Mechanisms
that create beneficial plant associations: Trap
Cropping Sometimes,
a neighboring crop may be selected because it is more attractive to pests and
serves to distract them from the main crop. An excellent example of this is
the use of collards to draw the diamond back moth away from cabbage . Symbiotic
Nitrogen Fixation Legumes—such
as peas, beans, and clover—have the ability to fix atmospheric nitrogen for
their own use and for the benefit of neighboring plants via symbiotic
relationship with Rhizobium bacteria. Forage legumes, for example, are
commonly seeded with grasses to reduce the need for nitrogen fertilizer.
Likewise, beans are sometimes interplanted with corn. Biochemical
Pest Suppression Some
plants exude chemicals from roots or aerial parts that suppress or repel
pests and protect neighboring plants. The African marigold, for example,
releases thiopene—a nematode repellent—making it a good companion for a
number of garden crops. The manufacture and release of certain biochemicals
is also a factor in plant antagonism. Allelochemicals such as juglone—found
in black walnut—suppress the growth of a wide range of other plants, which
often creates a problem in home horticulture. A positive use of plant
allelopathy is the use of mow-killed grain rye as a mulch. The
allelochemicals that leach from rye residue prevent weed germination but do
not harm transplanted tomatoes, broccoli, or many other vegetables. Physical
Spatial Interactions For
example, tall-growing, sun-loving plants may share space with lower-growing,
shade-tolerant species, resulting in higher total yields from the land.
Spatial interaction can also yield pest control benefits. The diverse canopy
resulting when corn is companion-planted with squash or pumpkins is believed
to disorient the adult squash vine borer and protect the vining crop from
this damaging pest. In turn, the presence of the prickly vines is said to
discourage raccoons from eating the sweet corn. Nurse
Cropping Tall
or dense-canopied plants may protect more vulnerable species through shading
or by providing a windbreak. Nurse crops such as oats have long been used to
help establish alfalfa and other forages by supplanting the more competitive
weeds that would otherwise grow in their place. In many instances, nurse
cropping is simply another form of physical-spatial interaction. Beneficial
Habitats Beneficial
habitats—sometimes called refugia—are another type of companion plant
interaction that has drawn considerable attention in recent years. The
benefit comes when companion plants provide a desirable environment for
beneficial insects and other arthropods—especially those predatory and
parasitic species which help to keep pest populations in check. Predators
include ladybird beetles, lacewings, hover flies, mantids, robber flies, and
non-insects such as spiders and predatory mites. Parasites include a wide
range of fly and wasp species including tachinid flies, and Trichogramma and
ichneumonid wasps. Agroecologists believe that by developing systems to
include habitats that draw and sustain beneficial insects, the twin
objectives of reducing both pest damage and pesticide use can be attained. Security Through Diversity
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