5.3 Arthropod Pests: Feeding Guilds, Signs, Symptoms & Field Recognition
Key Takeaways
- Chewing damage removes tissue and leaves ragged margins and frass; sucking damage removes cell contents and leaves stippling, honeydew, and distortion without tissue loss.
- Stippling with fine webbing and a dusty bronzed appearance indicates spider mites, which are arachnids with eight legs as adults rather than insects.
- D-shaped exit holes indicate flatheaded borers such as emerald ash borer, while round exit holes indicate roundheaded borers and many bark beetles.
- Honeydew and the sooty mould growing on it are signs of phloem-feeding Hemiptera, and their presence on lower foliage or parked cars localizes the infestation above.
- Most borers of hardwoods are secondary, attacking trees already in carbohydrate deficit, so a borer finding should trigger a search for the predisposing site factor.
The outline requires the Board Certified Master Arborist to identify symptoms and signs of pest arthropods, pathogens, diseases, and wildlife which cause damage to trees. On a scenario-based exam you will not be asked to key a specimen. You will be given a description of damage and asked what caused it and what to do. That is a feeding guild problem.
Diagnose the Guild Before the Species
| Guild | Mouthparts / mechanism | Characteristic signature | Representative groups |
|---|---|---|---|
| Chewing | Mandibulate; removes tissue | Ragged or notched margins, holes, skeletonization, defoliation, frass pellets | Caterpillars, sawfly larvae, leaf beetles, weevils, grasshoppers |
| Piercing–sucking (mesophyll) | Stylets into cells | Stippling — fine pale flecks; bronzing; no tissue removed | Lace bugs, leafhoppers, thrips (rasping), spider mites |
| Piercing–sucking (phloem) | Stylets into sieve tubes | Honeydew, sooty mould, ants, distortion, chlorosis, dieback | Aphids, soft scales, mealybugs, adelgids, psyllids, planthoppers |
| Piercing–sucking (xylem) | Stylets into xylem | Little direct injury; vectoring of xylem-limited bacteria | Sharpshooters, spittlebugs |
| Boring | Larvae tunnel in phloem, cambium, or wood | Exit holes, galleries, frass and boring dust, sap or pitch flow, bark splitting, crown thinning from the top | Flatheaded and roundheaded borers, bark beetles, clearwing moths, carpenterworms |
| Mining | Larvae feed between epidermal layers | Blotch or serpentine mines; a blister that peels apart to reveal frass inside | Leafminer flies, moths, sawflies |
| Galling | Feeding stimulates abnormal host growth | Discrete swellings on leaf, petiole, twig, or bud, often species-specific in shape | Gall midges, gall wasps, eriophyid mites, adelgids |
| Root feeding | Larvae or adults feed on roots | Crown decline without above-ground signs; larvae in soil | Weevil larvae, white grubs, root aphids |
The single most useful discrimination is whether tissue was removed (chewing) or merely emptied (sucking). A leaf that is intact in outline but pale, flecked, and bronzed was not chewed.
Signs Worth Memorizing
Frass, boring dust, and exit holes
- Frass is insect excrement, often mixed with chewed tissue. Coarse pellets on the ground or in bark crevices come from caterpillars; fine sawdust-like boring dust extruded from a hole comes from a borer or from a bark beetle.
- Exit hole shape is a diagnostic shortcut: a D-shaped hole roughly 3–4 mm across is a flatheaded borer (Buprestidae — emerald ash borer, bronze birch borer, two-lined chestnut borer). A round hole is a roundheaded borer (Cerambycidae) or a bark beetle. Ragged holes with expelled frass and silk are clearwing moths or carpenterworms.
- Galleries read like a signature: bark beetles produce an egg gallery with larval galleries radiating perpendicular from it; flatheaded borers produce serpentine, frass-packed galleries in the phloem-cambium interface.
- Pitch tubes on conifers and sap or gummosis flow on hardwoods mark attempted or successful entry and often indicate a tree with enough vigour to respond.
Honeydew and sooty mould
Phloem feeders take in far more sugar solution than they can use and excrete the excess as honeydew. Its presence is a sign, and it localizes the infestation: honeydew and black sooty mould on lower foliage, understory plants, sidewalks, or vehicles means a phloem feeder is somewhere above. Ant trails ascending a stem often indicate ants tending aphids or soft scales for honeydew — and ants defending those colonies from predators and parasitoids is a real obstacle to biological control.
Webbing and casings
- Fine webbing on foliage with stippling and a dusty appearance: spider mites. Confirm with the tap test — strike foliage over a white sheet and watch for moving specks.
- Coarse silk tents: eastern tent caterpillar (branch crotches, spring), fall webworm (branch tips, late summer), and bagworms (spindle-shaped bags built of host foliage).
- Exuviae — cast skins — under leaves or on bark confirm that an infestation is active rather than historic.
Mites Are Not Insects
Mites belong to Acari within Arachnida. Adults have eight legs and no antennae or wings.
- Spider mites (Tetranychidae) are mesophyll feeders producing stippling, bronzing, and webbing. Populations explode in hot, dry, dusty conditions and after broad-spectrum insecticide applications that eliminate predatory mites. This is the classic secondary pest outbreak, and it is a favourite exam scenario.
- Eriophyid mites are microscopic, wormlike, with four legs, and cause rust, erineum (felt-like) patches, and galls. They are a cause of abnormal growth that is easily mistaken for disease.
Primary versus Secondary Status
This distinction converts a pest finding into a correct recommendation.
| Behaviour | Examples | Recommendation implication |
|---|---|---|
| Primary / aggressive — attacks apparently healthy trees | Emerald ash borer on North American ash; gypsy-type defoliators; Asian longhorned beetle | Direct suppression of the pest is legitimate and often mandatory under regulation |
| Secondary / opportunistic — attacks trees already stressed | Two-lined chestnut borer, bronze birch borer, most Agrilus on native hosts, many bark beetles, Armillaria | Find and correct the predisposing site factor; treating the insect alone fails |
A borer finding on a native host should therefore trigger a site investigation — rooting volume, bulk density, drainage, grade change, recent construction, drought history — before any product recommendation.
Distinguishing Arthropod Damage from Abiotic Injury
| Feature | Arthropod | Abiotic |
|---|---|---|
| Distribution across the population | Often species-specific, patchy, or clustered | Frequently affects multiple unrelated species along a gradient |
| Signs present | Frass, exuviae, webbing, honeydew, the organism | None |
| Pattern on the individual | Related to feeding site and access | Related to exposure — prevailing wind, road, sun, grade |
| Margins of affected tissue | Irregular, following feeding | Often sharply defined by exposure or spray drift line |
Wildlife Damage
The outline explicitly includes wildlife. Recognize the signatures:
- Deer: browse line at a uniform reachable height; torn rather than clipped twig ends because deer lack upper incisors; antler rubbing shredding bark on small-diameter stems in autumn.
- Rabbits and voles: clean 45-degree clipped twigs (rabbits); girdling at or just below the ground line under mulch or snow, with paired incisor grooves (voles).
- Sapsuckers: neat horizontal and vertical rows of shallow rectangular wells, often on thin-barked trees; repeated years of wells can girdle.
- Woodpeckers other than sapsuckers: irregular excavation, usually indicating insect larvae beneath — the bird is a diagnostic clue, not the cause.
- Squirrels: bark stripping on upper branches, often in strips, sometimes girdling limb tips.
- Beaver: characteristic chiselled, pointed stumps near water.
Wildlife management is largely exclusion: trunk guards clear of the stem and extended above expected snow depth, hardware cloth against voles, and fencing or repellents against deer.
An arborist inspects a linden showing pale flecked foliage with a bronzed cast, fine webbing between petioles, and abundant dust from an adjacent gravel drive. Broad-spectrum insecticide was applied for aphids six weeks earlier. What is the most probable situation?
A consulting arborist finds crisp D-shaped exit holes about 3.5 mm wide, serpentine frass-packed galleries at the phloem-cambium interface, and thinning that began in the upper crown of a native birch growing in a compacted, west-facing lawn. What should the primary recommendation address?
A client reports that cars parked beneath a row of mature maples are coated in a sticky film and that the lower foliage of shrubs beneath the trees is turning black. What do these observations indicate?
Which observation most reliably distinguishes deer browsing from rabbit feeding on a young tree?