CSI For Bugs, Part 3: Diagnosing Dieback Caused by Insects

This article is the third in a series by the author designed to help arborists diagnose and understand 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. In this article, we will learn about insects associated with dieback in the canopies of woody plants.

Dieback in the canopies of woody plants is associated with several different types of insects but primarily those from four orders of insects: three that have chewing mouthparts, including Coleoptera (larval and adult beetles), Lepidoptera (larvae, aka caterpillars, of moths) and Hymenoptera (larvae of wasps); and one with sucking mouthparts, Hemiptera (nymphs and adults of scales).

Dieback of shoots, twigs or branches

Plants are an integrated living system of structures and tissues working in coordination to collect the raw materials for photosynthesis from the environment. Plants produce materials for growth, defense and reproduction. Water, primary nutrients such as nitrogen, phosphorous and potassium, plus trace minerals are collected from the soil and transported by xylem elements to the tree’s canopy, where leaves build the complex molecules of life powered by the sun’s energy. Photosynthetic products move via phloem tissue throughout the tree and sustain not only above-ground portions but also the roots as they grow and capture water and minerals. Vascular cambium divides to create both xylem and phloem tissues. Damage to any element of this highly integrated vascular system can result in dieback in the tree’s canopy.

Photo 1: Before concluding that a pest is causing dieback on these ash trees, consider if restricted root zones, deicing salts, volcano mulch, deep planting, girdling roots, damage to bark caused by weed whackers or other abiotic factors are in play. Once you eliminate the obvious, look for symptoms and signs of insects and disease. Unless otherwise noted, photos by the author.

Whenever you notice the symptom of dieback in a tree, consider looking down as well as up. Before implicating biotic agents like insects or pathogens as causal agents, first eliminate abiotic factors. (Photo 1) Recent construction can create numerous problems. Installation of utilities or hardscape can sever roots. Undisturbed soil contains air spaces vital to root growth. Heavy construction equipment can compact soils and compress air spaces, thereby limiting the penetration of air and water needed by roots.

One very common cause of tree dieback and failure is the installation of trees too deeply in the ground or planting them in sites prone to waterlogging. Roots need oxygen, and planting poorly adapted trees to saturated soils can condemn roots to an anaerobic death. This death will soon be reflected in canopy dieback.

Girdling roots are often spawned when woody plants are grown in containers that restrict normal root extension and expansion. When roots contact the surface of the container, they will turn and begin to circle the container. When trees are installed, circling roots may eventually restrict radial expansion of the trunk and compromise vascular development and growth. Planting trees too deeply may also promote development of shallow surface roots that can encircle and girdle the trunk. Application of excessive amounts of mulch, whimsically called volcano mulch, fosters conditions for abnormal root development.

When roots die, so, too, will the branches they support. When dieback is observed, inspect the root collar and bole (trunk, stem) of the tree to rule out girdling roots, embedded wires or indestructible, plastic planter bags as the cause of dieback. Inspect the bole for mechanical damage to bark and underlying tissues caused by weed whackers, lawn mowers or vehicles.

Once obvious abiotic factors have been eliminated as a cause, look for symptoms and signs left behind by insects. Some of the easily observed visual clues used to identify insects responsible for dieback include entry and exit holes in bark; accumulation of sawdust-like waste products, called frass, on leaves, bark and the ground; bleeding wounds; split and sloughing bark on the tree trunk; and shed exoskeletons of insects emerging from the tree.

Dieback of shoots

As the name implies, shoot dieback means the death of the most distal portions of actively growing branches. Many caterpillars and beetles and some sawfly larvae use powerful jaws to bore down the centers of shoots from the tip toward the base, causing tips of branches to wither and die. Tip moths and shoot moths, including Nantucket pine tip moth and European pine shoot moth, are common causes of shoot dieback in landscapes, nurseries and Christmas-tree farms. Eggs laid at the tips of branches hatch and larvae tunnel from the tip toward the base, causing terminal branch ends to wither and die. Clues associated with these caterpillars include masses of pitch at the tips of branches that may contain frass, larvae or pupa.

While some pathogens like Diplodia tip blight may also cause shoot dieback on pines, this can be easily distinguished from tip-moth dieback by snapping open the dying shoot and observing a hollow gallery in the center created by the caterpillar. Diplodia produces no such gallery.

Photo 2: Dead, curled terminals called shepherd’s crooks in the top of white pine trees are commonly caused by white pine weevil borers. Photo by John Davidson.

Several other species, such as beetle larvae, may cause similar shoot dieback on conifers and deciduous trees. For example, adult white pine weevils chew small holes in terminal shoots (candles) of pine and spruce and deposit eggs within these holes. (Photo 2) Eggs hatch into legless grubs that tunnel down the center of the shoot, causing a classic dieback symptom known as a shepherd’s crook. Larvae of small, longhorned beetles like the dogwood twig borer cause similar shoot dieback on terminals of dogwood, viburnum, azalea and fruit trees. Several other relatives of the dogwood twig borer attack various trees, shrubs and cane fruit.

Dieback of branches and entire trees

As we move from the tips of branches to more inclusive components of trees, including the bole, several potentially lethal perpetrators can be responsible for dieback. Clues associated with dieback include the presence, absence, color and shape of insect frass and the shape and size of exit holes created as adult insects leave the plant.

Several strong-jawed beetles in the family Buprestidae, known as jewel beetles or metallic wood-boring beetles, attack not only trees under environmental stress such as drought but also otherwise seemingly healthy trees. Metallic wood-boring beetle larvae are called flatheaded borers. Emerald ash borer is the quintessential example of a lethal borer capable of attacking and killing seemingly healthy trees. Emerald ash borer is the single-most economically important pest of natural and managed forests in North America, where losses of ecosystem services and timber revenues and costs of control measures and tree replacement amount to billions of dollars annually.

Larvae hatching from eggs in bark crevices of ash and fringe trees bore through the bark and tunnel into nutrient-rich phloem and cambium where they feed. Winding, serpentine larval galleries filled with frass are the hallmark of flat-headed borers. Their feeding destroys meristematic cambium and vascular phloem and xylem, effectively girdling the tree and stopping the movement of nutrients, ultimately resulting in dieback in the canopy and, if left untreated, killing the entire tree. Larvae pupate beneath the bark, and, as adult beetles chew their way out, they create an exit hole shaped like the letter “D.” (Photo 3) D-shaped exit holes are signs of flatheaded borer attack.

Photo 3: Exit holes shaped like the letter “D” are the hallmark of metallic wood-boring beetles, including emerald ash borer. Their larvae are called flatheaded borers.

Native bronze birch borer larvae cause similar dieback on paper bark birches and can be equally lethal, especially to species of European and Asian provenance. Serpentine galleries packed with frass and D-shaped exit holes associated with canopy dieback all point to flatheaded borers as the perpetrator.

A second major group of wood-boring beetles responsible for dieback is longhorned beetles, whose larvae are called roundheaded borers. Generally, these beetles attack trees already under stress and colonize those already dead. Roundheaded borers achieved infamy when Asian longhorned beetles were discovered in New York City in 1996. Although many local populations of this rascal have been eradicated, some remain active and regulated in parts of New York, Massachusetts, Ohio and, most recently, South Carolina.

Dieback results when larvae bore through the bark and cambium and enter sapwood and heartwood. In addition to destroying vascular tissue, structural damage to the canopy results from larvae as they bore numerous large holes in branches. Additional clues related to attack by roundheaded borers include production of sawdust-like frass expelled from galleries that collects on or beneath infested trees and round or oval exit holes created by adults as they escape from the tree upon completing development. (Photo 4) Roundheaded apple tree borer and its cousin, the red-edged saperda, are common borers causing dieback, especially in saplings under stress or planted too deeply. Two species of non-native longhorned borers cause dieback and death of eucalyptus trees in California.

Photo 4: Exit holes of longhorned beetles, including Asian longhorned beetles, are circular. Their larvae are called roundheaded borers.

Weevils are a third major cabal of beetles causing dieback. Two of the most damaging groups of weevils are bark beetles and their cousins, ambrosia beetles. Bark beetles feed beneath the bark on phloem tissue. Massive outbreaks of western and mountain pine bark beetles related to changing weather patterns have resulted in losses of millions of pine trees in western states. In southern states and along the Eastern Seaboard, southern pine beetle has extended its range northward from the Deep South to New England.

Ambrosia beetles dig deeper into the sap and heartwood and inoculate their galleries with fungi, which they transport from tree to tree in specialized sacs called mycangia. Both larval and adult ambrosia beetles consume mats of fungus known as ambrosia, from whence they get their name. These fungi may be pathogenic, like blue and brown staining fungi, including Ophiostoma species, the causal agent of Dutch elm disease. By destroying vascular tissue, plugging xylem elements with fungi and disrupting transport of nutrients, ambrosia beetles cause dieback of branches and sometimes entire trees.

Photo 5: Piles of fine, sawdust-like frass that can be white, brown or reddish at the base of a tree are a sign of bark beetle borers.

Diagnosing attacks by bark and ambrosia beetles is relatively straightforward. Many bark beetles expel fine-grained frass as they feed. (Photo 5) Depending on the species and condition of the host, frass can range in color from brown to reddish to white. Frass often accumulates on bark or the ground. Frass of some ambrosia beetles, including granulate ambrosia beetles, may form toothpick-like projections jutting from galleries on the bole of a tree or shrub. (Photo 6)

Photo 6: Strange, toothpick-like accumulations of frass often project from the trunks of trees attacked by granulate ambrosia beetles.

Holes in the bark created by bark beetles are round and range in size from a few to less than one millimeter in diameter. They are often described as shot holes. Several recently imported non-native bark and ambrosia beetles, including polyphagous shot-hole borer, Kuroshio shot-hole borer, granulate ambrosia beetle and redbay ambrosia beetle, are wreaking havoc on a diverse assortment of trees in natural and managed landscapes and nurseries.

Larvae of several species of moths also bore in trees and shrubs. Some of the most common found in landscapes are caterpillars of clearwing moths in the family Sesiidae. Eggs of sesiid borers are laid on tree bark. Upon hatching, larvae tunnel through the bark and feed on cambium and associated vascular tissues. Common clearwing borers include peach tree and lesser peach tree borers, banded ash clearwings, ash/lilac borer, oak borer, sycamore borer, viburnum borer and western poplar borer, to name just a few.

Clues associated with these borers include the production of course frass (Photo 7) that accumulates on the bark and beneath the tree, oval or round exit holes, cracked and sloughing bark, empty pupal cases protruding from holes where adults emerge and gummy exudates from borer galleries, especially on trees in the genus Prunus. Caterpillars of several species of moths in the family Cossidae known as carpenter worms and leopard moths attack a wide variety of ornamental and shade trees. These caterpillars make large galleries in the sapwood of branches, trunks and roots of trees, and may cause severe dieback and death.

Photo 7: Course frass at the base of an ash tree with dieback is a sign of infestation by clearwing borers like banded ash clearwing or ash/lilac borer, rather than emerald ash borer. Photo by Paula Shrewsbury.

Primitive wasps in the family Siricidae develop as larvae boring into branches and trunks of trees. Adult siricids are called horntails by virtue of an impressive egg-laying appendage on their rear-end. A native horntail called the pigeon tremex is a minor pest attacking stressed deciduous trees including beech, elm, hickory, maple, oak and several others. However, in 2004, an exotic horntail called the Sirex woodwasp was discovered in Oswego County, New York, attacking native and non-native pines.

The Sirex woodwasp is a major pest of pine plantations throughout the world and poses a major threat to North American pines grown for pulp and those growing in landscapes. Since 2004, it has spread to Vermont, Connecticut, Michigan and Ohio. A combination of a toxin injected into the tree and the introduction of a fungus when eggs are deposited can cause rapid wilting, needle drop and foliage color change from green to red within months, with possible tree death ensuing in a single year. Woodwasp larvae tunnel through sapwood into heartwood. Signs and symptoms of Sirex attack include brown staining of vascular tissue, heavy pitch flow on the bark, round exit holes and frass-packed galleries.

Photo 8a: Heavy infestations of armored scales, like obscure scale, can cause dieback in the canopy of oak trees in stressful sites like parking lots. Photo by John Davidson.

Sucking insects like armored scales, including obscure scale on oak and gloomy scale on maple, achieve high densities on branches and trunks. (Photos 8a & 8b) Armored scales insert sucking stylets into cells and remove cell contents, thereby killing underlying tissues. This injury may be sufficient to cause dieback of branches and can eventually lead to severe canopy decline. Hemlock woolly adelgids pierce the tree’s parenchyma ray cells, storage sites for nutrients. This injury robs the tree of its food reserves, leading to a steady and potentially lethal decline.

Photo 8b: Obscure scales often reach densities high enough to cause dieback on pin and willow oaks. Photo by John Davidson.

In addition to canopy dieback, armored scales and hemlock woolly adelgid leave waxy “footprints” behind. Armored scales produce tight-fitting waxy covers and woolly adelgids, as their name implies, produce fluffy, white wax covering their bodies and eggs produced by the female.

Although many other insects are responsible for dieback of shoots and branches, one deserves special mention in 2021, and that is the periodical cicada. From Georgia to New York in many locations east of the Mississippi, trillions of Brood X cicadas will emerge in May and June. Adult females will slice small branches with sharp ovipositors and deposit eggs within these egg nests. Where cicadas are abundant and many egg nests occur, branches distal to the injury may turn brown, flag, break and drop to the ground. (Photo 9) While large, well-established trees will shrug off this injury with no apparent reduction in growth or survival, recently installed, small-caliper trees may be severally damaged.

Photo 9: Egg laying by Brood X periodical cicadas will cause shoot and branch dieback in the spring of 2021.

Reports of large-scale sapling death in nurseries, orchards and landscape plantings have been reported in cicada emergence years. The best-management tactics include delaying planting until the fall of 2021, and, for recently installed trees, netting with 1.0 cm mesh has been shown to be far more effective than insecticide sprays or soil drenches with systemic insecticides. No clues are needed to diagnose flagging and dieback caused by periodical cicadas. The perpetrators will make their presence known.

Part 4 of this series will cover diagnosing symptoms of discoloration and deformation of plant parts caused by insects and mites.

References

Davidson, J. A. and M. J. Raupp. 2014. Managing Insect and Mite Pests on Woody Plants: An IPM Approach. 2014. Third Edition. Revision. Tree Care Industry Association. Londonderry, N.H.

Johnson, W. T. and H. H. Lyon. 1988. Insects that Feed on Trees and Shrubs. Second Edition. Cornell University Press. Ithaca, N.Y.

Solomon, J. D. 1995. Guide to Insect Borers in North American Broadleaf Trees and Shrubs. United States Department of Agriculture Forest Service Agricultural Handbook AH-706.

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 www.youtube.com/user/BugOfTheWeek.

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