Michigan State University Extension
Ornamental Plants plus Version 3.0 - 00001731
11/12/99
Causes Of Insect Outbreaks And Plant Injury
Trees and shrubs are often planted in unfavorable
environments that predispose them to insect damage. For
example, the pine needle scale is seldom destructive in
the forest, where it is found in low population densities,
but in nursery and landscape situations, the same insect
often reaches outbreak proportions and can ruin the
aesthetic value of the plants and even kill them. The
unnatural conditions in the man-made environment induce
unusually high and destructive insect populations.
Despite quarantine regulations, many foreign insect
species have been accidentally introduced into the United
States. Away from their natural environment, many of
these become pests. The black vine weevil, the European
pine shoot moth, the gypsy moth and the mimosa webworm are
examples of introduced species that have become serious
problems to ornamental plants. In this country, these
insects became pests when they found an abundant food
supply, a favorable climate and few or no effective
natural enemies.
Many ornamental trees and shrubs are chosen for their
beauty and planted without proper consideration of their
suitability for a particular site. Some trees and shrubs
planted in the north central states are native to more
southern climates and become severely stressed because of
winter injury. Examples of these plants include southern
dogwood and azaleas. Winter injury can make these plants
susceptible to attack by insects.
Most of the commonly used shade trees are native species
from the forest that have been moved into the city. These
trees, while visually healthy, are often in a low state
of vigor because of exposure to full sunlight and moisture
stress. Insect pests of native species always abound in
neighboring woodlots, but there they are usually only
secondary problems. In weakened shade trees, the same
insects may build up to damaging levels.
Many of our popular, widely used ornamental trees and
shrubs are asexually propagated varieties and clones of
identical genetic composition. Strains of insects can
quickly develop and adapt to such genetically homogeneous
cultivars. For example, the Sunburst honey locust clone
is more susceptible to attack by the mimosa webworm than
other commonly planted clones of the same species.
Drought stress or nutrient stress may also weaken trees
and shrubs and make them more susceptible to insect
attack. Birch trees adequately watered and fertilized are
less likely than stressed trees to be attacked by bronze
birch borer. Proper watering and fertilizing of Euonymus
and Taxus plants allow them to compensate for root pruning
by root weevils, while stressed shrubs may be seriously
injured by the same amount of root pruning.
Sudden outbreaks of aphids, spider mites and scale insects
may occur when pesticides wipe out their natural enemies.
Ladybird beetles, lacewings, predaceous mites, parasitic
wasps and other natural enemies devour or parasitize
aphids, scales and mites, keeping populations under
control. Some insects are more tolerant of insecticides
than their natural enemies are. Applications of
insecticides may stimulate a rapid outbreak or a delayed
resurgence of mites and aphids, depending on the relative
toxicity of the pesticide applied. Outbreaks of scale
insects, leaf miners and other insects protected inside
galls or under waxy secretions are frequently associated
with pesticide application because the pests are
physically protected from insecticides but their natural
enemies are not. (Natural enemies of all plant feeding
insects are adversely affected by pesticides. Aphids,
scale insects and mites have been used as examples
because they reproduce rapidly and have the most obvious
outbreaks associated with pesticides.)
This information is for educational purposes only. References
to commercial products or trade names does not imply
endorsement by MSU Extension or bias against those not
mentioned. This information becomes public property upon
publication and may be printed verbatim with credit to MSU
Extension. Reprinting cannot be used to endorse or advertise
a commercial product or company.
This file was generated from data base ZZ on 01/19/00.
Data base ZZ was last revised on 11/12/99.
For more information about this data base or its contents please contact
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