This is part one of a four-part series of articles that has been one component of workshops on integrated pest management and plant health care sponsored by the Tree Care Industry Association over the past decade. It is designed to introduce the basics of diagnosing problems caused by insects and mites on woody landscape plants.
Insects and mites injure plants as they feed, make protective refuges and lay eggs on or in plants. Often this activity damages the leaves, stems or roots of plants. In many cases, when you arrive at a client’s property – the crime scene, as it were – the perpetrator of this injury will not be present on the plant. Only symptoms and signs may remain. Like a crime-scene investigator (CSI), you can use clues left behind to help identify the perpetrator.
Accurate identification enables you to design the right strategy for managing the pest in a timely and efficacious way. Symptomatology – using the clues left behind by a pest to help identify the culprit – was first described for use in arboriculture by one of the founders of arboricultural pest management, Carlton S. Koehler (1987). This approach is discussed in greater detail in Davidson and Raupp (2014). In this series of articles, we will learn the ways insects and mites injure plants and identify the types of damage caused by different groups of pests. Symptoms will be placed into five general categories: chewing injury, discoloration, distortion, dieback and insect products. Specific types of damage within each category will be described and linked to their causal agent. For each specific example, we will discuss possible approaches to mitigate damage caused by the pest.
What are the steps in the diagnostic process?
Using our CSI analogy, the first step in making a diagnosis is to identify the victim of the crime. Specifically, what kind of plant or plants are suffering damage? Why is plant identification so important? Most insects and mites specialize on a limited number of plants as their food source. Most often this happens at the level of plant family or genus. For example, both emerald ash borer and lilac/ash clearwing borer specialize on plants in the olive family, Oleaceae, and most commonly in the genus Fraxinus, ash trees. Observing dieback in the canopy of a tree and knowing the tree is an ash immediately reduces the number of potential insects that might cause dieback. Using signs such as the shape of exit holes coupled with frass beneath the tree would confirm the perpetrator to be the larvae of a clearwing borer like banded ash clearwing, rather than emerald ash borer. A second practical reason to know the taxonomic identity of the tree is that most reference books and internet identification tools will be indexed by the genus, species or common name of the plant under attack.
Step two is to identify patterns of damage. If several plants of different species in the same location are affected, then the problem might be related to the physical environment. Compacted soils, flooding or extreme temperatures rather than a pest may be the root cause of the problem. Stress factors like these may make trees more susceptible to attack by insects and pathogens.
On an individual plant, is the damage uniform or patchy? Often, in the early stages of an insect infestation or infection by a disease, damage will first appear in isolated patches on a tree rather than uniformly distributed throughout the canopy. (Photos 2 & 2a) Later in the progression, the entire canopy may become involved. If damage appears uniformly throughout an entire canopy, sometimes an abiotic problem may be the culprit.
To gain insight into the timing and progression of the problem, input from the client will be very useful. Questions such as, “When did you first notice the damage?” or “How long has your plant looked this way?” are important and relevant. Many insect and mite pests of woody plants have well-defined periods of activity when they injure trees and shrubs. Knowing the time of year that a pest is active will help shape monitoring and intervention activities. Every pest has one or more windows of vulnerability, those times at which intervention activities have the best chance of disrupting or killing the pest. Correctly identifying the host plant and understanding the pattern of damage and when the injury occurred can help the diagnostician to identify the cause of a problem.
The third piece of the diagnostic puzzle is to find clues left behind that can help identify the perpetrator. These clues are symptoms and signs.
What are symptoms and signs?
How do they relate to biotic and abiotic disorders?
Let’s spend a few moments discussing the terms “symptoms” and “signs” and their link to two broad categories of plant problems – biotic and abiotic disorders.
Symptoms are the clues telling us how a tree responds to non-living factors, also known as abiotic factors, such as drought, nutrient deficiencies, temperature extremes and mechanical injuries. Symptoms also can be caused by biotic factors, living organisms including insects, mites, fungi, bacteria, viruses and phytoplasmas. Symptoms can be non-specific and may have many causes. Take, for example, dieback in the canopy of an ash tree. Dieback is a symptom, but many agents could be at work singly or in concert to create dieback. Perhaps roots of the tree are buried under a “volcano” of several inches of mulch. Perhaps a plastic liner was not removed from a root ball when the tree was installed. Maybe weed whackers debarked the tree at the root flare. Perhaps the tree has a pathogen such as ash yellows or a borer like emerald ash borer.
Each of these agents could result in the outward symptom of dieback in the canopy of an ash tree. (Photos 3 & 3a) To pin down the true agent or agents underlying the symptom of dieback, we have to dig a little deeper, and this is where signs come into play.
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 and shed skins of insects. Signs also can include specific, easily recognized objects or other clues left behind by an insect or mite. Silk, frass (a suave term for insect excrement), wax coverings, characteristic feeding patterns on leaves and exit holes in bark also can serve as signs. In the aforementioned example of dieback on ash, round or oval exit holes, the presence of frass beneath the tree or a papery pupal case extended from an exit hole are clear signs of clearwing borers, like banded ash clearwing, as the cause of the dieback, rather than dieback caused by emerald ash borer.
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.
How do insects injure and damage plants?
Although the terms “injury” and “damage” are often used synonymously, they are somewhat different and distinct. Injury happens when an insect or mite feeds on or infests a plant. Damage is how that injury affects the vitality, quality, value or appearance of the plant. The primary way insects and mites injure plants is by feeding on them. As you might guess, the type of injury caused by insects and mites is determined largely by its type of mouthparts. Using our CSI analogy, a broad sword leaves a very different signature on a murder victim than an ice pick. Learning the signatures of different types of insect mouthparts, the weapons they use to attack plants, can help identify the perpetrator. Understanding how different types of insect mouthparts function helps in understanding the type of injury produced as insects feed.
We can broadly classify insect mouthparts as either chewing or sucking. The more primitive type of mouthparts, chewing mouthparts, consist of six distinct external parts. Behind a front lip called a labrum are two powerful jaws, called mandibles, designed to cut and tear leaves, bark and wood. (Photos 4 & 4a) They create chewing injury. Maxillae are paired appendages located just behind the mandibles. They are loaded with sensory structures that evaluate food quality and help shove cut leaves or other plant tissues into the insect’s digestive tract. A lower lip, called a labium, rests behind the maxillae.
An evolutionary breakthrough in mouthparts came after millions of years when mandibles and maxillae elongated and fused into a structure called a proboscis or, more commonly, a beak, that could be inserted into plant tissue and used to suck fluids from a plant. This type of mouthpart can be described many ways, but for our purposes we will call it piercing or sucking. Within the proboscis are two channels. One connects with salivary glands, the source of saliva laced with enzymes capable of breaking down and liquefying plant tissues. The other channel is connected to a tiny hydraulic pump in the head of the insect used to suck liquid plant tissues into the insect’s digestive tract. Butterflies also have highly modified sucking mouthparts to sip nectar from blossoms on which they feed. Plants are not the only targets of sucking insects. Mosquitoes, deer flies, lice and fleas use their unique mouthparts to suck blood from animals.
While mouthparts are the usual weapon causing injury to plants, one more appendage, the ovipositor, also can injure plants. An ovipositor is an egg-laying tube used to insert eggs into plant tissue. Cicadas are the most common culprit here. They insert eggs into small branches, causing jagged tears in the bark. During massive emergences of periodical cicadas – every 13 or 17 years in specific locations – egg laying can cause small branches to die and break, producing a symptom called flagging. (Photo 5, 5a & 5b) While well established large trees may withstand this assault, canopies of young, recently transplanted saplings may be severely injured. Even when trees recover from the initial injury, structural damage will remain as gouty deformations on branches.
Who has what mouthpart?
Anyone who has taken an entomology course may have been overwhelmed by the great diversity of different kinds of insects. This diversity has been organized into a classification system that includes groups called orders, families, genera and species. Textbooks typically list more than two-dozen orders of insects, but here is some good news – only a few orders of insects create the vast majority of problems on woody trees and shrubs. These orders include those with chewing mouthparts; immature stages of moths and butterflies (order: Lepidoptera) commonly called caterpillars or sometimes, inappropriately, worms; larvae of primitive wasps called sawflies (order: Hymenoptera); and larvae (grubs) and adults of beetles (order: Coleoptera). In addition to these three orders of chewers, grasshoppers (order: Orthoptera) and earwigs (order: Dermaptera) have chewing mouthparts, and some species occasionally damage plants.
The premier insect order with sucking mouthparts wreaking havoc on trees and shrubs is a very large collection of pests in the order Hemiptera. They suck plant sap as both nymphs and adults.
While most of the plant eaters fall neatly into the categories of chewers and suckers, a few have mouthparts and modes of feeding that stretch the boundaries of this dichotomy. Maggots, the larvae of flies (order: Diptera), have tiny jaws or mouth hooks capable of rupturing plant cells, the contents of which are then slurped up into the maggot’s digestive tract. Thrips (order: Thysanoptera) have rasping mouthparts that rupture plant cells, releasing cell contents that are imbibed. Although not insects, tiny plant-eating arachnids called mites (order: Acari) use needle-like stylets to rupture cells, releasing cell contents that are ingested.
Using clues left behind to pin down potential perpetrators
We previously mentioned that different types of injury caused by insects and mites could be grouped into five categories using the symptomatology approach. These categories are chewing injury, discoloration, distortion, dieback and products. Recognizing these different injury clues and associating them with different types on insect mouthparts will help diagnosticians narrow the search for a perpetrator. Understanding the type of mouthpart that caused the injury can help rule out or rule in insects or mites that caused the injury.
When making a diagnosis, one must also be aware that many agents may cause injury to a plant part or tree more or less simultaneously. Since different pests attack a plant during the course of a single growing season, several different categories of injury may be present on the same leaf or branch at the same time. A spring cankerworm may have partially chewed an oak leaf in April that was later discolored by spider mite in August. Conversely, one pest may produce several types of injury. Aphids may distort and discolor leaves and excrete honeydew as they feed. Their shed skins are often a clue to their presence on a plant. Here a single pest has created three different symptoms of plant injury.
In the next article in this series, we will dive into symptoms caused by insects with chewing mouthparts. A preview of chewing injury is seen in Photo 1, which shows what cankerworms can do to an oak tree early in the season.
Koehler, C. S. 1987. Symptomatology in the instruction of landscape ornamentals entomology. Journal of Arboriculture. 13(3): 78 – 80.
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.
Michael J. Raupp, Ph.D., is professor emeritus at the University of Maryland, in College Park, Maryland. He has published numerous articles, presentations and books, and he frequently appears 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 website at www.bugoftheweek.com.