PESTS AND PEST CONTROL - Agrochemical and Pesticide Safety - page 91

trol of St. Johnswort by the Chrysolina beetle in the western United States.
To be effective. biological control requires two things:

• the insect or disease must be specific to the weed to be controlled;

otherwise, it may spread to other species—such as crops and orna-
mentals—and become a pest itself

• the insects must have no natural enemies that interfere with their ac-

tivity.

Grazing is another form of biological control sometimes used to control
plant growth along ditches, fence rows, and roadsides. Sheep and goats are
used most often, but geese are used for weeding some crops.

b. Cultural Control

Tillage —This is an effective and often-used method to kill or control
weeds in row crops, nurseries, and forest plantings. However, tillage may
bring buried seeds to the surface where they can either germinate and compete
with the newly planted crop or be spread to nearby fields. Tillage also may
increase soil erosion and may help to spread established plant diseases to unin-
fected areas of the field.

Time of Planting —Crops and turfgrass planted in the spring compete
well against winter annual weeds. Sometimes the planting date can be delayed
until after weeds have sprouted and have been removed by cultivation or by
herbicides.

Nurse Crops —Plant species (usually annuals) which germinate quickly
and grow rapidly are sometimes planted with a perennial crop to provide com-
petition with weeds and allow the crop to become established. The nurse crop
is then harvested or removed to allow the perennial crop to take over. For
example, oats are sometimes used as a nurse crop to help establish alfalfa or
clover. Annual ryegrass is sometimes used in mixtures to provide a nurse
crop for perennial rye, fescue, or bluegrass.

Burning —Fire may be used to control limited infestations of annual or
biennial weeds. Fire destroys only the above-ground parts of plants and is
usually not effective against many herbaceous perennial weeds.

Mulching —Mulching is used to prevent light from reaching weed seeds,
thus preventing weed growth between rows, around trees and shrubs, or in
other areas where no plants are desired.

Mowing —Mowing may be used to reduce competition between weeds
and crops and to prevent flowering and seeding of annual or biennial weeds.
Mowing is often used in orchards to control weeds and prevent soil erosion.
To be most effective, mowing height must be adequate to ensure control of