Sap-Sucking Insects: How They Feed, and the Damage They Cause

A dirty dozen or so families of sucking insects are key pests of woody plants. Who are these sucking insects?

Suckers belong to a large clan of insects called Hemiptera. More than 10,000 species of Hemiptera are found in North America alone. Most are terrestrial, but many, such as water striders, are aquatic. Some, such as assassin bugs and minute pirate bugs, are highly beneficial predators that kill insect pests. Due to limitations of space, we will visit just a few groups (called families) of suckers that regularly damage trees and shrubs.

Hemiptera are characterized by having gradual metamorphosis with three distinct life stages (egg, nymph and adult) in contrast to those with complete metamorphosis such as beetles, butterflies and bees, which have four life stages (egg, larva, pupa and adult). Unlike caterpillars and beetles that have jaws for removing large and small areas of leaf tissue, Hemiptera have piercing-sucking mouthparts enclosed in a structure called a beak or proboscis that houses sharp stylets used to pierce and access contents of plant cells and vascular tissues.

Within the stylets are two ducts, one for infusing saliva laced with histolytic enzymes and other compounds into the plant, and another for imbibing the fluid contents of plant cells. The food duct connects to a pump in the head of the insect that creates negative pressure to suck liquids from the plant. Direct damage that results from feeding of sucking insects is related to the type of plant tissue attacked and how the plant responds to this attack. Indirect damage results from waste products like honeydew, excreted by some sucking insect pests.

Plants are comprised of several types of tissues. Xylem and phloem are vascular tissues conducting fluids throughout the plant. Others, such as cambium, rapidly divide and give rise to new vascular tissues or bark. Parenchyma cells photosynthesize, produce food and store photosynthetic products. Epidermal cells prevent moisture loss and help in the exchange of gases.

Let’s look at several common families of sucking insects, see which plant tissues they attack, learn how plants respond to their attacks and see how this insult manifests as plant damage.

CELL BURSTERS

Adelgids (Adelgidae)

These close relatives of aphids include some of our most egregious pests. Perhaps the worst of the lot in the eastern United States is the hemlock wooly adelgid. This invasive pest from Asia inserts its stylets deep into a branch to access contents of cortical storage cells, thereby robbing the hemlock of stored nutrients. Hemlocks respond to this attack by walling off damaged tissue where stylets are inserted. The combined effects of stolen nutrients and damaged tissues result in characteristic yellowing and loss of needles, branch dieback and, in chronic cases, tree death. (Photo 1)

Pine bark adelgid has a similar modus operandi, but through a long evolutionary history with this pest, pines usually exhibit no severe damage symptoms. Other adelgids, including eastern and Cooley spruce gall adelgids, inject saliva into developing plant meristems, which induces host plants to produce abnormal, pineapple-shaped growths called galls, within which adelgids develop. In addition to discoloration of leaves, distortion of shoots and dieback of branches, adelgids and many other sucking insects produce wax that can be used as a diagnostic clue to their identity.

Armored scales (Diaspididae)

The term “scale insect” is given to more than a dozen families of sucking insect pests. In this article, we have space to examine only four of the most important and common ones. (Photo 2)

The most diverse group of scale insects are the armored scales, with more than 1,500 known species worldwide. Like all scale insects, the first nymphal stage that hatches from an egg is called a crawler. These highly mobile nymphs colonize new areas of plants and may be transported by wind or birds to new plants. Once they find a suitable place to feed, they settle down and become sessile for the remainder of their lives. Depending on the species and geographic location, armored scales may have one, two or more generations annually.

Armored scales insert highly flexible, elongated stylets into plants and extract the contents of cells. Because they do not feed on vascular tissues, armored scales do not produce liquid waste called honeydew, which is associated with vascular feeders like soft scales, aphids, whiteflies and several other hemipterans we will discuss later. Armored scales feeding on broad-leaved plants, such as euonymus scale, cycad scale, tea scale and lantania scale, create a halo of ruptured cells around their feeding site. Leaf cells damaged by feeding on narrow-leaved plants, such as elongate hemlock scale on Tsuga, maskell scale on Cryptomeria and pine-needle scale on Pinus, turn needles yellow or brown and may cause needles to drop.

Some scales, like euonymus and lantania scale, feed on both leaves and bark. Others feed primarily on one tissue. Common bark-feeding armored scales that can severely damage trees include gloomy scale (mostly on Acer), obscure scale (mostly on Quercus), white prunicola scale (mostly on Prunus), San Jose scale (broad host range), oyster shell scale (broad host range), white peach scale (broad host range) and Japanese maple scale (broad host range), to name just a few. Some, like obscure scale, settle in dense clusters on green wood. Their feeding can cause depressions in the bark beneath encrustations, thus creating permanent deformations of branches and boles of trees. Densities of bark-feeding armored scales can become very high, causing severe canopy dieback and contributing to tree death in stressful sites.

The bodies of armored scales are covered with wax. The color and shape of this wax cover provide an important clue to the identity of the scale.

Pit-making scales (Asterolecaniidae)

These strange scales feed through the bark and induce their host to create a pit, where adults and nymphs live and suck fluids from the underlying plant tissues. Several species of eastern and western oaks are attacked by one or more species of pit scales, while holly and pittosporum are attacked by their own unique pit makers. Depending on the species, scale colors vary from green to pink to golden yellow, and some stages sport a waxy fringe. Densities can become great enough to distort branches and disrupt vascular flow of nutrients, which results in leaf loss and dieback. (Photo 3)

Lace bugs (Tingidae)

Leaves have small openings in their surface called stomata that allow gas exchange. Lace bug nymphs and adults have a bizarre mode of feeding. They insert their stylets through the stomata on the undersurface of a leaf and pierce palisade parenchyma cells near the upper leaf surface. This creates a type of discoloration called stippling, which is diagnostic for these sap suckers. (Photo 4)

Stipples coalesce when lace bugs are abundant, turning leaves white. Fecal spots on the undersurface of leaves deposited by lace bugs as they feed provide an excellent additional diagnostic clue useful for distinguishing damage caused by lace bugs from discoloration caused by spider mites or abiotic chlorosis associated with basic soils.

Several introduced and native species of lace bugs feed on common landscape trees and shrubs, including oak, sycamore, elm, hawthorn, birch, walnut, willow, azalea, rhododendron, andromeda, cotoneaster and pyracantha. Depending on location and seasonal temperature regimes, they may have one or many generations annually.

Plant bugs (Miridae)

Plant bugs, such as the four-lined plant bug, use their robust beaks to slash open cells of many herbaceous plants and douse them with saliva with histolytic enzymes that liquify plant tissues. Fluids are then imbibed from the feeding site. Dark necrotic lesions associated with these enzymes remain on the leaf after the bug has fed. These are often misidentified as damage caused by a microbial agent.

Other plant bugs, including honey locust plant bug and ash plant bug, create coarse stippling as they feed and burst cells. Feeding by nymphs on undifferentiated leaf tissue can cause curled, deformed leaves and leaflets that wear their damage for the entire season. Blades of yucca can be severely discolored by yucca plant bugs. Some mirids, like honeylocust plant bug, have a single generation, while others, like yucca plant bug, have several generations annually in warm locations. (Photo 5)

XYLEM FEEDERS

Xylem is the vascular system that transports water and minerals from roots of plants to the canopy, where leaves use these raw materials in the process of photosynthesis. Xylem sap is very low in nutrient content, and xylem-feeding insects like leafhoppers (Cicadellidae) and cicadas (Cicadidae) have special adaptations to deal with this nutrient-poor food. Powerful pumps in their heads allow them to remove xylem fluid from pressurized xylem elements. Microbes living in their gut convert raw materials in xylem into nutrients essential for growth and development. To gain sufficient nutrients, xylem feeders suck large quantities of xylem fluid from plants. This results in copious amounts of liquid waste being excreted by xylem feeders. If one stands beneath a tree with many xylem feeders, as CCR (Credence Clearwater Revival) lyricized, it will “rain on a sunny day.”

Leafhoppers (Cicadellidae)

Leafhoppers damage plants in several ways. Potato leafhoppers migrate from southern states to northern states annually. They reach extraordinarily high densities, feeding on a wide variety of woody and herbaceous plants. Their piercing mouthparts damage leaves, which become discolored, distorted and withered with a type of injury called hopper burn. Acer species are particularly susceptible to this damage. Leafhoppers also vector serious diseases of many crops including Xylella fastidiosa, the bacterium responsible for bacterial leaf scorch, a serious pathogen of oaks and other trees. (Photo 6)

Spittlebugs (Cercopidae)

A close relative of the leafhoppers are spittlebugs, also known as froghoppers (Cercopidae). Spittlebugs create frothy, spittle-like masses on the stems and branches of herbaceous and woody plants. While most cause no serious injury to plants, the pine spittlebug may reach densities damaging to pines such as mugo pine, and can be economic pests in Christmas-tree plantations. (Photo 7)

Cicadas (Cicadidae)

Damage associated with feeding by annual and periodical cicadas is usually minor. However, periodical cicadas can seriously damage trees in landscapes, nurseries and orchards by slicing branches with an egg-laying appendage called an ovipositor as they deposit eggs in woody tissues. (Photo 8)

PHLOEM FEEDERS

While xylem is transporting water and minerals in one direction, from roots to leaves, phloem is multi-directional, transporting the products of photosynthesis to nourish all parts of the plant. This nutrient-rich tissue is used as a food source for many of our most destructive and problematic insect pests. In addition to direct forms of plant damage caused by suckers as they disrupt tissues with their mouthparts, steal the products of photosynthesis and enslave plant tissues to galls, phloem feeders also excrete waste products that disfigure plants, but are sometimes useful in diagnosing plant problems. Honeydew is a sugar-rich liquid excreted by phloem-feeding insects such as aphids, soft scales, felt scales, psyllids, whiteflies and many other hemipterans. It rains down on foliage, the ground and human-made objects, and forms the substrate for the growth of black, nonpathogenic fungi called sooty mold.

Aphids (Aphididae)

Aphids are the most diverse group of phloem-feeding tree pests, with more than 4,000 species known worldwide. Some species of aphids feed only on one species of plant, while others feed on several. Almost any part of a plant may have an aphid that feeds on its roots, stems, branches, leaves, blossoms and fruit. Most common trees and shrubs are susceptible to one or more species of aphids. Their beaks tap into phloem elements and remove sap, which causes direct damage to plants by robbing them of the vital products of photosynthesis. Heavy infestations may cause leaves to turn yellow. Undifferentiated leaf tissues and branch terminals may be distorted by aphid feeding as mouthparts damage tissues and cells, resulting in small, twisted, gnarly leaves. Many aphids, like the elm cockscomb gall aphid, produce strange galls on leaf blades. Wooly aphids, including those that feed on beech, alder and apple, produce remarkable amounts of fluffy white wax, characteristic for their species. (Photo 9)

As with other phloem feeders, aphids produce vast amounts of honeydew that fouls leaves, bark, blossoms and fruit and encourages the growth of sooty mold, thereby reducing the aesthetic value of trees, shrubs and underlying vegetation. Sugar-rich honeydew attracts a variety of stinging insects such as yellow jackets, honey bees and paper wasps that pose a health risk to people and pets.

Soft scales (Coccidae)

Scales in this family can be smooth, cottony or waxy and are usually larger, rounder and more convex than their close relatives, the armored scales. What you observe on the plant when finding a soft scale is the body wall of the insect itself. Soft scales lack the wax cover associated with armored scales. Sometimes a waxy egg sac is associated with the scale, as in cottony maple and cottony camelia scales. (Photo 10)

Soft scales feed on phloem sap and directly injure plants in much the same way as do aphids. Soft scales can be mobile as nymphs and adults until the female settles down and begins to produce eggs. Some, like tulip tree and magnolia scales, feed solely on woody tissues, while others feed on woody tissues and leaves during different times of the year. On deciduous trees, nymphs of oak lecanium and calico scales spend the summer months feeding on leaves and then return to their birthplace on woody tissues in autumn, where they will mature and feed through the bark on phloem the following spring.

Unlike female armored scales, which usually produce fewer than 100 eggs, female soft scales can lay a thousand eggs or more. Heavy infestations can result in reduced growth of branches and small, distorted, discolored leaves. Vast amounts of honeydew, produced as they feed, attract stinging insects, disfigure plants with sooty mold and foul furniture, vehicles and other human-made objects below, creating a nuisance.

Felt scales (Eriococcidae)

A few species of felt scales are important pests of trees and shrubs in managed landscapes and forests. Historically, European elm scale, azalea bark scale and beech bark scale have been the most problematic species. However, a relatively recent arrival, crepe myrtle bark scale, is now creating havoc in more than a dozen southern and Mid-Atlantic states. These scales can be smooth, cottony or waxy, and at maturity they are usually about 0ne-eighth-of an-inch long. Adults are covered with felt-like wax or ringed with a waxy boarder. Like other phloem feeders, they excrete large amounts of honeydew and create attendant problems with stinging insects and sooty mold. (Photo 11)

Chronic heavy infestations can cause leaf yellowing and branch dieback. Wounds created by beech bark scale as they feed facilitate infections by the Neonectria fungus, which can result in tree death.

Psyllids (Psyllidae)

Psyllids are small sucking Hemiptera that resemble tiny cicadas in their adult stage. The life cycle of some psyllids is closely linked to the production of new foliage, where their feeding distorts leaves in characteristic ways. Cupped leaves at the terminals of boxwood are a sure sign of boxwood psyllids. Psyllids attacking boxwoods, eucalypts, eugenia, acacia, hackberry and peppertree distort foliage and may cause premature leaf drop. Many species produce copious amounts of honeydew and wax. (Photo 12)

Whiteflies (Aleyrodidae)

In cooler regions of the United States, mulberry whiteflies often are found on holly, mountain laurel, magnolia, maple and mulberry. Maple whitefly feeds only on maple. Both species feed on the undersurface of leaves and occasionally reach levels that cause leaves to yellow and drop. (Photo 13)

In warmer regions of temperate zones and in tropical and subtropical regions, whiteflies are major pests of trees and shrubs. Across the southern tier of the continental U.S. and in Hawaii, whiteflies, such as the citrus whitefly, spiraling whitefly and giant whitefly, attack a wide variety of agricultural crops and landscapes plants. Whiteflies can produce large quantities of honeydew and create problems with sooty mold. Invasive ash whitefly, once a major pest in California, has been brought under control by tiny parasitic wasps.

Lanternflies (Fulgoridae)

Prior to 2014, lanternflies were not considered to be important pests of woody plants in the U.S. However, with the discovery of spotted lanternfly in Berks County, Pennsylvania, it has achieved pest status in a dozen eastern states where breeding populations now exist. It is a direct pest of grapes, where high densities can kill vines. Spotted lanternflies have more than 100 known hosts and can be pests of commercial fruit trees, where extraordinary amounts of honeydew foul fruit and foster the growth of sooty mold.

Dieback and death of young saplings has been reported, but spotted lanternfly is generally not considered to be a killer of large, well-established trees. Honeydew on street trees and trees in residential and commercial landscapes may become heavily infested, and attendant honeydew can create a major nuisance and attract stinging insects that pose a hazard to humans and pets.

Managing sucking insect pests

A previous article of mine in TCI Magazine (“There’s a Sucker Born Every Minute: A Whirlwind Tour of Sucking Insect Pests of Woody Plants,” February 2012,) provided additional information on the biology and management of several insect pests reviewed in this article. That article summarizes plant-health-care strategies for dealing with sucking insects, including plant selection, cultural practices to reduce outbreaks of suckers, biological control agents attacking sucking insects and the use of conventional and organic insecticides for managing these pests. In addition, several insecticides listed as organic and labeled for use against sucking insects were highlighted in the September 2022 issue of TCI Magazine in an article I wrote entitled, “Organic Insect Management in Arboriculture: Theory, Practices and Pesticides.”

Acknowledgements

Grants from the TREE Fund, USDA – NIFA and McIntire Stennis Programs support my research and outreach. Thanks to the Tree Care Industry Association for inviting me to share information and thoughts on sucking insects, and to Dr. Paula Shrewsbury for contributing photographs and reviewing a draft of this manuscript.

References

Cranshaw, W., and D. Shetlar. 2018. Garden Insects of North America. Second Edition. Princeton University Press. Princeton, NJ. 704 pp.

Davidson, J. A., and M. J. Raupp. 2014. Managing Insect and Mite Pests on Woody Plants: An IPM Approach. Third Edition. Tree Care Industry Association. Londonderry, NH. 174 pp.

Johnson, W. T., and H. H. Lyon. 1988. Insects that Feed on Trees and Shrubs. Second Edition. Cornell University Press. Ithaca, NY. 556 pp.

Michael Raupp, Ph.D., is a professor emeritus of entomology in the Department of Entomology at the University of Maryland in College Park, Maryland. He works closely with arborists to develop and implement sustainable management approaches for insect and mite pests in urban forests. His most recent book, Managing Insects and Mites on Woody Plants: An IPM Approach, is in its third printing and is available through TCIA, as is his children’s book, 26 Things That Bug Me. Visit his blogs and www.youtube.com/user/BugOfTheWeek.

This article was based on his presentation on the same subject during TCI EXPO ’21 in Indianapolis, Indiana. To listen to an audio recording of that presentation, go to this page in the digital version of this issue of TCI Magazine online at tcimag.tcia.org and, under the Resources tab, click Audio. Or, under the Current Issue tab, click View Digimag, then go to this page and click here.

Raupp will be speaking on “Scales in the City: Biology, Ecology and Management” at TCI EXPO ’22 this November in Charlotte, North Carolina. For schedule details or to register, click here.