Tree Services Magazine - February, 2009

FEATURES

The Leaf Munchers

By John C. Fech; Frederick P. Baxendale
Photos courtesy of Dennis Ferraro, UNL.
A sawfly on needles.
Beetles and caterpillars comprise the two largest groups of leaf-feeding insects. These leaf munchers can cause significant damage to broadleaf plants and conifers, alike. The signature consequence of an infestation of leaf munching insects is a tree or shrub with parts missing from their leaves. Your best bet for success in controlling these insects is to follow an integrated pest management (IPM) approach, including accurate pest identification, determination of damage potential and selection of the least toxic control measures.

Damage symptoms

Leaf-munching insects damage plants by removing the parts of the plants (leaves) that make necessary sugars and carbohydrates, causing the infested plant to be disfigured and weakened. The extent of their damage depends on the number of insects feeding, the state of health of the plant, and the number of insect generations per year. Plants placed in the wrong location or suffering from herbicide drift, compacted soils or lawn mower injury are more readily injured by leaf feeders than otherwise healthy specimens.

Below, you’ll learn to identify the various patterns of consumption. This damage pattern is important because in many situations the actual insects are difficult to find. Looking to the leaves for clues can be a helpful first step in controlling these unwanted pests.

Photo courtesy of Dennis Ferraro, UNL. Photo courtesy of Dennis Ferraro, UNL.
A bagworm on a stem. Sawfly on a stem.

Caterpillars

Caterpillars are the larvae (immatures) of moths and butterflies, and there are scores of different species that feed on the leaves of ornamental trees and shrubs. Most cause little injury, but a few can damage the vigor of trees by continued defoliation over a period of years. Caterpillars vary widely in their biology and behavior, but in general, after mating, the adult female lays her eggs on the host plant. The eggs may be laid singly or in masses. After a few days, or even months, the eggs hatch and the young caterpillars migrate to feeding sites on the tree or shrub. Most caterpillars feed voraciously and grow rapidly, shedding their skins several times before becoming pupae. Some caterpillar species spin a protective cocoon in which to pupate, while others overwinter as pupae, with moths or butterflies emerging several days or months later. The number of generations per season depends on the species and the weather conditions.

Spring Cankerworms: Cankerworms feed on most species of deciduous trees and some shrubs, but elm and hackberry are their favorites. Spring cankerworms overwinter in the soil, and wingless adults emerge in the spring, mate and climb a nearby tree to deposit eggs under flakes of bark on the trunk and branches. Adults emerge as early as late February and early March during warm periods. Upon hatching, the caterpillars or “measuring worms” feed voraciously on the leaves, at times completely stripping the tree. Severe defoliation over a number of consecutive years may weaken, but is unlikely to kill the tree. After feeding, larvae enter the soil near infested trees to overwinter. There is only one generation per year.

Photo Courtesy of James A. Kalisch , UNL.
Spring cankerworm.

Bagworms: Bagworms are common pests of evergreens, junipers and occasionally deciduous trees and shrubs. The case, or bag, that provides a home for the bagworm caterpillar, and which gives the insect its name, is constructed of silk and fragments of leaves or needles. Bagworms overwinter as eggs within the bags. During the first or second week of June, tiny larvae hatch from eggs and immediately begin construction of small protective bags. Caterpillars feed from within their bags and move along the branch in search of food. If food is depleted in one area they simply move to another area.

Tent caterpillars: Tent caterpillars are voracious feeders on numerous deciduous trees including ash, oak, box elder and cottonwood, but wild plum and cherry are their favorite hosts. In late summer, female moths lay eggs in shiny brown frothy masses that encircle twigs. These eggs overwinter and hatch in the spring, when leaves begin to expand. Eastern tent caterpillars construct a “tent” in crotches of branches. These caterpillars are hairy, almost black with a white stripe down the back and a row of blue spots on each side. They can reach 2 inches in length when fully grown. Infested trees are often completely defoliated and their growth and vigor are reduced. There is only a single generation each year.

Photo Courtesy of James A. Kalisch , UNL.
Bagworm-interior.

Fall webworms: Fall webworms feed on nearly all deciduous trees and shrub species. Larvae construct silken webs around branches, feeding on leaves and living within these shelters. Webs are enlarged as the caterpillars grow. Fall webworms are covered with long hairs and vary in color from white to yellow, orange to gray. All color phases have a series of distinct black dots along the body. Fall webworms typically appear in late summer and fall. While they can cause significant defoliation of trees and shrubs, damage is generally minimal since defoliation occurs late in the season.

Photo Courtesy of James A. Kalisch , UNL.
Bagworm on juniper

Sawflies: Sawfly larvae resemble caterpillars, but are actually the immatures of primitive stingless wasps. Sawflies feed on a wide variety of evergreens and broadleaf trees, and therefore are quite variable as a group in behavior, feeding habit and life cycle. Pine sawfly larvae vary in color from grayish to yellowish-green. Some species of pine sawflies have one or more lengthwise stripes. Larvae typically reach .75 to 1 inch in length and rear up in a characteristic defensive “S” shape when disturbed. Most species overwinter as pupae, but a few spend the winter as eggs inserted in the needles. In the spring, sawfly larvae feed in groups on needles, starting at the needle tip. Feeding habits among species vary, but those that feed only on old or young needles weaken trees or slow their growth, while species that feed on both young and old needles may kill or cause severe injury to trees. After feeding for several weeks, the larvae of most species drop to the ground, spin cocoons and pupate in the soil.

Beetles

A second important group of leaf munchers are beetles. Most beetles overwinter as adults in leaf litter layers under trees. They emerge in spring and begin feeding on new leaves of trees and shrubs. Some species have several generations each year. Beetles are characterized by thick, hardened hindwings that form a protective shield over the body of the insect.

Photo Courtesy of James A. Kalisch , UNL.
Cottonwood leaf beetle.

Elm leaf beetles: A good example of a leaf-feeding beetle is the elm leaf beetle. These insects feed on the foliage of American and Siberian elms. Adults are .25 inch long, yellowish-green and have a black stripe on the outside of each wing cover. Larvae reach about 3/8 inch in length and are yellow with black spots and stripes. Elm leaf beetles overwinter as adults in sheltered areas, especially in and around buildings, and in litter and bark crevices. They emerge from overwintering sites as leaves begin to expand. After feeding on leaves, adults lay clusters of yellow eggs on the foliage. Eggs hatch in late spring, and the first signs of larval feeding are skeletonized areas between veins, giving leaves a lacy appearance. As damage accumulates, leaves take on a ragged appearance and turn brown. Heavily infested trees often prematurely lose their leaves. There are two generations per year. Interestingly, a parasitoid wasp, Tetrastichus gallerucae, which attacks elm leaf beetle eggs, has significantly reduced elm leaf beetle numbers over the past decade.

Photo Courtesy of James A. Kalisch , UNL.
Eastern tent caterpillar.

Other leaf beetles: Cottonwood leaf beetle adults are about .25 inch long, and are yellow with black stripes. The head area is black. Larvae reach 1 inch in length and have dark bodies with two white spots on each side. These spots are scent glands that give off a disagreeable odor when larvae are molested. Larvae feed in groups, skeletonizing leaves. As they grow, they feed individually and consume entire leaves. These leaf beetles overwinter as adults under bark or debris on the ground. There may be several generations each year. In addition to cottonwood, these beetles will also feed on willow and other poplar species.

Photo Courtesy of James A. Kalisch , UNL.
European sawfly larvae.

Control strategies

• Inspection—Arguably, the most important step in minimizing damage from leaf munchers is a thorough inspection of your customer’s trees on a regular basis. Every three weeks or so, have one of your technicians walk the property looking for any signs of unusual symptoms or abnormal developments. Regular inspections are important for effective leaf muncher control because infestations can explode, going from just a few bugs here and there to a large mass of chewing critters. Control is always much easier and more likely to be successful if the infestation is caught early in its development.

• Tree species selection—When your customers are planning new plantings or filling in voids in their current landscape, suggest trees that are less often infested with leaf munchers. This will vary from year to year and location to location, but certain tree species—such as mulberry, Scot’s pine and cedar—have been known to attract more than their fair share of unwanted pests. Balance the desirable attributes of a tree selection (shade pattern, disease resistance, lack of surface rooting, fall color, winter appeal) with the likelihood of insect infestation.

• Physical removal—This method will work quite well for certain leaf munchers (bagworms, sawflies), but not so well for others. This means actually picking and removing by hand or using a moderate blast of water to dislodge pests from the tree. For example, hand removal of bagworm cases before overwintered eggs have hatched is an effective way to control bagworms. Of course, the efficiency of this method depends somewhat on available resources. Unfortunately, physical removal doesn’t work well for large trees, where pests may be hard to reach and water blasts are not usually strong enough to dislodge them.

• Pest control agents—There are numerous pesticide products to choose from that will effectively control leaf munchers. Some are designed to be applied systemically and others topically. For topically applied products, thorough coverage of the leaf surface is important for effective control. Examples of topically-applied products include insecticidal soaps, horticultural oils, Bacillus thuringiensis (Bt), carbaryl (Sevin) and the synthetic pyrethroids (e.g., bifenthrin, cyfluthrin, cypermethrin, deltamethrin, lambda-cyhalothrin and permethrin). Examples of systemic insecticides include disyston, imidacloprid, halofenozide and thiamethoxam.

Photo Courtesy of James A. Kalisch , NL.
Elm leaf beetle.

There are pros and cons of each application approach. Topically applied insecticides provide ease of application, are relatively low cost and offer quick results, however, they include the possibility of spray drift to nontarget organisms and the potential for phytotoxicity (foliar burn). Systemic applications avoid the spray drift problem if the tree is in a “wind tunnel” or near a parking lot, but may experience uneven or inconsistent uptake, potential trunk damage when insecticides are injected, lack of soil for drench or broadcast applications, and in most cases slower results.

John C. Fech is a horticulturist, certified arborist and frequent contributor located in Omaha, Neb. Frederick P. Baxendale is a professor and extension entomologist with the University of Nebraska-Lincoln.