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Diagnosing Plant Damage


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  MG Manual Reference
Ch. 5, pp. 16 - 20
[Determine Causes: determine | symptoms and signs | distinguishing | chemical injury]



The location of the feeding damage on the plant caused by the insect’s feeding, and the type of damage (damage from chewing or from sucking mouth parts) are the most important clues in determining that the plant damage is insect-caused and in identifying the responsible insect.
An insect’s life cycle (complete or incomplete) is important when attempting to detect the insect or design a control program.


Chewing Damage or Rasping Damage:
Entire Leaf Blade Consumed by various caterpillars, canker worms, and webworms. Only tougher midvein remains.
Distinct Portions of Leaf Missing. Distinct notches cut from leaf margin (black vine weevil adult), circular holes cut from margin of leaf (leaf cutter bees), small randomly scattered holes in leaf (beetles, chafers, weevils, grasshoppers).
Leaf Surfaces Damaged: "Skeletonization" of leaf surface. Slugs, beetle larvae, pearslug (pear sawfly larvae), elm leaf beetle, and thrips.
Leaves "rolled": Leaves that are tied together with silken threads or rolled into a tube often harbor leafrollers or leaftiers, i.e. omnivorous leaftier.
Leaf Miners Feed Between the Upper and Lower Leaf Surfaces. If the leaf is held up to the light, one can see either the insect or frass in the damaged area (discolored or swollen leaf tissue area), i.e. boxwood, holly, birch, elm leaf miners.
Petiole and Leaf Stalk Borers burrow into the petiole near the blade or near the base of the leaf. Tissues are weakened and leaf falls in early summer. Sectioning petiole reveals insect-larva of small moth or sawfly larva, i.e. maple petiole borer.
Twig Girdlers and Pruners, i.e. vine weevil and twig girdling beetle.
Borers Feed under the Bark in the cambium tissue or in the solid wood or xylem tissue, i.e. Mountain pine beetle and smaller European elm bark beetle galleries. Damage is often recognized by a general decline of the plant or a specific branch. Close examination will often reveal the presence of holes in the bark, accumulation of frass or sawdust-like material or pitch, i.e. raspberry crown borer, Sequoia pitch moth.
Root Feeders, larval stages of weevils, beetles and moths cause general decline of plant, chewed areas of roots, i.e. sod webworm, Japanese beetle, root weevil.
Sucking Damage:
In addition to direct mechanical damage from feeding, some phloem-feeding insects cause damage by injecting toxic substances when feeding. This can cause symptoms which range from simple stippling of the leaves to extensive disruption of the entire plant. Insect species which secrete phytotoxic substances are called toxigenic (toxin-producing) insects. The resulting plant damage is called "phytotoxemia" or "toxemia". (Chapman, R.K. 1985. Insects that poison plants. American Vegetable Grower 33-10:31-38, October 1985).
Spotting or Stippling result from little diffusion of the toxin and localized destruction of the chlorophyll by the injected enzymes at the feeding site. Aphids, leafhoppers, and lygus bugs are commonly associated with this type of injury.
Leaf curling or Puckering – More severe toxemias such as tissue malformations develop when toxic saliva causes the leaf to curl and pucker around the insect. Severe aphid infestations may cause this type of damage.
Systemic Toxemia – In some cases the toxic effects from toxigenic insect feeding spread throughout the plant resulting in reduced growth and chlorosis. Psyllid yellows of potatoes and tomatoes and scale and mealy bug infestations may cause systemic toxemia.
  • General (uniform) "stipple" or Flecking or Chlorotic Pattern on leaf i.e. adelgid damage on spruce needles and bronzing by lace bugs.
  • Random Stipple Pattern on leaf, i.e. leafhoppers, mites.
  • Leaf and Stem "distortion" associated with off-color foliage = aphids (distortion often confused with growth regulator injury), i.e. rose aphid, black cherry aphid, leaf curl plum aphid.
  • Galls, Swellings on leaf and stem tissue may be caused by an assortment of insects, i.e. aphids, wasps, midge, mossyrose gall wasp, poplar petiole gall midge, azalea leaf gall.
  • Damaged Twigs = Split: Damage resembling split by some sharp instrument is due to egg laying (oviposition) by sucking insects such as tree hoppers and cicadas. Splitting of the branch is often enough to kill the end of the branch, i.e. cicada.
  • Root, Stem, Branch Feeders – General Decline of Entire Plant or Section of a Plant as indicated by poor color, reduced growth, dieback. Scales, mealy bugs, pine needle scale.


Knowledge of life cycles assists in identifying the damaging insect.
Incomplete Life Cycle:

Insects resemble the adult upon hatching, except they are smaller and without wings. As the insect grows, it sheds its skin or molts leaving cast skins as a diagnostic sign. Adult stage is most damaging.
Lygus bugs, leafhoppers, and grasshoppers are examples of insects with incomplete life cycles.
Complete Life Cycle:

Eggs, larva (wormlike or grub-like creature that may feed on various plant parts), pupa (relatively inactive, often enclosed in some form of cocoon), ADULT INSECT COMPLETELY DIFFERENT IN APPEARANCE. The larval stage with chewing and rasping feeding is most damaging.
Examples of insects with complete life cycles are butterflies, moths, weevils, beetles and flies.


Arachnids have sucking mouth parts and have 8 legs instead of six like the insects. Spider Mites, incomplete life cycle (mite resembles adult throughout life cycle). Damage is often a CHARACTERISTIC STIPPLE PATTERN ON LEAF which then becomes pale color on underside (severe infestation causes leaf bronzing and death). Presence of "dirty" foliage = small fine webbing on the underside of the foliage mixed with eggs and frass. Eriophyid Mites = DISTORTED NEW GROWTH, leaf margins roll, leaf veins swell and distort the leaf, (symptoms often confused with growth regulator damage).
Crustacea — Sow bugs and pill bugs feed on decaying vegetation. NOT CONSIDERED TO BE DAMAGING TO LIVE PLANTS.
Mollusca — Slugs and snails. Feeding injury to low growing foliage resembles SKELETONIZING or ACTUAL DESTRUCTION OF SOFT TISSUE. Signs: Presence of `silvering’ and slime trails on foliage.
Miscellaneous Animals — Millipeds and centipedes (arthropods) feed on decaying plant vegetation (many small legs, brownish or white in color, vary in size from 1/2 – 2"). NOT CONSIDERED INJURIOUS TO LIVE PLANTS.
Small Mammals — Chewing of bark and cambium tissue on small trees and shrubs is most frequently by rodents (mice, rabbits, squirrels, and possibly beavers). Signs: Note teeth marks.
Large Mammals — Branches torn or clean cut by cattle, goats, deer, and horses.
Birds — Yellow-bellied sapsucker (even rows of holes in the tree trunk). Missing flower petals, puncture splitting of bark.

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