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Rotation, Tillage and Insect
Pests
The severity of insect related
problems is strongly influenced by changes in crop cultural practices.
Cultural practices can be used to reduce or limit exposure to insect pressure.
Planting date, crop rotation, varieties planted, cultivation and harvest
date all have an effect on insect pests.
Tillage often was used by growers
as a control method for decreasing soil insect pests. It literally crushes
white grubs and wireworms. The problem with this approach is that very
few of these pests are in the plow layer at any one particular time. Spring
or fall tillage usually misses the pests which winter deeper in the soil.
Limited control of European corn borer can be achieved by plowing infested
stalks. To be effective, large tracts of land (counties) must be plowed
which exposes soil to wind and water erosion.
Rather than physical destruction
of a pest, tillage probably modifies the soil temperature and soil moisture,
influencing the behavior and growth of soil insects. As the soil warms
up, the soil moisture levels usually decrease. Insects such as white grubs
and wireworms tend to move deeper in the soil and feed on less important
plant parts when this occurs. Since soil under conservation tillage can
be slower to warm up and have higher soil moisture levels, crop growth
may be reduced and a larger window for insect damage on the young seedlings.
The presence or lack of crop
residue and weeds also has an effect on insects. Black cutworm moths prefer
to lay their eggs in fields with crop residue or green vegetation present,
while greenbugs are more likely to colonize fields under conventional
tillage systems or field with more bare ground.
The table below is a summarization
of the possible effects of reduced tillage systems on common insect pests.
How conservation tillage effects
pests in row crops:
|
Crop
|
Insect
|
Effect
|
|
CORN
|
Armyworm
|
0 to +++
|
|
|
Black cutworm
|
+ to +++
|
|
|
Corn earworm
|
0 to +
|
|
|
Corn rootworm
|
0
|
|
|
European corn borer
|
0 to +
|
|
|
Stalk borer
|
0 to +++
|
|
|
Western bean beetle
|
0 to +
|
|
|
Wireworms
|
0 to +
|
|
SOYBEANS
|
Bean leaf beetle
|
0 to +
|
|
|
Grasshoppers
|
0 to +
|
|
|
Spider mites
|
- to 0
|
++++ = Substantial increase
in pest population.
+ = Some increase.
0 = No effect.
- = Some decrease in pest population.
Fortunately for most growers,
the use of more conservation tillage systems has not vastly increased
insect pest problems. Early season pests can still be a problem and will
require growers to monitor these pests.
Crop rotation has the potential
to greatly reduce insecticide use. Rotation can also reduce the damage
to adjacent or subsequent crops by insect pests. Crop rotations were proven
many years ago to have an effect on some insect pests. The ability of
an insect to migrate and the number of different host plants available
to it are two factors which influence the success of crop rotation. Some
insect pests are very specific in the range of plants they will use at
various stages of their life cycle. If host plants are removed, this type
of insect will either die or migrate. Other insects with a wide range
of host plants will switch to available plants for feeding and egg laying.
Crop rotations are most effective
against insect pests which survive in the soil. These insects seldom move
more than several inches in the soil during their life span. Corn rootworms
are pests that have been known for years to be controlled by crop rotation.
Corn rootworms have a narrow range of grassy plants that are preferred
hosts and an annual life cycle. Eggs are laid in corn fields in the mid
to late summer, overwinter and hatch in the spring. If there are no corn
plants in the field, the young larvae die from starvation.
The mobility of an insect determines
how far an insect pest will travel looking for a suitable host plant,
especially when crop rotation reduces the preferred plant. The species
and stage of development determines how far an insect will travel. Some
insects will move only a matter of inches while others will travel several
miles.
The European corn borer is
not greatly affected by crop rotation since it will fly to another field.
Corn planted downwind from wheat or other grasses can be readily colonized
by spider mites earlier than usual in the growing season. Chinch bugs
are a pest of corn and sorghum, but the first generations of this pest
occur on wheat. As the wheat matures, these pests then move to other nearby
grass crops. If wheat is not grown in nearby fields, the potential for
the first generation of chinch bugs to damage sorghum or corn is greatly
reduced. Soybeans grown near alfalfa fields are at increased risk from
bean leaf beetle damage. Removing the first cutting of alfalfa forces
these insects to seek other food.
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