Agriculture Reference
In-Depth Information
heterogeneity inherently reduces the probability of counter-resistance in rapidly evolving path-
ogens and phytophagous arthropods, and increases the durability of plant defences over time. A
mixed cropping or mixed varietal scheme reduces the concentration of suitable food plants for
insects and pathogens that specialise on a subset of the plants or varieties grown in the mixture
(Finckh 1997, Mundt 2002). Herbivores, particularly specialised feeders, have a lower probabil-
ity of finding their host plant under these conditions, and tend to leave the field at greater rates
than when suitable hosts are concentrated in monocultures. However, the searching efficiency
of parasitoids may also be reduced (Bukovinszky 2004), while indirect effects through plant
quality and emission of volatiles may also have a role in the effects of crop mixtures on herbiv-
ore suppression (Bukovinszky
et
al
. 2004). Spread of plant pathogens is inhibited by resistant
components in the mixture forming obstacles and traps. Andow's (1983, 1991) reviews of the lit-
erature on pest population densities in mixed cropping versus monoculture showed that 56% of
the herbivores had lower population densities, 16% had higher population densities, and 28%
had similar or variable densities in polyculture compared to monoculture. Intercropping is an
integral part of many low-input, traditional cropping systems in the tropics, but is only occa-
sionally used for products destined for the organic market, especially in temperate regions.
Community resistance - pathogens and herbivores
The degree to which competition among herbivores or among microbes ultimately reduces
plant injury in natural systems is debated. In agroecosystems, where the reduction of target
pest numbers is often the goal, it is conceivable that the guilds of plant exploiters could be
shifted to include more neutral invaders and prevent the build up of the few most injurious
species. Theoretically, organic practices that promote the richness of plant-supported microbes
and herbivores in the community can cause such a 'dilution effect' of the pestiferous taxa, thus
reducing crop injury levels and yield loss. However, innovative practices aimed at specific taxa
have not always been successful. For example, supplemental feeding of rodents aimed at
increasing the numbers of competing, non-pestiferous species in Canada did not reduce vole
densities or damage (Sullivan and Sullivan 2004).
Community resistance - biological control
Organic crop production relies on the suppression of pathogens and pests through the intro-
duction, conservation or enhancement, or augmentation of predators (or parasitoids). Natural
biological controls of pests and pathogens are enhanced in organic systems that foster and
maintain biodiversity through limited use of disruptive curatives coupled with vegetation
management (Barbosa 1998). Plants growing within and near the crop field offer resources for
natural enemies such as alternate prey or hosts, pollen or nectar, as well as microhabitats that
are not available in weed-free monocultures (Letourneau and Altieri 1999) or extensive
cropping operations with little non-crop vegetation. Non-crop vegetation serves to increase
faunal biodiversity, which increases the potential for ecosystem services to growers. Because
organic growers tend to rely more on ecosystem services for crop health than do growers
employing chemical input intensive schemes, vegetation management and farmscaping have
become key crop protection tools in some areas. The challenge is to encourage natural enemies
without overly favouring pest organisms. Detailed knowledge of animal behaviour, resource
use and movement patterns with respect to non-crop vegetation can aid in vegetation manage-
ment schemes for biological control and biodiversity conservation. For example, studies of
bird communities in riparian strips in Quebec suggested that woody vegetation increased
richness of some insectivorous birds, but did not increase pestiferous red-winged blackbird
densities in adjacent crop fields (Deschenes
et
al
. 2003). Microbial communities in organically
managed soils are often highly diverse compared to simpler systems managed with low vegeta-
