CSI for Bugs, Part 5: Insect Products Provide Clues

This article is the fifth in a series for TCI Magazine by the author designed to help arborists understand and diagnose problems caused by insects and mites on woody landscape plants. The first article, “Basics for Diagnosing Problems Caused by Insects and Mites” (TCI, November 2020), focused on using signs and symptoms to help pin down the identity of the perpetrating pest. Article two, “Diagnosing Injury Caused by Insects with Chewing Mouthparts” (TCI, February 2021), discussed symptoms caused by defoliating insects. Part 3, “Diagnosing Dieback Caused by Insects” (TCI, March 2021), discussed insects associated with dieback in the canopies of woody plants. Part 4, “Diagnosing Discoloration and Distortion Caused by Insects and Mites” (April 2021), discussed what changes in the normal color of leaves can reveal about the damage being done and the perpetrators causing it.

In this final installment of “CSI for Bugs,” one additional collection of clues useful in diagnoses is presented. In addition to symptoms associated with plant injury, insects and mites leave behind products that serve as clues to their identity much the same way a hair, clothing fiber or bodily secretions can be used by crime-scene investigators to identify the human perpetrator of a crime. These are grouped into three major categories.

Products associated with waste

Honeydew and sooty mold – All living creatures excrete waste products as they grow and develop. In the case of insects and mites that damage plants, waste products provide excellent clues to the identity of a pest. As we learned in the article dealing with discolored and deformed plant parts, several sucking insect pests, including aphids, soft scales, whiteflies, psyllids, lanternflies and mealybugs, suck phloem sap as their primary source of nutrition. Often, vast quantities of sap are consumed to obtain nutrients to support growth and development. Similarly, vast quantities of waste liquid are often excreted as a product called honeydew. (Photo 1) Sugar-rich honeydew is highly attractive to stinging insects such as yellow jackets, hornets and paper wasps, which then become secondary pests in a client’s landscape.

Photo 1: Sooty mold grows on carbohydrate-rich honeydew excreted by sucking insects, including aphids, soft scales and whiteflies. Cottony camellia scale, a soft scale, is the culprit here. All photos courtesy of Michael Raupp, unless otherwise noted.

In addition, honeydew becomes a substrate for the growth of non-pathogenic fungi known as sooty mold. Accumulations of sooty mold often cloak leaves and underlying vegetation with an ugly black coat. In some cases, this may suppress growth and development on underlying vegetation. While not directly pathogenic, sooty mold may reduce the photosynthetic capacity of leaves. When sooty mold is observed on leaves, inspect leaves and stems above those discolored to detect the sucking insects excreting honeydew.

Frass and tar spots – Frass is the polite term for insect excrement and plant debris often mixed with the excrement. In some cases, such as tiny engraver beetles that attack pine trees, frass may be quite dry and sawdust-like in appearance and texture. Other wood-boring beetles, including granulate ambrosia beetles and black ambrosia beetles, produce strange, toothpick-like tubes of frass that protrude from the trunk of infested trees. (Photo 2) The highly destructive Asian longhorned beetle is another borer whose presence can be detected in infested trees by the presence of frass accumulating near galleries or beneath an infested tree. In other cases, frass of large caterpillars, such as gypsy-moth caterpillars or orangestriped oakworms, can be large and pellet-like.

Photo 2: Strange, toothpick-like cylinders of frass are the hallmark of granulate ambrosia beetles. Photo courtesy of Paula Shrewsbury.

Frass of caterpillars is often lined with distinctive ridges left by the caterpillar’s digestive tract. (Photo 3) Frass accumulating on the bole or at the base of an ash tree can be used to differentiate between attack by emerald ash borer and one of the clearwing borer moths, such as banded ash clearwing or lilac/ash borer. Emerald-ash-borer larvae do not push frass from their galleries beneath the bark, but clearwing borers do.

Photo 3: Large pellets of frass beneath a tree are a good clue that caterpillars, such as orangestriped oakworm, may be dining on foliage above.

Fecal spots, sometimes called tar spots, are excellent clues for the presence of many sucking insects such as lace bugs, thrips and some plant bugs, as well as chewing insects such as leaf beetles. These insects produce semi-liquid excrement called tar spots on the surface of leaves. These excellent clues may aid in pinning down the identity of pests, especially when coupled with other damage symptoms. For example, azaleas often become discolored due to nutrient imbalances or pest attack. Close examination of leaves may reveal stippling associated with the discoloration. Could this be spider mites or lace bugs? An inspection of the undersurface of leaves discloses black tarry spots. Spider mites produce no such spots, so the culprit behind the damage is azalea lace bug. Now the appropriate corrective therapy can be implemented. (Photo 4)

Photo 4: Discolored azaleas? Turn over a leaf, and if you see tar-like fecal spots, the culprit is azalea lace bug.

Products associated with defense, transit and thermoregulation

Silk – Caterpillars, the larvae of moths and butterflies, produce silk, as do several other groups of pest insects including sawflies, such as dogwood sawfly and butternut woollyworm, and spider mites. Silken galleries or tents provide protection from predators and parasites, and form trails and highways for pests like eastern tent caterpillars to locate food. Tents made by caterpillars trap heat, enabling these early-season pests to digest their food more efficiently and develop more rapidly on chilly spring days.  Silken webs allow spider mites to move quickly from one part of a plant to another.

Photo 5: Eastern tent caterpillars build silken tents in the crotches of trees.

Location and the structure of tents are useful clues to help identify tent-making caterpillars, webworms and leaf tiers. Eastern tent caterpillars are defoliators with a single generation annually. In early spring, they construct small tents that line branches and clusters of leaves near the eggs from which they hatched. Later, as caterpillars grow and develop, they build large tents in branch crotches of rosaceous trees including cherries, hawthorns, crabapples and apples. (Photo 5) Fall webworms often are mistaken for tent caterpillars. These defoliators, with two generations annually, build ever-expanding tents at the tips of branches of several species of trees in summer and autumn. (Photo 6)

Photo 6: Fall webworms build expanding webs on the terminals of branches that may engulf entire trees.

Pine webworm caterpillars build messy, silken galleries filled with pellets of frass on several species of pines, including mugo, Virginia and Scotch pines. Mimosa webworms build similarly nasty, frass-packed webs on mimosa and honeylocust. These rascals have two generations in parts of their range, and canopies of honeylocusts planted along roadways often look like they have been singed with a blowtorch. Euonymus caterpillar builds expanding nests on terminals of several species of euonymus, including Euonymus europaea, E. kiautschovicus and E. alatus. This invasive pest is purported to kill euonymus shrubs in a single season when densities are high.

Several species of caterpillars fold leaves, roll edges of leaves or tie leaves together with silk, creating protected shelters in which caterpillars feed. For example, azalea leafminers fold the edge of an azalea leaf and anchor it to the leaf blade with silk to form a protected feeding site.

Insects are not the only arthropod pest to integrate silk into their feeding repertoire. At high densities, spider mites also produce webs of fine silk. These webs connect adjacent parts of plants, creating a highway for mites to move rapidly from one part of a plant to another.

Protective cases – Bagworms are the quintessential example of an insect that produces a protective case. Incorporating pieces of vegetation into a strong silken cone surrounding their bodies, bagworms look nothing like a caterpillar. This may help them fool the hungry eyes of birds and other predators that recognize caterpillars as a tasty meal. Bagworms transport their silken refuge as they move about plants to feed. (Photo 7) The bagworm’s bag also shelters the female moth and serves as a refuge for her eggs in winter. Bagworms are one of the most important pests of landscape plants, especially conifers, in the eastern United States. Smaller caterpillars known as case bearers also construct silken sacks in which they reside and carry about as they dine on leaves and buds.

Photo 7: Plant parts incorporated into a silken bag provide protection for bagworm caterpillars, female moths and eggs.

Wax – One universal challenge faced by terrestrial insects is water retention. Due to a very large surface-area-to-
volume ratio, insects tend to lose water at a much higher rate than larger animals. To conserve water, the outer surface of most insects is coated with waxes or lipids. In addition to reducing moisture loss, wax helps insects shed water droplets from rainfall or dew that might be detrimental to developing eggs. Hemlock woolly adelgids and cottony soft-scale insects, such as cottony maple scale, enclose their eggs in fluffy, waxy egg sacs. (Photo 8)

Photo 8: Hemlock woolly adelgids produce fluffy cloaks of white wax to enclose females and their eggs.

Dense filaments of wax, like those of woolly aphids, may confound predators as they attempt to eat an aphid and, instead, wind up with a mouthful of wax. Wax is believed to provide some defense against parasitic wasps as they attempt to lay eggs on or in an insect’s body. Rays of solar and ultraviolet radiation harmful to soft-bodied insects are reflected by wax. Surface waxes coating insects also are thought to trap and release chemicals important in helping members of the same species recognize each other.

Photo 9: Florida wax scales are common pests of hollies in the eastern United States.

The shape, texture and color of wax provide important diagnostic clues for key pest groups such as soft- and
armored-scale insects. Several species of soft scales produce copious amounts of wax and are known as wax scales. (Photo 9) Armored scales, such as white peach and white prunicola scales, have whitish waxy covers, while obscure scales and San Jose scales have black or gray waxy covers. (Photo 10) Tiny Japanese maple scales and oystershell scales have covers resembling oysters.

Photo 10: Encrustations of gray, round scales on the bark of pin oaks are the clue to identifying obscure scales. Note the small exit holes on the scale cover where lethal parasitic wasps have emerged.

Sometimes, wax covers of armored-scale insects blend with the color of the tree’s bark, complicating detection and diagnosis. While wax is a hallmark of many sucking insects, including adelgids, mealybugs, some aphids, whiteflies, psyllids and scale insects, other pests, such as larvae of sawflies like dogwood sawfly or woolly alder sawfly, have impressive cloaks of wax.

Shed skins – Due to the presence of an exoskeleton, insects must shed their skins on a regular basis to grow. Shed skins of pests such as aphids and leafhoppers often remain on the surfaces of leaves for long periods of time after the insect has developed. (Photo 11) Due to their small size, shed skins are sometimes misidentified as living, active pests. A simple poke with a pencil will differentiate between a living, moving insect or an immobile, shed skin. Even when pests are no longer present, shed skins are useful clues in assessing incipient infestations. Shed skins of aphids or leafhoppers on deformed leaves of a Japanese maple in July may warrant an inspection of the same plant early in the spring of the following year.

Photo 11: Even though the leafhoppers on this plant are gone, their shed skins are a clue that they may be back next year.

Spittle – Frothy masses of spittle may occur on a wide variety of herbaceous and woody plants. Adult spittlebugs are called froghoppers and are close relatives of leafhoppers. By mixing air with liquid waste products, they produce a spit-like mass. While most spittlebugs do little or no damage to plants, two-lined spittlebug is reported to cause leaf distortion on hollies.

Plant products useful in diagnosing insect pests

Pitch tubes, pitch masses and gummosis – Attacks by insects often cause plants to respond with clear, visual clues that are not directly produced by an insect. These same responses can be the result of mechanical wounding caused by tools or machinery, or in some cases, by attack pathogens such as bacteria or fungi.

Pressurized resin canals are part of the defensive arsenal of conifers such as pine trees. When bark beetles breach the outer bark defense of pines, they puncture resin canals and release resin. Tiny Ips engraver beetles may cause small dribbles of sap to appear near points of entry. Larger beetle borers, such as turpentine beetles and southern pine beetles, may induce larger flows of sap. (Photo 12) These large exudates of resin mass can entrap and sometimes physically expel the invader from the plant.

Photo 12: Pitch tubes are a good clue that bark beetles are attacking a tree.

As resin oozes from the tree, it may accumulate and harden to form pitch tubes extending beyond the bark. Pitch tubes are excellent indicators of attack by bark beetles. Attack by other insects, such as Nantucket pine tip or Zimmerman pine shoot moth, may cause pines to respond with large masses of pitch beneath which these caterpillars feed. Deciduous trees such as cherries may respond to infestations of boring caterpillars, such as peach tree borer and lesser peach tree borer, with flows of thick sap called gummosis.

The products insects and mites leave behind can aid in identifying the perpetrators of injury to woody landscape plants. We hope these five articles will help you hone your diagnostic skills, make the right call on identification and provide appropriate interventions for your clients.


Cranshaw, W. and D. Shetlar. 2018. Garden Insects of North America. Second Edition. Princeton University press. Princeton, N.J. 704 pp.

Cullen, P. J. and P. S. Cranston. 2005. The Insects: An outline of Entomology, third edition 2005. Chapman and Hall. New York, New York. 491 pp.

Davidson, J.A. and Raupp, M.J. 2009. Managing Insects and Mites on Woody Plants: An IPM Approach. Tree Care Industry Association, Londonderry, N.H. 197 pp.

Koehler, C. S. 1987. Symptomatology in the instruction of landscape ornamentals entomology. J. Arboric.13: 78-80.

Russel, H. and H. Stevens. 2008. Euonymus caterpillar. Michigan State University Diagnostic Services, Michigan State University Extension.

Michael J. Raupp, Ph.D., is professor emeritus in the Department of Entomology at the University of Maryland, College Park, Maryland. He has published many articles, made numerous presentations and frequently appears on television and radio. His most recent book, 26 Things that Bug Me, published by ISA, introduces youngsters to the wonders of insects and natural history, while Managing Insects and Mites on Woody Landscape Plants, published by the Tree Care Industry Association, is a standard for the arboricultural industry. His websites include www.bugoftheweek.com and

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