This article is the second in a series by the author designed to help arborists 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 the basics of diagnostics. By using clues, signs and symptoms left behind by a pest, like crime-scene investigators, arborists can link signs and symptoms with the species of plant, time of year, habitat and geographic location to help pin down the identity of the pest. In this article, we will learn about injury to leaves and blossoms caused by insects with chewing mouthparts.
Although various sources list more than two-dozen orders of insects, the orders of insects responsible for almost all chewing damage on trees and shrubs can be winnowed to three, Coleoptera (larval and adult beetles), Lepidoptera (larvae, aka caterpillars of moths and butterflies) and Hymenoptera (primarily larvae of primitive wasps known as sawflies). Some Orthoptera (nymphs and adults of grasshoppers and crickets) damage leaves, but the impact of these on trees is minor. Dermaptera (nymphs and adult earwigs) also have chewing mouthparts and will injure leaves of herbaceous plants.
In the broad sense, when insects with chewing mouthparts consume leaves, they cause defoliation. However, defoliation can be further reduced into several categories that can help identify specific perpetrators associated with the injury. Easily recognized categories include shot-holes, notched leaves and skeletonization.
In temperate regions of North America, large caterpillars, sawfly larvae and beetles are the usual suspects responsible for removing large portions of leaf blades or conifer needles, classic defoliation by a chewing insect. In tropical and subtropical regions of North America, Mexico and Central and South America, leafcutter ants are important defoliators of trees and shrubs. All of these groups have the potential to strip trees of their leaves.
First, let’s take a look at defoliation caused by caterpillars. Suppose you visited a client in mid-May in the mid-Atlantic region of the United States and found leaves of an oak tree with the injury depicted in Photo 1. Clearly, perpetrators with chewing mouthparts did the deed. This symptom appeared on several branches, and beneath the tree were large and small pellets of frass. Some of the leaves bore traces of silk. Who are some potential perpetrators of the crime?
The first big clue is, of course, the type of mouthpart responsible for the injury. Chewing mouthparts are found in beetles, sawfly larvae and caterpillars, but frass produced by leaf-feeding beetles and sawflies does not usually accumulate as pellets beneath a tree. Beetles and sawflies drop down in the ranking of potential perps. Furthermore, beetles and most sawfly larvae do not make silk. Caterpillars surely do.
Having narrowed the field to caterpillars, what comes next? Well, as Photo 2 reveals, a few minutes of searching turned up three different perpetrators involved in the crime – forest tent caterpillar, Malacosoma disstria; fall cankerworm, Alsophila pometaria; and European gypsy moth, Lymantria dispar. Narrow-leaved evergreens such as pines, firs and spruces also may be defoliated by chewing caterpillars such as bagworms and tussock moth caterpillars that consume vast quantities of needles, eventually leading to defoliation of isolated branches of entire trees when populations are high (Photo 3).
Sawflies are primitive members of the bee and wasp clan (Hymenoptera). Larvae of several sawfly families have powerful jaws capable of removing large sections of leaf tissue. Some common species, such as dogwood sawfly, curled rose sawfly and dusky birch sawfly (Photo 4), consume leaves of deciduous plants, while others, including redheaded pine sawfly and European pine sawfly, feed on a variety of pines. Many species are gregarious, and these hordes may result in substantial defoliation. However, this grouping behavior often makes management easy by simply destroying the clustered larvae by hand or by pruning them out.
While sawflies are the most common leaf-chewing Hymenoptera, some bees and ants also defoliate. Leafcutter bees remove circular sections of leaf to construct nests for their larvae. This defoliation is often seen on roses and redbuds, and it is highly diagnostic (Photo 5). However, in light of the important role these bees perform as pollinators, it is worthwhile to explain to clients the nature of this injury and dissuade attempts to disrupt or kill these pollinators. In tropical and semitropical areas of the Americas, leafcutter ants remove vast amounts of leaf biomass and may be major pests of fruit trees, stripping entire plants of foliage (Photo 6).
One subtending category of defoliation is shot-hole injury. Shot-holes are created most commonly by the early developmental stages (instars) of caterpillars such as gypsy moths. Due to their small size, their jaws can only remove small pieces of leaf tissue, leaving behind holes in the leaf blade as they feed (Photo 7). Other shot-hole defoliators include many small beetles.
First reported in the Midwest in 2003, the European elm flea weevil, Orchestes alni, has become an important pest of Siberian elm (Ulmus pumila) and several other elms (Photo 8). Adults cause extensive shot-hole injury. When compounded by larval leaf mines, this weevil can cause severe defoliation and premature leaf drop.
Whereas shot-holes produced by European flea weevils are birdshot in size, shot-holes in viburnum caused by the adult of viburnum leaf beetles, Pyrrhalta viburni, are more akin to buckshot in size (Photo 9). This invasive pest native to parts of Europe and Asia was first discovered in New York in 1996. Its unique shot-hole feeding injury to viburnum makes it an easy diagnosis, even when the perpetrator is not present.
In addition to shot-holes created directly by insects feeding, some leaf miners, including the small caterpillar called the tupelo leafminer (Antispila nysaefoliella), create small windows on the leaf surface of tupelo as they escape from their mine to pupate.
Insects are not alone in creating shot-holes. Lesions caused by foliar pathogens also cause localized death of plant tissue, often resulting in shot-holes.
Leaf notching results when insects feed preferentially or solely on the margins of leaf blades. Among the most damaging of leaf notchers are beetles known as weevils. Preeminent of these weevils is the black vine weevil, Otiorhynchus sulcatus, a native of Europe now common in North America. Black vine weevil removes regular and irregular notches from the margins of more than 100 kinds of annual and perennial plants (Photo 10). In landscapes, broad- and narrow-leaved evergreens, including Rhododendron, Camellia, Euonymus, Heuchera, Astilbe, Bergenia and Taxus, often fall victim to this pest. Their larvae are root feeders on many woody plants, and may prove lethal by girdling trees and shrubs at the root collar. Large notches caused by adults are very definitive and a strong sign that black vine weevil is the culprit.
Another common leaf-notching weevil widely distributed in the Northeastern United States is the two-banded Japanese weevil, Callirhopalus bifasciatus. Notches caused by this weevil are similar to those of black vine weevil, but typically are much smaller due to the smaller size of the beetle (Photo 11). Larvae of this weevil also damage roots of trees and shrubs.
Several other weevils in the genus Otiorhynchus, including strawberry and rough-strawberry root weevils, clay-colored weevil, lilac root weevil and cribate root weevil, also notch leaves of landscape and agricultural plants. In Florida, a milky-white-colored weevil known as the Sri Lankan weevil, Myllocerus undecimpustulatus undatus, starts notches at the leaf margin and then makes deep notches into the blade of many ornamental, fruit and vegetable plants.
Some caterpillars also concentrate their meals at the margins of leaves. A native of Asia, the euonymus leaf notcher, Pryeria sinica, was first discovered in the U.S. in Virginia in 2002. It now is established in several locations in Maryland and Virginia, where it notches leaves of several species of Euonymus but also has been reported to consume bittersweet, Celastrus.
As leaves age, they become hardened, or sclerophyllous, through the deposition of lignin, especially along vascular bundles. This toughening of leaves serves as an impediment to feeding by chewing insects, as the insects are forced to contend with a matrix of hard leaf veins interspersed with tender, nutritious leaf tissues. Many large caterpillars have jaws powerful enough to bisect tough leaf veins. Smaller caterpillars lacking this ability may be forced to “feed between the lines,” removing tender epidermis and mesophyll with leaf veins left behind. This type of chewing injury is called skeletonization.
Large beetles with relatively powerful jaws, such as Japanese beetle, Popollia japonica, often chew through leaf blades and secondary vascular tissue, leaving behind only stronger primary veins and creating course leaf skeletons (Photo 12). Their hosts range in the hundreds, but some of their favored hosts include lindens, roses, apples and sassafrass during their period of feeding in June and July.
Smaller beetles and their larvae, such as elm leaf beetle, Xanthogaleruca luteola, leave behind small vascular bundles plus a translucent layer of epidermis (Photo 13). The presence of an epidermal layer accompanying skeletonization is sometimes called “windowpaning” or etching.
Beetles are not the only skeletonizers. Several small caterpillars, including the rather innocuous oak-leaf skeletonizer, Bucculatrix ainsliella, and the destructive Asian mimosa webworm, Homadaula anisocentra, also skeletonize leaves. In the latter case, the presence of silken webs littered with frass are other clues to the identity of this pest.
To complete the trifecta of chewing-mouthpart skeletonizers, several species of sawfly larvae join beetles and caterpillars as important pests. Roses – key elements of residential, commercial and institutional landscapes – are perennially pestered by a pair of skeletonizing sawfly larvae – roseslug sawfly, Endelomyia aethiops, and bristly roseslug sawfly, Cladius difformis. These small larvae are quite slug-like in appearance. By removing tender cellular material between leaf veins and one layer of epidermis, they strip leaves of photosynthetic machinery. Skeletonized leaves wither and crinkle, giving the rose the appearance of having had an encounter with a blowtorch (Photo 14).
In the next installment of “CSI for Bugs,” get ready for two more categories of symptoms caused by insects and mites. We will explore the deadly realm of dieback and the strange phenomenon of distorted plant parts.
Cranshaw, W. and D. Shetlar. 2018. Garden Insects of North America. Second Edition. Princeton University Press. Princeton, N.J.
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. 1991. Insects that Feed on Trees and Shrubs, Second Edition Revised. Cornell University Press. Ithaca, N.Y.
Raupp, M. J. 2020. CSI for Bugs, Part 1: Basics for Diagnosing Problems Caused by Insects and Mites. Tree Care Industry Magazine. Volume XXXII, Number 11, pages 54-59.
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.