Michigan State University Extension
Ornamental Plants plus Version 2.0 - 00001706
01/01/98
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 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.