8.2 Biotic Signs, Symptoms, and Host Patterns

Reading Living-Agent Evidence

Biotic disorders are caused by living organisms: insects, mites, fungi, bacteria, viruses, phytoplasmas, nematodes, and animals. Correct diagnosis depends on the host species, the plant part affected, the season, the distribution pattern, and any signs of the organism. The same leaf spot may be cosmetic on one host and lethal on another, so host identification comes first.

Insects by Feeding Style

Insect damage is sorted by how the insect feeds, and each style leaves a recognizable signature:

• Chewing insects (caterpillars, beetles, sawflies) remove tissue, leaving holes, notched margins, or skeletonized leaves. Heavy chewers are defoliators.
• Sucking / sap-feeding insects (aphids, scales, leafhoppers, lace bugs, spider mites) pierce tissue and remove fluids, causing stippling, chlorosis, leaf curl, and a sticky exudate called honeydew that supports black sooty mold.
• Boring insects (bark beetles, flatheaded and roundheaded borers, emerald ash borer) feed under bark or in wood, leaving exit holes, frass, serpentine or D-shaped galleries, sap flow, and progressive dieback.
• Gall makers (some wasps, mites, adelgids) induce abnormal swellings; most are cosmetic.

Pathogens by Plant Part

Vascular wilts such as Dutch elm disease and oak wilt move through the water-conducting tissue and can kill quickly; the streaked sapwood is a key sign. Root and butt-rot fungi produce conks at the base and raise structural-failure concerns, linking this domain to Tree Risk (domain 8).

Host Pattern and Life-Cycle Timing

Host pattern is decisive. If only one genus declines in a mixed planting, suspect a host-specific pest or pathogen. If unrelated species share symptoms in one spot, shift toward abiotic causes. If one side of one tree is affected, look for canker, borer, trenching, salt spray, or sun exposure.

Life-cycle timing governs treatment windows. Many scale insects are controllable only at the mobile crawler stage; once they secrete a waxy cover, contact controls fail. Many foliar diseases are best managed preventively through sanitation, airflow, and resistant species, not by curative spraying after infection. The exam rarely asks for product labels, but it frequently asks whether monitoring is aimed at the susceptible life stage.

Primary Versus Secondary Pests

Many biotic agents are secondary: borers, canker fungi, and decay organisms typically attack trees already weakened by drought, deep planting, root injury, or compaction. Treating the organism while ignoring the underlying stress gives poor, short-lived results. Conversely, not every insect found is damaging; some are incidental, some are beneficial predators, and many sit below an action threshold. On the exam, choose the answer that confirms the host, separates signs from symptoms, targets the vulnerable stage, and addresses predisposing stress, rather than the answer that sprays because a leaf is yellow.

Mites, Bacteria, Viruses, and Nematodes

Beyond insects and fungi, several agent groups appear in scenarios. Spider mites are not insects (they have eight legs) and thrive in hot, dusty, drought conditions; their feeding causes fine stippling and bronzing, and broad-spectrum insecticides can worsen outbreaks by killing predatory mites. Bacterial diseases such as fire blight produce wilted, blackened "shepherd's-crook" shoots and oozing cankers; sanitation pruning well below the visible margin, with disinfected tools, is the standard cultural response.

Viruses and phytoplasmas cause systemic symptoms like mosaic patterns, witches'-broom, or yellows and generally cannot be cured, so management focuses on removing the source and controlling insect vectors. Nematodes are microscopic roundworms; some parasitize roots, producing decline that mimics drought, while the pinewood nematode causes lethal pine wilt.

Distinguishing Look-Alikes

These pairings are common exam traps because each pair shares a surface symptom (browning, dieback, wilt) but demands a different management response. The discipline is the same throughout the domain: confirm the host, find the most diagnostic evidence, and let that evidence, rather than the first obvious symptom, drive the conclusion. When two candidates remain plausible, the higher-value or higher-consequence cause (a lethal vascular wilt over a cosmetic leaf spot) usually deserves the more cautious, confirm-before-acting response.