In Part 1 of this article (TCI Magazine, October 2025), we introduced four foundational elements useful in diagnosing problems with trees and shrubs, represented by the acronym SPSD, and discussed the first “S,” which stands for “site,” and the “P,” which stands for “plant.” In part 2, we’ll discuss the second “S,” which stands for the “symptoms.” Knowledge of signs and damage symptoms is a powerful tool in making an accurate diagnosis and developing an effective intervention. We’ll also discuss the “D,” which stands for “date.” Date can be defined by an actual Gregorian date, such as May 15, or by using a proxy such as a growing-degree-day value or phenological plant indicator that is linked to the seasonal development of pests and plants.

Photo 1: Black vine weevil. All photos courtesy of the authors.

“S” stands for “symptoms” – zeroing in on causes of plant problems
Between November 2020 and June 2021, Tree Care Industry Magazine published a five-part series of articles focused on honing the diagnostic skills of arborists. This series detailed a forensic approach to diagnostics called symptomatology that uses clues left behind by a pest to help identify the culprit. Symptomatology was first described for use in arboriculture by Carlton S. Koehler (1987) and later by Davidson and Raupp (2014).

Photo 2: Birch-tree dieback caused by bronze birch borer.

Using the analogy of crime-scene investigators, Part 1 of the series, entitled “CSI for Bugs: Basics for Diagnosing Problems Caused by Insects and Mites,” explored the fundamental steps in the diagnostic process. The second installment, “CSI for Bugs, Part 2: Diagnosing Injury Caused by Insects with Chewing Mouthparts,” zoomed in on key insect pests with chewing mouthparts, including beetle larvae and adults, caterpillars and larval sawflies.

We learned that insects with chewing mouthparts caused several categories of defoliation as they munched leaves, including course defoliation where entire leaves often disappear and smaller “shot holes” made by young insects and those with tiny mouths. We also learned about notches on leaf margins made by beetles called weevils (Photo 1), and interveinal feeding creating damage known as skeletonization or etching, where leaf eaters remove all soft tissues between leaf veins or soft tissues and one epidermal layer of a leaf.

“CSI for Bugs, Part 3: Diagnosing Dieback Caused by Insects,” dealt with four insect clans: larvae and adult beetles, caterpillars of moths, larvae of wasps and nymphs and adults of scales. Feeding by these pests has the potential to damage vital meristematic tissues like cambium and vascular elements, particularly phloem, so severely that branches and sometimes entire trees die. Well-known examples include flatheaded borers, such as bronze birch borer (Photo 2) and emerald ash borer, and round-headed borers, such as Asian longhorn beetle.

Part 4 of the series, “CSI for Bugs, Part 4: Diagnosing Discoloration and Distortion Caused by Insects and Mites,” explored how chewing and sucking insects and mites change the fundamental color of leaves and the normal shape of buds, leaves, branches and boles of trees. Categories of discoloration include stippling, leaf-mining, yellowing, bronzing, silvering and russeting of leaves. If you said these symptoms often are caused by sucking insects like scales and aphids or mites, you would be correct. However, some leaf-mining larvae of flies, beetles, sawflies and moths (caterpillars) also get into the act. (Photo 3) Distorted buds, leaves, branches and boles reflect damage to undifferentiated tissues, meristematic tissues of leaves, buds, shoots and cambium that result in abnormal growth. Damage symptoms include leaf cupping and curling and galls of leaves and twigs. (Photo 4)

The final installment of the series, “CSI for Bugs, Part 5: Insect Products Provide Clues,” details how products associated with insect waste like honeydew and sooty mold can help pin down perpetrators like scale insects or lanternflies. Frass and tar spots, and fecal deposits of caterpillars and lace bugs, provide solid diagnostic evidence for these pests. Fine, sawdust-like frass beneath trees is the tell-tale sign of bark beetles, and toothpick-like waste poking out of the bark of a tree is a sure sign of attack by ambrosia beetles.

Products associated with defense, transit and thermoregulation include silk produced by caterpillars, sawflies and spider mites. Wax is a product associated with many kinds of sucking insects, including scales, adelgids and aphids. As aphids and leafhoppers grow and molt, their shed skins leave behind telltale clues of their presence. Trees respond to attack by beetle and moth borers by extruding masses of resin called pitch tubes in conifers. Deciduous trees like cherries produce gummy masses of sap in response to attack by pathogens and borers.

Please note that these categories of symptoms are not exclusive to attack by biotic agents. The same symptom can have multiple underlying causes. While leaf discoloration can be caused by sucking insects, discoloration also results from nutrient deficiencies in soil, viral or bacterial pathogens in leaves and abiotic stressors like drought or flooding.

Photo 3: Hawthorn leafminer.

Signs
However, one other category of clues called signs can help pin down the true agent or agents underlying a symptom. Signs are quite definitive and can include the organism itself or direct products left behind by the organism.

Signs include adults, eggs, nymphs, larvae, pupae, cocoons, adult insects and their shed skins. Signs also can include specific, easily recognized objects and other clues left behind by an insect or mite. Silk, frass, wax coverings, characteristic feeding patterns on leaves and exit holes in bark also can serve as signs. For example, dieback in a green ash with D-shaped exit holes and no frass on the bark or ground is almost certainly caused by attack of emerald ash borer. However, if exit holes are round or oval, frass is present beneath the tree or a papery pupal case hangs from an exit hole, these are clear signs of attack by clearwing borers, like banded ash clearwing.

By linking symptoms and signs to a causal agent, an accurate diagnosis can be made and an appropriate strategy for therapy can be developed and implemented.

Photo 4: Horned oak gall.

“D” is for “date” – seasonal activity also can be a diagnostic clue
For insect pests, there is a season when they are active and readily observed. Knowing the time frame or date when an insect is observed on a plant can help pinpoint the identity of a pest and develop tactics to manage it. Reference books and online resources frequently list dates or seasons when pests are most active. Specific times or dates when control measures are most effective are called “windows of vulnerability.”

Traditional methods for targeting treatments for pests, including those used in our publications, attempted to bracket periods of pest activity and vulnerability. Statements like “apply insecticides to bark when adults are active and depositing eggs in May” are commonly seen in textbooks, bulletins and online references. However, general recommendations using dates might work in one location like Atlanta but not in a different location like Milwaukee, where May temperatures might arrive in June. Regional differences in latitude and altitude, as well as local differences in aspect and exposure, will affect temperatures and insect activity.

A more precise way to track insect activity and plan intervention strategies involves the use of growing degree days and plant phenology. Insects and mites are cold-blooded organisms, and therefore their development rate is directly related to temperature. Life cycles of plants in temperate regions also are strongly related to temperature. Seasonal phenological events like bud burst and flowering are linked to temperature. As temperatures get warmer, insects and plants develop faster. In addition, most insects have a lower and upper temperature-development threshold, below or above which, respectively, they do not develop.

Photo 5: Ash trees with dieback in planting island.

Growing degree days (GDD or DD) are a measure of the “heat units” (related to temperature and the amount of time per day that an insect spends actively growing) that accumulate over time. By keeping track of the number of DD that accumulate in a specific location over time, we can predict the activity of different insects and target vulnerable life stages for control measures. Important life-cycle events – such as the seasonal appearance of insects feeding on a plant, the hatching of insect eggs, the appearance of scale crawlers or the emergence of borers from a tree – will be driven by temperature and can be correlated to degree days and plant phenology.

Degree days and plant phenological indicators (PPI) are based on biological events influenced by temperature. Therefore, DD and PPIs can be used to more accurately predict when a susceptible life stage of a pest will be active compared to historical and relatively static Gregorian calendar dates. In an upcoming issue of TCI Magazine, we will provide additional information detailing methods to determine and use DD and PPIs to predict the timing of pest activity and target-susceptible life stages of insect and mite pests that can result in more effective pest management.

Plant problem unveiled using S, P, S, D
Let’s look at an example of how this all works.

Suppose your client has a row of small trees installed in an island in a parking lot. Your assessment of the site (S – site) indicates these trees may be stressed due to excessive heat and lack of water created by the impervious surface surrounding the trees. De-icing salts that accumulate when snow is piled on the island in winter may contribute to plant stress. (Photo 5)

Photo 6: Round exit hole. The shape and size of the hole as well as the tree species all aid in diagnosis.

You know that abiotic stress can predispose trees to attack by borers. You identify the trees as green ashes (P – plant) and note dieback (S – symptom) in the canopies of several trees. Green ash has several boring pests, including native clearwing borers and an exotic flatheaded borer. Round or oval exit holes and the absence of D-shaped holes are signs that this is not an emerald ash borer attack. (Photo 6) Frass beneath the tree is a symptom of a different type of borer, a caterpillar rather than a beetle. (Photo 7)

Larvae of two native moth borers, banded ash clearwing caterpillars and ash/lilac clearwing caterpillars, attack ash trees. Adults of the former emerge from the tree in late summer and adults of the latter in spring. By observing when pupal cases poke out of the tree (Photo 8), either in spring or late summer, date (D), will define which pest it is and allow you to apply bark applications of insecticides for optimal efficacy.

Photo 7: Coarse frass made by clearwing borer.

Conclusion
Honing diagnostic and management skills is only one aspect of plant health care, but an important one. Applying the four foundational elements represented by SPSD – site, plant, symptoms and date – in diagnosing problems with trees and shrubs can help.

Michael J. Raupp, Ph.D., is professor emeritus at the University of Maryland in College Park, Maryland. His writing, research and scientific outreach have received a dozen national and international awards. He is a regular guest on television and radio. His most recent book, “26 Things that Bug Me,” introduces youngsters to the wonders of insects and natural history, while “Managing Insects and Mites on Woody Plants,” published by the Tree Care Industry Association, is a standard for the arboricultural industry. Visit his websites at www.bugoftheweek.com and https://www.youtube.com/user/.

Paula M. Shrewsbury, Ph.D., is a professor of entomology at the University of Maryland in College Park, Md., specializing in ornamental integrated pest management (IPM), sustainable agriculture and invasive species management.

This article is based on their Pre-Conference workshop entitled “Plant Health Care Pathways” at TCI EXPO ’24 in Baltimore, Md.

Photo 8: Pupal case of clearwing borer.

Resources and references
Please visit these links to the five-part series designed to help arborists hone their diagnostic skills presented in previous issues of TCI Magazine.
“CSI for Bugs, Part 1: Basics for Diagnosing Problems Caused by Insects and Mites.” https://tcimag.tcia.org/training/csi-for-bugs-part-1-basics-for-diagnosing
-problems-caused-by-insects-and-mites/ (November 2020)
“CSI for Bugs, Part 2: Diagnosing Injury Caused by Insects With Chewing Mouthparts.” https://tcimag.tcia.org/training/csi-for-bugs-part-2-diagnosing-
injury-caused-by-insects-with-chewing-mouthparts/ (February 2021)
“CSI For Bugs, Part 3: Diagnosing Dieback Caused by Insects.” https://tcimag.tcia.org/training/csi-for-bugs-part-3-diagnosing-dieback-caused-by-insects/ (March 2021)
“CSI For Bugs, Part 4: Diagnosing Discoloration and Distortion Caused by Insects and Mites.” https://tcimag.tcia.org/training/csi-for-bugs-part-4-diagnosing-discoloration-and-distortion-caused-by-insects-and-mites/ (April 2021)
“CSI for Bugs, Part 5: Insect Products Provide Clues.” https://tcimag.tcia.org/training/csi-for-bugs-part-5-insect-products-provide-clues/ (June 2021)
Costello, L. R.; Perry, E. J.; Matheny, N. P.; Henry, J. M.; and Geisel, P. M. 2014. Abiotic disorders of landscape plants. University of California Agriculture and Natural Resources.
Davidson, J. A., and Raupp, M. J. 2014. Managing Insect and Mite Pests on Woody Plants: An IPM Approach. 2014. Third Edition. Revision. Tree Care Industry Association.
Grime, J.P. 1989. The stress debate: Symptom of impending synthesis? Biological Journal of the Linnean Society.
Johnson, W. T., and Lyon, H. H. 1988. Insects that feed on trees and shrubs. Cornell University Press.
Koehler, C. S. 1987. Symptomatology in the instruction of landscape ornamentals entomology. Journal of Arboriculture.
Raupp, M. J.; Davidson, J. A.; Holmes, J. J.; and Hellman, J. L. 1985. The concept of key plants in integrated pest management for landscapes. Arboriculture and Urban Forestry.
Raupp, M. J. 1984. Effects of exposure to sun on the frequency of attack by the azalea lace bug. Journal of the American Rhododendron Society.
Schutzki, R. E., and Cregg, B. 2007. Abiotic plant disorders symptoms, signs and solutions: A diagnostic guide to problem solving. Michigan State University Press.