From the early 1970s on, the literature provides hundreds of examples of experiments documenting that diversification of cropping systems often leads to reduced herbivore populations (Andow, 1991; Altieri, 1994). Most experiments that have mixed other plant species with the primary host of a specialized herbivore show that, in comparison with diverse crop communities, simple crop communities have greater population densities of specialist herbivores (Root, 1973; Cromartie, 1981; Risch et al., 1983). In these systems, herbivores exhibit greater colonization rates, greater reproduction, higher tenure time, less disruption of host finding, and lower mortality by natural enemies.
There are various factors in crop mixtures that help constrain pest attack. A host plant may be protected from insect pests by the physical presence of other plants that may provide a camouflage or a physical barrier. Mixtures of cabbage and tomato reduce colonization by the diamondback moth, while mixtures of maize, beans, and squash have the same effect on chrysomelid beetles. The odors of some plants can also disrupt the searching behavior of pests. Grass borders repel leafhoppers from beans, and the chemical stimuli from onions prevent carrot fly from finding carrots (Altieri, 1994).
Alternatively, one crop in the mixture may act as a trap or decoy — the "flypaper effect." Strips of alfalfa interspersed in cotton fields in California attract and trap Lygus bugs. There is a loss of alfalfa yield, but this represents less than the cost of alternative control methods for the cotton. Similarly, crucifers interplanted with beans, grass, clover, or spinach are damaged less by cabbage maggot and cabbage aphid. There is less egg laying on the crucifers, and the insect pests are subject to increased predation (Altieri, 1994).
The two hypotheses that have been proposed to explain lower herbivore abundance in polycultures, the resource concentration hypothesis and the enemies hypothesis (Root, 1973), identify key mechanisms of pest regulation in polycultures. They explain why there may be differences in mechanisms between cropping systems, and suggest plant assemblages which enhance regulatory effects and those which do not, and under what management and agroecological circumstances. According to these theories, a reduced insect pest incidence in polycultures may be the result of increased predator and parasitoid abundance and efficiency, decreased colonization and reproduction of pests, chemical repellency, masking and/or feeding inhibition from nonhost plants, prevention of pest movement or immigration, and optimum synchrony between pests and natural enemies (Andow, 1991).
A recently conducted, well-replicated experiment, where species diversity was directly controlled in grassland systems, found that ecosystem productivity was increased and that soil nutrients were utilized more completely when there was a greater diversity of species, leading to lower leaching losses from the ecosystem (Tilman et al., 1996). In agroecosystems, this same pattern applies to insects as herbivore regulation increases with increasing plant species richness. Evidence suggests that as plant diversity increases, pest damage tends to reach acceptable levels, thus resulting in more stable crop yields (Figure 6). Apparently, the more diverse the agroecosystem and the longer this diversity remains undisturbed, the more internal links develop to promote greater insect stability. It is clear, however, that the stability of the insect community depends not only on its trophic diversity but on the actual density-dependence nature of the trophic levels (Southwood and Way,
Figure 6 Hypothetical trend of pest regulation or damage reduction as species richness increases in agroecosystems. "X value" represents the level at which a functional assemblage of species with natural control attributes is established.
1970). In other words, stability will depend on the precision of the response of any particular trophic link to an increase in the population at a lower level. Thus, selective diversity, rather than just a random collection of species, is crucial to achieve desired pest regulation (Dempster and Coaker, 1974).
From a practical standpoint, it is easier to design insect manipulation strategies in polycultures using the elements of the natural enemies hypothesis than those of the resource concentration hypothesis. This is mainly because we cannot yet identify the ecological situations or life history traits that make some pests sensitive (i.e., their movement is affected by crop patterning) and others insensitive to cropping patterns (Kareiva, 1986). Crop monocultures are difficult environments in which to induce the efficient operation of beneficial insects because these systems lack adequate resources for the effective performance of natural enemies, and because of the disturbing cultural practices often utilized in such systems. Polycultures already contain specific resources provided by plant diversity and are usually not disturbed with pesticides (especially when managed by resource-poor farmers who cannot afford high-input technology). They are also more amenable to manipulation. In polycultures, the choice of a tall or short, early or late maturing, flowering or nonflowering, legume or nonlegume companion crop can magnify or decrease the effects of particular mixtures on specific pests (Vandermeer, 1989). Thus, by replacing or adding the correct diversity to existing systems, it may be possible to exert changes in habitat diversity that enhance natural enemy abundance and effectiveness.
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