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Managing Aboveground Habitat
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Cover crops in a California
orchard reduce soil erosion and contribute to overall
farm diversity. Photo by Gary Kramer, USDA NRCS |
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A mixture of perennial
rye and chewings fescue helps moderate vigorous vine growth
in deep valley soils. Grasses go dormant in the summer
and begin growing again in the fall. See page 30. Photo
by Zach Berkowitz |
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Diversify plants within agroecosystems. You can attract natural
enemies and improve biological pest control by planting polycultures
of annual crops — two or more crops simultaneously growing
in close proximity. You can also let some flowering weeds reach
tolerable levels or use cover crops such as buckwheat or sunflowers
under orchards and vineyards.
For three decades, Dick Thompson has planted cover crops, managed
weeds like covers instead of like pests, and lengthened and expanded
his crop rotation. “I’m not saying we don’t have
any insect problems, but they do not constitute a crisis,”
says Thompson, who farms in Boone, Iowa. “We don’t have
to treat for them. We haven’t done that for years.”
Numerous researchers have shown that increasing plant diversity
— and thereby habitat diversity — favors the abundance
and effectiveness of natural enemies:
 In
the Latin American tropics, lower numbers of leafhoppers and leaf
beetles have been reported in small farms where beans are intercropped
with corn. Corn earworm populations were reduced when corn was
intercropped with legumes.
In
Canadian apple orchards, 4 to 18 times as many pests were parasitized
when wildflowers were numerous compared to when they were few.
In this research, wild parsnip, wild carrot and buttercup proved
essential to maintaining populations of a number of parasitoids.
In
California organic vineyards, growing buckwheat and sunflowers
between the vines attracts general predators as well as the leafhopper
egg wasp (Anagrus species) to help manage grape leafhoppers and
thrips. When these summer-blooming cover crops flower early, they
allow populations of beneficials to surge ahead of pests. When
they keep flowering throughout the growing season, they provide
constant supplies of pollen, nectar and alternative prey. Mowing
every other row of cover crops is a management practice that forces
those beneficials out of the resource-rich cover crops and into
vines.
Georgia
cotton fields strip-cropped with alfalfa or sorghum had higher
populations of natural enemies that attack moth and butterfly
pests. Beneficials reduced pest insects below economic threshold
levels in cotton that was relay-cropped with crimson clover, eliminating
the need for insecticides.
At
Michigan State University, researchers discouraged potato leafhoppers
in alfalfa by adding forage grasses to alfalfa stands. The grasses
don’t provide the leafhoppers with enough nutrition to develop
eggs, but the leafhoppers feed on them anyway for 5 to 8 minutes
before trying another plant and eventually flying away. By diverting
leafhoppers from alfalfa and by increasing their chances for dispersal,
alfalfa-orchardgrass mixtures held 30 percent fewer leafhoppers
than pure alfalfa stands. Because potato leafhoppers are often
controlled later in the season by a naturally occurring fungus,
this strategy may reduce leafhopper damage below threshold levels.
TIP:
Grow two or more crops — such as corn and beans, or cabbage
and fava beans — simultaneously to boost beneficial populations.
CAUTION:
Before introducing additional crop(s) into your system, be sure
to consider the possibility that the new crop may compete for water,
nutrients, light or other resources needed by the cash crop for
optimal growth. Seek local knowledge and/or test the new system
on a small portion of your acreage. How might a very wet or a very
dry year affect your results?
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