Department of Entomology

University of Maryland

Defoliation has different meanings in different contexts. Literally it means the removal of leaves from a plant. Some arthropod pests will cause plants to drop leaves prematurely. Heavy spider mite or scale infestations may cause euonymus leaves to abscise prematurely. However, another form of defoliation results from the feeding of insects with jaws. Most insects have mandibles. These opposable jaws are used to chomp, nibble, and scrape the blades of leaves. The removal of leaf tissue by insects with jaws is called defoliation.

Diagnosis

When small, mandibulate insects feed on leaves somewhat older and tougher, they may not be able to penetrate leaf surfaces and tough leaf veins and sometimes a layer of epidermal cells remains. This type of defoliation is called skeletonization. Larger caterpillars and sawfly larvae have powerful jaws and many can remove gaping chunks of very tough leaves. In contrast, when leaves are small and tender small defoliators will be able to remove large areas of the blade. In both cases big areas may be gone.

Who are these defoliating insects? Several small and large taxa or groups of insects have chewing mouthparts. Defoliators include the grasshoppers and their relatives (Orthoptera), walking sticks (Phasmatodae), earwigs (Dermaptera), beetles (Coleoptera), sawflies and kin (Hymenoptera), and caterpillars, the larvae of moths and butterflies (Lepidoptera).

Although grasshoppers, walking sticks, and earwigs can be problematic from time to time, the defoliators that regularly cause big problems in landscapes are beetles, caterpillars, and sawfly larvae. It is curious that of these three only beetles have both immature and adult stages damaging to plants. Caterpillars metamorphose into moths and butterflies that may be beneficial pollinators of flowering plants. Adults of sawflies feed on nectar and pollen and in some cases they may be predators of plant pests.

Unless the jawed perpetrator entirely consumes a leaf, the evidence left behind often provides a clue to the identity of the herbivore. Shot holes in the young leaves of maples or oaks are often a clue that cankerworms or gypsy moth caterpillars are present in the canopy. Large areas of missing leaf blades indicate that larger caterpillars have been at work.

The wise plant health care technician or IPM monitor will remember that damage caused by a defoliator early in the season will remain with an individual leaf during its tenure on the plant. This means that severe damage may be present but pests are no longer active or accessible to destruction by pesticides. I call this residual damage the "ghost of herbivory past." The ghost of herbivory past should not be used as a justification for a pesticide application. Always look and be sure that a pest is present before making an application.

Leaf notching is another type of defoliation injury caused by insects with jaws. Perhaps the most frequently encountered type of leaf notching results from the feeding adult weevils. Weevils are a large family of beetles characterized by profoundly long snouts at the end of which are tiny jaws. Many weevils feed on the margins of leaves and produce readily identifiable notches. In fact the defoliation is so distinctive that this group is often collectively referred to as leaf notching weevils. Perhaps the most widespread and well known of the leaf notching weevils is the black vine weevil, a.k.a. Taxus weevil. Other close relatives include the strawberry root weevil, rough strawberry root weevil, Fuller rose beetle and two-banded Japanese weevil.

Adults of all of these weevils are important pests in landscapes and nurseries. Damage by adults can be disfiguring and is sometimes severe. However, larvae of weevils are the real killers of plants. The legless, C-shaped grubs of some species of leaf notching weevils such as the black vine weevil consume the roots of plants. In heavy infestations larvae may sever roots and completely girdle woody plants at the root collar. Attack on herbaceous plants can involve larvae tunneling into the stems. Severe root and stem damage can result in plant death.

Another common and curious leaf-notching insect is the leaf cutter bee. These small bumble-bee-like insects cut disks, almost perfectly round disks, from a variety of woody plants including rose, ash, and redbud. The disks are used to line and plug the galleries in which the leaf cutter bees lay their eggs. These bees cause little damage to plants and control is not needed, but their handiwork is unmistakable.

One type of collateral damage to plants and a sure clue that defoliators are at work is the presence of large silken structures commonly referred to as tents, webs, or nests. Many of our defoliating caterpillars including eastern and western tent caterpillars produce silken galleries where larvae reside between bouts of feeding. These tents may first be found surrounding small branches but later are constructed in the branch crotches near the bole of the tree as larvae grow. Tent caterpillars are among the earliest defoliating caterpillars to appear each spring.

Later, in summer and fall, the fall webworm makes its presence known by constructing webs at the tips of branches. Hatchling fall webworm larvae make their first web on a single leaf -- the birth place of larvae as they hatch from eggs. As larvae grow, they engulf an ever-widening collection of leaves at the distal end of branches. In large colonies, webs can be almost a meter in length.

Other web-making caterpillars include the mimosa webworm and juniper webworm. These clever larvae use silk to web several leaves together to form a protected refuge, the perfect spot for consuming leaves while avoiding enemies. Heavy infestations of these caterpillars rapidly ensnare entire canopies of their hosts with the resultant damage looking like a close encounter with a blow torch. Many other caterpillars web, tie or roll leaves to provide a protected refuge from which to feed. In the case of bagworms, pieces of plant tissue are incorporated into a silken bag that is carried by the caterpillar as it moves from meal to meal.

Management

The first step in management is always to design pests out of the landscape by using resistant plants whenever possible. For example, maples and ash tend to be less suitable hosts for gypsy moth caterpillars than oak or apple. In areas where gypsy moth is endemic, try to use trees and shrubs that are less favored by this pest. Several species and cultivars of elm are quite resistant to elm leaf beetle. The same is true for species of roses, lindens, and crabapples with respect to Japanese beetle. Your local Cooperative Extension Service office should be able to assist with selection of resistant plants in your region.

Keep plants healthy by planting them in good soils, using appropriate amounts of mulch and fertilizer, and providing supplemental water in times of drought. Avoid practices that wound the bark and damage branches. Never burn tent-making caterpillars out of trees (How would you like a burning torch placed in your armpit?).

Several of our common defoliators over-winter as eggs deposited on the branches or boles of the tree. This is the case with tent caterpillars and gypsy moth. Problems can be dramatically reduced in isolated trees by simply removing egg masses from the tree prior to egg hatch in the spring. Many caterpillars and sawfly larvae are gregarious and some live in tents or webbing. Clusters of gregarious larvae can be removed by hand or pruning. Bagworms overwinter as eggs inside the bag. Thorough removal and disposal of these bags prior to hatch in late spring can all but eliminate this pest from individual plants.

Many caterpillars, sawfly larvae, and beetles are attacked by a vast array of natural enemies including vertebrate predators such as birds and mice, and invertebrate predators such as lady beetles, predatory bugs, and parasitic wasps and flies. Sometimes epidemics of viruses and fungi sweep through pest populations, causing them to collapse on local and regional scales. The fungus Entomophaga is responsible for dramatic reductions of the gypsy moth throughout the eastern United States.

When natural enemies are unable to keep defoliator populations in check, insecticide applications may be warranted. In general, defoliators are among the easiest of insect pests to kill with insecticides. Their habit of feeding externally on plant leaves exposes their bodies directly to pesticides that may be absorbed through the cuticle. Insecticides may be ingested as they eat or encountered as they move about contaminated plant surfaces. Some systemic insecticides may be applied to leaves where they are absorbed. Others are injected or implanted into vascular tissueor absorbed by roots when applied to the soil. These are distributed throughout the canopy of plants and ingested by the defoliator.

As a general rule, it is easier to kill a small or early instar larva than an older one. The biological truth here is that it generally takes less poison to kill a small organism than a large one. This makes the timing of insecticide application critical. Careful observation of events such as egg hatch, the first signs of defoliation, flight or adult moths, or emergence of adult beetles are important. The use of degree-day models or correlating pest activity with plant phenology may also be helpful in timing applications for maximum effect.

In a fascinating twist, mother nature has provided a lethal solution to some of our most damaging defoliators in the chemical arsenal of naturally occurring soil microbes. Strains of the soil bacterium Bacillus thuringiensis (Bt) produce compounds called endotoxins that are lethal to caterpillars and the larvae of leaf beetles. Bt toxins are relatively specific to these pest groups and thereby minimally disruptive to beneficial insects such a predatory bugs and parasitic wasps that cohabit the landscapes with defoliating pests.

Insecticides containing Bt are effective against many of our most important defoliators. Another soil microbe, Saccharopolyspora spinosa, produces a class of compounds called spinosyns that kill caterpillars and the larvae of beetles and sawflies. Spinosyns are the active ingredient of several commercial insecticides and are reported to have little impact on beneficial natural enemies and non-target organisms commonly found in the landscape.

For the grubs of soil-dwelling scarabs and leaf notching weevils, microscopic assassins await underground. Tiny roundworms called entomopathogenic nematodes stalk these pests and enter their bodies through small breathing ports and openings of the digestive tract. Once inside, they release lethal bacteria that invade the insect's cells turning their hapless victim into nematode nirvana. The nematodes reproduce, multiply and move out to find another host, leaving a deflated corpse behind.

To learn more about nematodes visit the nematode website at www2.oardc.ohio-state.edu/nematode/. Commercial suppliers of entomopathogenic nematodes can be found in the beneficial suppliers catalog at www.cdpr.ca.gov/ipminov/bensupl.html.

Equally fascinating and no less lethal are several types of fungi that attack insects in the soil. Spores of insect-eating fungi land on the cuticle of the insect. If conditions are favorable, spores germinate and send out an advanced strike force, a thread-like hypha. This fungal strand produces a sticky holdfast with all the capabilities of a tiny chemical weapons factory. The holdfast manufactures enzymes capable of eating a hole through the protective armor of the insect.

Once the skin is breached the fungal invasion proceeds in full force. A penetration peg is rammed through the hole and grows into the body cavity of the insect. The insect's internal defenses are disabled by a variety of chemicals including venom similar to that of a rattlesnake. The innards of the insect are transformed into a fungus garden-end of story. These fungi occur naturally and a few strains are available commercially.

As a final thought, remember that defoliators, especially the larvae of butterflies and moths are important members of several terrestrial food webs. They support birds, mammals, and invertebrate predators and parasitoids higher in the food chain. They are important members of urban ecosystems, despite their nefarious behaviors. They are easy to kill, but should only be eliminated when the vitality and beauty of plants are seriously in question.