Michigan State University Extension
Ornamental Plants plus Version 3.0 - 00001706
11/12/99
Symptoms Caused by Insects
Leaf spots
Although leaf spots are most frequently caused by plant
pathogens, they may occasionally be caused by sucking
insects, such as plant bugs and leafhoppers. When the
insect's saliva is toxic to the plants, a dead spot may
develop around the point where the insect feeds.
Branch dieback
Wood-boring insects such as the bronze birch borer may
cause significant damage to plant vascular tissue that
results in a dieback of the infested limbs or branches.
Branches damaged by other causes and weakened trees in
general may be particularly susceptible to insect borers.
Branch dieback may also be caused by diseases,
environmental factors, cultural factors, insects, or a
combination of two or more of these factors. Another type
of branch dieback is twig dieback caused by twig girdlers
or twig pruners. Twig dieback may be caused by beetle
larvae boring inside twigs or by the feeding of adult
beetles that completely girdles the twigs. This type of
damage occurs most frequently to oaks, maples, hickory,
pecan and flowering fruit trees.
Stem and leaf galls
Several large groups of insects have many gall-forming
species. These include the gall wasps, gall midges,
aphids or adelgids, eriophyid mites and sawflies. Some
families of gall- forming insects are so diverse that an
insect species exists for almost every common tree
species. Stem and leaf galls may also be caused by plant
pathogens. Leaf galls, however, are usually caused by an
insect or mite. If you open fresh leaf galls with a
knife, you can usually find a small aphid or midgelike
larvae.
Witches'-broom
Some of the eriophyid mites and the honeysuckle aphid
(Vis. 1) cause a witches'-broom effect in infested plants.
A good example is the dense, twisted growth of hackberry
twigs induced by infestations of Eriophyes mite and a
powdery mildew fungus. These dense, irregular growths can
be caused by mites or plant pathogens.
Cankers and swellings
Many beetle larvae and caterpillars bore into tree
trunks or limbs, causing them to swell. When you cut
these spindle-shaped galls open, insect tunnels and frass
should be visible. Insect borers often attack trees that
are weakened or damaged by other causes. In some cases,
borers and plant pathogens are associated with the same
canker.
Leaf drop
Leaf drop is generally caused by plant pathogens or
environmental problems. One exception is the early summer
leaf drop of maples caused by the maple petiole borer.
Diagnose leaf drop caused by insects by looking for
tunneled or chewed leaf petioles.
Bronzing
From a distance, trees heavily infested with spider mites
may appear discolored. Closer examination of the affected
foliage reveals a bronze discoloration of infested leaves.
Confirm a diagnosis of spider mite injury by tapping
infested branches over a white piece of paper. Dislodged
spider mites will appear as tiny specks moving on the
paper.
Leaf miners
Trees heavily infested with leaf miners may appear brown,
as if portions of the tree are dying. When you examine
damaged leaves individually, the leaf miners become
apparent. Leaf miners feed inside the leaves between the
upper and lower leaf surfaces, leaving only a paper thin
cuticle covering the mined areas. If you open the mines,
you may find brown frass and the wormlike larva itself.
Chewed or skeletonized leaves
Some insects, such as leaf beetles and some sawflies,
chew the plant tissue off one side of the leaf while
leaving the opposite leaf cuticle and the veins intact.
The damaged leaf then looks like a lacy skeleton of a
leaf.
Most caterpillars and adult beetles chew through the
entire leaf. The damage may appear as tiny or large holes
in the leaf, or as irregularly shaped leaves with jagged
edges. If plant leaves are still growing when the insect
feeds, the damage may later appear to have smooth edges
around the feeding holes. Only insects cause these types
of damage.
Leaf curling, puckering or rolling
The saliva of some sucking insects, especially aphids, may
cause plant leaves to fold, curl or pucker. These insect
symptoms can be confused with some plant diseases that
cause similar symptoms. You can diagnose the damage as
insect damage if you find the aphids themselves inside the
curled leaves. Some caterpillars, called leafrollers, use
silk threads to hold leaves in a curled or rolled shape.
These leafroller caterpillars may also be found inside the
rolled leaves.
Erinea leaf patches
Some microscopic mites, called eriophyid mites, give off
substances that cause plant tissues to grow in an
irregular way. One common type of induced growth is the
fuzzy or spongy patches on leaves called erinea. These
patches usually appear between leaf veins, are irregularly
shaped, and may be red or silver. Because the mites are
too small to see without a microscope, the damage may be
incorrectly blamed on a mysterious plant disease.
Pitch flow
Many trees respond to trunk or twig injury by producing
sap or pitch around the injured area. The pitch flow is
believed to be a tree defense mechanism to prevent
additional injury from insects and diseases. Wood-boring
insects and bark beetles usually induce the plant to
produce pitch where the insects are actively causing
injury. Plant pathogens and mechanical injury can also
induce pitch flow.
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