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Crimson clover is often
grown in California nut groves and orchards to provide nitrogen
to tree crops. Its eye-catching flowers produce abundant nectar
for bees and contain pirate bugs that prey on small pests like
thrips. Photo by Ray Weil. |
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Create Multiple Stresses on Pests
Maximizing the impacts of many “little hammers” takes an understanding
of the life cycles of pests and of beneficial organisms. In ecologically
based systems, farmers scrutinize the life cycles of pests and beneficial
organisms, looking for times and places where small control measures
can add up to big results. A good opportunity might arise during
a pest’s overwintering stage, for example, and another while it
is first colonizing the crop. A beneficial organism may offer protection
at one stage and need protection at another. Even in small increments,
pest mortality can eventually pare a big problem to a low level.
In Sentinel Butte, N.D., cattle producer Dennis Dietz is battling
a 75-80 percent leafy spurge infestation with several species of
imported flea beetles after herbicides had little effect on a pest
that plagues farmers in the western Plains. The flea beetles, however,
are leaving their mark.
“I’ve seen dramatic changes in stem count and flowering,” says
Dietz. “My feeling is that the control is excellent, and it’s long
term.” Established in his own on-site insectary plantings, the flea
beetles “will be there forever,” he says. “Chemical control, in
my opinion, is way too expensive and it doesn’t last as long.”
Not only do the adult beetles feed on the tops of leafy spurge,
but flea beetle larvae burrow into the weed’s roots, exposing them
to a second stress: opportunistic disease organisms.
Cooperating in a “Team Leafy Spurge” project through the USDA Agricultural
Research Service in Sidney, Mont., Dietz has begun to add a third
stress — sheep — to his biocontrol strategies. Their mission: graze
off the tops of the plant while the beetles work on the roots.
Discourage the pests’ dispersal or connection with crops.
Interspersing non-host plants can hinder the movement of insect
pests and crop disease organisms by altering light and humidity
and jumbling the critical visual and chemical signals insect pests
use to recognize their hosts. Non-host plants put distance between
susceptible plants and, like fly paper, intercept spores to limit
the spread of diseases.
Flea beetles that attack cabbage and other crucifers are less abundant
when clover — a non-host species — is sown between cabbage rows.
Crop losses to mildew are higher in pure stands of barley than in
mixtures of barleys that differ in their susceptibility to disease
races.
Disrupt pest populations by destabilizing habitat.
When pests are adapted to some crops but not to others, rotations
that include non-host crops can help with control. Placing a non-host
crop in a rotation sequence often destroys the habitat a pest needs,
limiting population growth. Infestations of Colorado potato beetles,
for example, are more severe in continuous potatoes than in potatoes
that follow winter wheat or rye. Similarly, rotating soybeans with
such non-hosts as corn reduces charcoal rot.
To be suppressed by rotation, a target pest must have specialized
feeding habits that restrict it to a narrow host range. Its ability
to move to other locations also must be low. Finally, its dormant
and resting stages must be shorter than the time gap between susceptible
crops.
Rotations that include diverse crops and management practices tend
to have fewer weed problems than simple rotations and crop monocultures.
By continually changing the “rules of the game,” complex rotations
discourage the selection and adaptation of weeds.
Giant foxtail, for example, is less of a problem in a three-year
corn-soybean-winter wheat rotation than in corn-soybean or continuous
corn. Differences in the timing of germination, growth and competitiveness
among the three crops, plus the suppressive effects of wheat straw,
are likely reasons.
| Percentage reduction
of various pest populations/ Increase of beneficial insects
(averaged over the 1997–1998
growing season at USDA-ARS, Beltsville, Md.) |
| Pest Species |
% Reduction/increase
by a hairy vetch cover crop |
| Annual grass weed number |
-83 |
| Early blight disease severity |
-52 |
| Colorado potato beetle number |
-82 |
| Beneficial lady beetle number |
+250 |
|
| Comparisons made from experiments with tomatoes
grown in hairy vetch versus black polyethylene mulch for disease
and insect data and from experiments with corn grown in hairy
vetch versus unmulched soil for weed data. |
Reduce weeds’ access to resources. Many crops
lack canopy cover and substantial root growth when they are young,
limiting the plants’ ability to fully capture sunlight, water and
nutrients. Instead, these crops give invading weeds a foothold.
Later in the season, weeds will compete with the crop and reduce
its yield. Narrower row spacings, higher crop population densities
and intercropping may all rob weeds of the resources they need to
grow.
On the other hand, these same weed-choking practices can increase
disease levels in some crops. Denser stands of wheat, for example,
are more susceptible to mildew. Use disease-resistant varieties
with such practices.
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