This article is the fourth 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. Part 3, “Diagnosing Dieback Caused by Insects” (TCI, March 2021), discussed insects associated with dieback in the canopies of woody plants.
Discoloration is a symptom defined by a change in the normal color of a plant part to an abnormal or unusual color. Let’s focus on leaves.
Chlorophyll is the magnificent molecule responsible for photosynthesis. Due to chlorophyll’s inherent quality to reflect green wavelengths of light, the normal color of leaves is green. Exceptions to this norm include cultivars with non-green variegation, such as Euonymus japonicus, “picta,” or foliage with other pigments that reflect different wavelengths, such as purple in Cercis canadensis, “forest pansy.”
Nutrient deficiencies linked to the inability of plants to obtain proper levels of nutrients due do improper soil pH or elemental deficiencies also can cause foliage to be abnormally colored. Classic examples commonly observed in landscapes include chlorotic yellowing of acid-loving oaks and azaleas planted in soils with an alkaline (basic) pH.
Microbial pathogens, including viruses and bacteria, also discolor leaves. When it comes to insects and mites, some of the more common insects responsible for discoloration are ones with piercing-sucking mouthparts in the order Hemiptera and their tiny cousins, spider mites and rust mites, which are members of the clan called Acari. These rascals pierce leaves with sharp, elongated mouthparts that rupture cells. Nutritious cell contents are sucked into the insect’s digestive tract, assisted by a muscular pump in the insect’s head.
Spider mites use tiny needle-like stylets to burst cells, and then they imbibe liquid cell contents. Larvae of several species of flies, beetles, caterpillars and primitive wasps called sawflies tunnel within leaves as they feed, creating discoloration.
Discoloration of leaves can be grouped into several categories. Here are a few of those most commonly seen.
Stippling – Picture this: a sucking insect or mite jabs its mouthparts into a cell on the surface of a leaf. It activates the tiny pump in its head and slurps the cell contents into its digestive tract. A cell once green with chlorophyll now turns white as its contents are removed. These empty white cells on an otherwise green leaf create a symptom known as stippling. Perpetrators infamous for this type of injury include nymph and adult lace bugs and leafhoppers. (Photo 1) Areas of stippling produced by these pests are relatively large.
Another clue associated with lace bugs is tar-like deposits of frass on the undersides of leaves where lace bugs feed. Leafhoppers excrete a clear liquid called honeydew that may accumulate on objects below, fostering the growth of sooty mold.
Now imagine creatures much smaller than lace bugs or leafhoppers – arthropods less than a millimeter in length, spider mites and their even tinier cousins, eriophyid mites. Spider mites are infamous worldwide as a major pariah of vegetables grown in fields and greenhouses, large and small fruits, ornamental trees and shrubs and herbaceous ornamental plants. When spider mites and eriophyid mites are abundant and their feeding is prolonged, leaves may turn white as thousands of stipples coalesce. Eventually, leaves discolor and may turn yellow, bronze or brown before dropping from the plant. Heavily infested plants often appear to have sustained an unfortunate encounter with a blowtorch. (Photo 2)
Mining – The surface of a leaf where hungry predators hunt is a hazardous place to make a living if you are a plant-eating insect like a caterpillar or aphid. Several groups of insects have solved this problem in part by foraging between the upper and lower epidermal surfaces of leaves. This clever adaptation helps them avoid many creatures eager to eat them. Insects with this mode of feeding are called leafminers.
Some leafminers, including larvae of leaf-mining flies such as boxwood leafminer and holly leafminers, have hardened mouthparts that puncture cells, releasing nutritious cell contents. In turn, these fluids are slurped into the digestive tract of the maggot. Other leafminers, including caterpillars, beetle larvae and larvae of sawflies, have chewing mouthparts that slice open cells to access nutritious contents within. The handiwork of many leafminers is highly characteristic and often can be used to diagnose a specific pest, especially when the host plant is known.
Take, for example, American holly, Ilex opaca, the leaves of which are discolored by winding serpentine galleries narrow at one end and growing ever wider at the other. (Photo 3) This unmistakable gallery is that of the native holly leafminer, Phytomyza ilicicola. In addition to the mines made by larvae, hollies also bear telltale small punctures in the leaf’s surface where female flies feed.
On closely related English holly, Ilex aquifolium, nearly identical galleries are caused by the holly leafminer, Phytomyza ilicis. Inkberry, Ilex glabra, is attacked by yet another member of this clan called the inkberry leafminer, Phytomyza glabricola, which creates an irregularly shaped blotch mine rather than the serpentine mine of its congeners. Other larvae responsible for blotch mines are found in the Hymenoptera (sawflies), Coleoptera (beetles) and Lepidoptera (caterpillars).
Sawfly larvae, including the birch leafminer, Fenusa pusilla, and hawthorn leafminer, Profenusa canadensis, create blotch mines along margins and at the tips of their host’s leaves. (Photo 4) Beetle larvae, like locust leafminer and European elm flea weevil, also create characteristic blotch mines on leaves of black locust and elms, respectively. Caterpillars like tupelo leafminer, Antispila nysaefoliella, and the azalea leafminer, Caloptilia azaleella, also create blotch mines. In the case of the azalea leafminer, after feeding as a leafminer in early stages of development, it vacates the mine, folds the azalea leaf and feeds within the shelter of the folded leaf.
Yellowing, bronzing, silvering and russeting – As aphids, scales, whiteflies and mealybugs feed on and damage interior tissues of leaves, leaves may change color, especially when pests are numerous. Diffuse yellow patches on the upper surface of euonymus leaves are a classic symptom of euonymus scales present on the lower leaf surface. (Photo 5)
Thrips have unusual mouthparts comprised of stiletto-like stylets that rupture cells, the contents of which are sucked into its digestive tract. Silvery patches or streaks on leaves are characteristic of thrips injury. Often, dark, tar-like deposits of frass accompany silvering caused by thrips.
Feeding by spider mites and their tiny cousins, eriophyid mites, may transform green leaves into leaves with shades of silver, bronze or russet. (Photo 6) Eriophyids also go by the common names of rust or russet mites.
When piercing-sucking insects or mites are abundant in great numbers, entire branches or trees may be discolored in shades of yellow, red, brown or white rather than green.
Distorted leaves, branches and trunks
Undifferentiated tissues on woody plants are typically found at tips of roots, tips of shoots and cambium, where meristems give rise to new cells and tissues. Injury to undifferentiated tissues can result in distorted plant parts. Subtending categories of deformation and associated perpetrators include the following.
Leaf cupping and curling – Think about an apical meristem giving rise to new leaves at the tip of a branch. Undifferentiated cells produced by the meristem will ultimately multiply and divide to form vascular tissues, epidermal cells of the leaf’s surfaces and underlying photosynthetic cells. Now imagine an insect like an aphid jabbing its piercing mouthparts into a leaf bud replete with undifferentiated and not-fully developed tissues. This mechanical damage may disrupt or destroy normal development of these tissues. Rather than expanding to their full shape, injured parts of the leaf curl and cup, resulting in a distorted leaf. The leaf bears this deformation for its life on the plant, not fully functional but still contributing, only in a limited way. Probing mouthparts of the boxwood psyllid damage leaves before they fully expand, causing them to cup or curl. (Photo 7) Once deformed, leaves remain so – even after the perpetrator is long gone – until the plant sheds them.
Leaf and twig galling – In addition to simple mechanical damage associated with direct injury caused by mouthparts, there is growing evidence that insects and mites cause distortion by producing and secreting chemicals that co-opt the genetic machinery of plant cells, instructing cells to grow in an abnormal way. Zombification of plants by insects, how strange is that? Classical examples include sucking insects like the Cooley spruce gall adelgid and tiny wasps called cynipids commonly found on leaves, buds and twigs of many species of oaks.
Adelgid nymphs feeding on the elongating branches of spruce in spring induce the tree to form strange pineapple-
like structures in which adelgids develop and reproduce. (Photo 8) These abnormal growths are called galls. Cynipid-
wasp larvae cause the otherwise normal leaf tissue to develop into strange structures shaped like balls of wool, small bullets or gnarly green apples festooned with tiny horns. (Photo 9, page 37) These, too, are types of galls. Many species of wasps, sawflies, flies, aphids, psyllids, thrips and mites cause galls on leaves, stems, flowers and roots.
Bark and trunk abnormalities: rippling, cracking, pitting and trunk asymmetries – Borers that consume cambium and vascular tissues beneath tree bark may cause deformation of branches or the trunk. Bark rippling on thin-barked trees, such as paper birch, may be a defensive response as a tree attempts to wall off and compartmentalize borers, such as bronze birch borer, tunneling just beneath the bark. Borers that kill cambium, such as the emerald ash borer, cause vertical bark cracks to form on the trunks of ashes. (Photo 10)
When cambium is killed by borers, bark is not produced by the underlying meristematic cells. As the rest of the tree increases in girth, circumferential bark separates and produces vertical splits where underlying cambium has died. Caterpillars of clearwing borers, such as dogwood borer and some metallic, wood-boring beetles like flatheaded apple tree borer, kill supportive tissues and cause large bark flakes to separate from the bole of a tree. In addition to splitting and flaking, borers may destroy structural tissues and cause differential rates of growth within the trunk, causing the bole of a tree to bend or become misshapen.
Several species of bark-feeding scale insects also disrupt normal tissue formation, causing pits and sunken areas on branches and trunks. Classic examples include pit-making scale insects in the family Asterolecaniidae, commonly found on oak and holly, and some Diaspididae, including obscure scale found on oaks. (Photo 11) A small number of insects, including some treehoppers and cicadas, lay eggs in twigs. Branches injured by cicadas often flag, break and drop from trees. However, those failing to break off immediately bear swollen, compartmentalized wounds, creating weakened places where branches may later break.
Conclusion
By understanding physiological and developmental processes underlying discoloration and deformation, these symptoms can be used to help pinpoint the mode of feeding of the offending insect or mite. When coupled with the identity of the plant and seasonal timing of the injury, these clues can help identify the perpetrator responsible for the crime.
References
Cranshaw, W. and D. Shetlar. 2017. Garden Insects of North America: The Ultimate Guide to Backyard Bugs – Second Edition. Princeton University Press, Princeton, New Jersey.
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, Manchester, New Hampshire.
Johnson, W. T. and H. H. Lyon. 1988. Insects that Feed on Trees and Shrubs. Second Edition. Cornell University Press, Ithaca, New York.
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.
