a
plant's apparency
is greatly increased in a monoculture and
defence mechanisms that might have been adequate in a more diverse setting may be
insufficient. (Price
et al.
, 1980; Andow, 1991).
Interactions among the host components
of a species mixture may affect,
physiologically, the suitability of the host plants as a food source (Vandermeer,
1989).
Associational resistance
refers to the reduced herbivore attack that a plant
experiences in association with genetically or taxonomically diverse plants and is
part of a plant's defensive system (Price
et al.
, 1980; Andow, 1991). Examples are
simple barrier effects of resistant plants or cover crops impeding access to
susceptible plant parts, olfactory effects, diversion of the pest to other plants and
effects of shading of susceptible plants on their attractiveness to the pest (Perrin,
1977; Price
et al.
, 1980; Risch, 1981). Thus, the concept of diversification for the
control of insect pests must go beyond the mixing of resistant cultivars or isogenic
lines. Deliberately retaining weeds in fields or planting borders to plant species that
support natural enemies, as proposed by Altieri and Letourneau (1982) and others,
could thus be understood as the deployment of associational resistance. Reduction of
disease or insect pests should reduce pesticide inputs and improve conditions for
natural enemies thus reducing the need for pesticides even further. The improvement
