Leaf-Feeding Insects

Determining what’s responsible for the demise or decline of your customer’s favorite tree can be difficult, especially when the causal agent is not readily visible. Fortunately, when dealing with leaf-feeding insects, there are usually visible clues to help with diagnosis.

Your best bet for success in controlling these insects is to follow an integrated pest management approach that includes accurate pest identification, assessing their damage potential, and selecting the least toxic control measures. Helping your clients manage their leaf insect populations provides a valuable service and offers the potential for profitability as well.

Consequences of leaf feeding

Not to downplay its importance, but just to make a distinction, leaf-feeding injury from chewing insects differs from damage to trees by wood-boring insects that tunnel in the cambium and sapwood, scale insects that extract plant sap through the bark, or gall insects that form unattractive or mysterious growths on tree leaves, but don’t generally cause any harm.

Leaf feeders typically cause defoliation and loss of photosynthetic capacity for one year; that’s a good thing and a bad thing. Good because if the tree is mature or at least more than a few years old, removal of one season’s growth usually won’t kill a tree. It’s bad because the defoliation robs the tree of the opportunity to produce adequate nutrients to defend itself against pests, expand the crown and root system and create shade for property owners.

Spotting leaf feeders

When leaf feeders damage a tree, the injury is often unique for individual insects. For example, Japanese beetles produce a characteristic honeycomb pattern in the damaged leaf. In other cases, the injury can consist of nondescript removal of leaf tissue, even though the outward appearance of the responsible parties can be quite different. When inspecting trees for leaf feeders, the main symptom to look for is the presence of leaves with parts missing. All leaf feeders create some sort of “leaf minus a piece” condition. Naturally, because various tree tissues vary from species to species, the damage can appear differently upon first glance. However, after a closer look, the signature characteristic of the missing leaf parts will remain constant. For example, Austrian pine and Caddo maple appear quite different in phenotype or outward physical appearance. When infested with leaf feeders, even though the pine needles are narrow and rounded and maple leaves are wide, thin and flat, noticing that parts are missing will guide you toward the culprit. Comparison of the affected leaves and uneaten portions of leaves can be helpful in spotting leaf-feeder damage. Of course, the presence of actual insects on the leaves or needles helps immensely with diagnosis.

Walnut caterpillars. Photo: John Fech

Walnut caterpillars. Photo: John Fech

Common leaf feeders

Caterpillars — Caterpillars are the larvae of butterflies and moths. There are scores of species that infest ornamental trees and shrubs. All caterpillars have chewing mouthparts; six small, true legs behind the head; and several false or prolegs on the abdomen. Most feeding damage by caterpillars is not serious, but a few species can damage the vigor of trees by continued defoliation over a period of years. Life cycles vary with each species.

Cankerworms — Cankerworms feed on most species of deciduous trees and some shrubs, but elm, oak and hackberry are their favorites. Spring cankerworms overwinter in the soil. Wingless adults emerge in the spring, mate, and then climb a nearby tree to deposit eggs under flakes of bark on the trunk and branches. Adults may 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 the tree, but most likely won’t kill it. After completion of feeding, larvae spin down from the canopy on a silken thread and enter the soil to overwinter. There is only one generation per year.

 Spring cankerworm. Photo: John Fech

Spring cankerworm. Photo: John Fech

Tent caterpillars — Tent caterpillars are voracious feeders on numerous deciduous trees including ash, oak, boxelder 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 are beginning to expand. Eastern tent caterpillars construct a “tent” in the 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.

Bagworms — Bagworms are common pests of evergreens, junipers, and occasionally deciduous trees and shrubs. The case, or bag, which provides a home for the bagworm caterpillar and gives the insect its name, is constructed of silk and fragments of needles or leaves. Bagworms overwinter as eggs within the bags. In the spring, 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.

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, however, have a series of distinct black dots along the body. Fall webworms typically appear in late summer and fall. While fall webworms can cause significant defoliation of trees and shrubs, damage is generally minimal, since defoliation occurs late in the season.

Mimosa webworm — Mimosa webworms are prevalent anywhere that honey locust trees are planted. Mimosa webworms overwinter as pupae in cracks or crevices of infested trees or in protected areas nearby. Adult females emerge in the spring and deposit their eggs. The larvae are gray to brown with five white longitudinal stripes. Larvae web leaflets together, feeding inside the webs and skeletonizing leaflets. The webs are unsightly and may cover entire trees.

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 are therefore quite variable as a group in behaviors, feeding habits and life cycles. Pine sawfly larvae vary in color from grayish to yellowish green. Some species have one or more lengthwise stripes. Larvae typically reach .75 to 1 inch in length and rear up in a characteristic defensive “S” profile 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 severely injure or kill trees. After feeding for several weeks, the larvae of most species drop to the ground, spin cocoons and pupate in the soil.

 Japanese beetle. Photo: John Fech

Japanese beetle. Photo: John Fech

Beetles — A second important group of leaf-feeder insects are the 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 hind wings that form a protective shield over the body of the insect.

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. In the early spring, they emerge from overwintering sites as leaves are beginning 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.

Cottonwood leaf beetles — Cottonwood leaf beetle adults are about .25 inch long. They are yellow with black stripes, and 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.

 Cottonwood leaf beetle and fall webworms. Photo: John Fech

Cottonwood leaf beetle and fall webworms. Photo: John Fech

Controlling leaf feeders — timing is everything

Applying a foliar insecticide to mature, fully grown leaf feeders is problematic on two fronts:

1. It’s not all that effective – large insects are more tolerant of insecticide applications than newly hatched ones.

2. The opportunity to prevent damage has been lost – by the time they are fully grown, chances are that they’ve already eaten most of what they are going to eat, and application does little more than provide revenge for the property owner.

Injections offer potential for “rescue” treatments, however, because of the potential for injury to the tree cambium, it’s best to consider this approach only when spray applications would cause injury to surrounding elements such as cars, buildings and other personal property.

Regular inspections

The importance of timing cannot be underestimated, this is why regular inspections are a hallmark of integrated pest management and plant health care. If an infestation is identified early in the insect’s life cycle, and an appropriate application is made, arborists win on two accounts. First, the applications tend to be more effective, and second, a greater amount of foliage loss is prevented, providing the customer with a better value for their investment.

Incorporating regular inspections into your tree program involves a paradigm shift. Traditionally, arborists and tree workers have been paid based on how much product they could spray on a tree. The change in thinking that must take place is that timely inspections are as (or more) important than pesticide applications. As such, it’s crucial that arborists make money from inspections by charging a reasonable hourly rate based on the experience and training of the inspector. Our advice: don’t give your inspections away, after all, you get what you pay for in life, and good tree care is no different than buying clothes, cars or carpet.

Editor’s note: This article was originally published May 2011 and has been updated.

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