As the work of tree care interfaces with the insect world, one thing is inevitable: at some point, in some location, for better or worse, insects feed on trees. A good share of the feeding is on the ultra-important food-making portions of a tree, the leaves. For better or worse is more appropriate for a wedding vow than for the care of trees; a more accurate phrase would be “for a little or a lot” of damage. When going about the nitty gritty of daily tree care, keep in mind that the presence of an insect or two doesn’t necessarily mean a tree is being severely affected. Sorting out when it is and isn’t should be the goal of a qualified arborist.

Inspection

Control of leaf-feeding insects is most effective when applications are made to actively feeding insects, so it’s important to inspect trees on a regular basis. Regular investigations are invaluable when it comes to spotting the critters that may need control. From a business standpoint, if a given activity isn’t a profit center or related to a profit center, it’s tough to justify the time and effort to accomplish it.

Eastern tent caterpillars feed in masses in mid-spring on many tree species. Photo courtesy of James Kalisch, UNL.

Eastern tent caterpillars feed in masses in mid-spring on many tree species.
Photo courtesy of James Kalisch, UNL.

Damage symptoms

Questions may arise during inspections, especially for greenhorns. A common one might be “What am I looking for?” Before we answer that, a more pertinent question is “What’s a sign and what’s a symptom?” This is an important distinction to make. A sign is the actual organism (or part of an organism) that is causing damage. In this case, a sign might be a caterpillar or a beetle. A symptom is the change in appearance that the sign (insect) has caused (i.e., chewed or stippled leaves). Signs and symptoms also apply to the disease world. Again, the sign is the organism such as a fungal spore or rust pustule. The symptom would be raised bumps or colored spots on the leaf surface. Getting familiar with the signs and symptoms of common insect pests is a step in the right direction.

Timing

Timing is everything. In an attempt to control a damaging insect, if the correct amount of an appropriate insecticide is applied, but it’s done after or before the insects are present at a vulnerable stage, then the application will fail to meet the objective. Without good timing, good control does not exist. Endeavor to research the best times for treatment and gear the treatment accordingly.

Classic leaf feeders

Aphids

Aphids occur on nearly all species of woody ornamentals. They feed predominately on the undersides of leaves, but are also found on the tender shoots of plants. Most are green, but their color can vary from bright red to yellow to black. Aphids possess piercing-sucking mouthparts, which they use to withdraw plant sap.

Ash aphids are able to hide beneath curled leaves and are protected from predators. Photo courtesy of James Kalisch, UNL.

Ash aphids are able to hide beneath curled leaves and are protected from predators.
Photo courtesy of James Kalisch, UNL.

The best time to control aphids is early in their life cycle for three reasons. First, smaller aphids succumb to treatments much more readily than older and larger ones. Second, as aphid colonies develop leaves often curl around the aphids, protecting them from predators and shielding them from insecticide applications. Finally, aphids excrete a sticky substance, called honeydew, that collects on lower leaves and branches and creates fertile growing areas for sooty mold. The end result is not only unsightly, but also reduces the photosynthetic surface of the tree leaves.

For maximum effectiveness, insecticidal sprays should be directed toward the undersides of leaves. Since aphids have a relatively short life cycle and can rapidly recolonize an area, multiple treatments may be necessary for acceptable control

Leafhoppers

Leafhoppers are easily identified by their characteristic wing pattern, size, shape and color. About 0.2 to 0.25 inch in length, leafhoppers hold their wings at a sharp angle, similar to a small tent. They often match the color of the plant tissue upon which they’re feeding, most commonly green, yellow or gray. Treehoppers often have extraordinary pronotum patterns that can resemble spines, horns or keels. Some species are thorn-like in appearance.

When in small numbers, leafhoppers and treehoppers rarely cause damage to trees. Photo courtesy of James Kalisch, UNL.

When in small numbers, leafhoppers and treehoppers rarely cause damage to trees.
Photo courtesy of James Kalisch, UNL.

Unlike many of the other sap-sucking insects, leafhoppers are reasonably good fliers and easily move from tree to tree to escape predators and arborists. These insects inhabit most broadleaf trees, but seem to prefer honey locust, ash and oak.

Because of their ability for flight, insecticidal soaps and horticultural oils are not always effective. Residual insecticides, including the synthetic pyrethroids, as well as systemic treatments are preferred. Fortunately, damage from leafhoppers is usually not severe enough to require treatment.

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 terms of their behavior, feeding habits and life cycles. 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 0.75 to 1 inch in length and rear up in a characteristic defensive “S” shape when disturbed. Most species overwinter as pupae in the soil beneath trees, but a few spend the winter as eggs inserted in the needles.

The leaf-feeding sawfly doesn’t look anything like a fly, at least in the feeding stage. Photo courtesy of James Kalisch, UNL.

The leaf-feeding sawfly doesn’t look anything like a fly, at least in the feeding stage.
Photo courtesy of James Kalisch, UNL.

In the spring, sawfly larvae feed in clusters 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.

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 is constructed of silk and fragments of leaves or needles. Bagworms overwinter as eggs within the bags. In the spring, during the first or second week of June, tiny larvae hatch from the 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.

Bagworms are cool because they use the foliage of the plant they’re feeding on to make their cocoon/covering. Photo: Mason Steinberg

Bagworms are cool because they use the foliage of the plant they’re feeding on to make their cocoon/covering.
Photo: Mason Steinberg

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. 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 is unlikely to kill it. After feeding is completed, larvae enter the soil near infested trees to overwinter. There is only one generation per year.

Look closely—the symptoms are stippled leaves and the signs are webs and actual insects.

Look closely—the symptoms are stippled leaves and the signs are webs and actual insects.

Mites

Although not technically insects — insects have six legs and three body parts, while adult mites have eight legs and two body parts — spider mites are the quintessential sap suckers. Under cool, moist springtime conditions, spruce spider mites can rapidly multiply on conifers including spruce and juniper. Damage is usually not noticed until the weather turns hot and dry later in the summer. As the summer progresses, two-spotted spider mites can become a problem on essentially all landscape ornamentals.

Mites can be detected in two ways. First, use a 10x hand lens to look for moving critters on the underside of a broadleaf tree leaf, or on the new growth of a conifer. The body and legs of motionless mites should be clearly visible, while mites in motion may simply appear as moving dots. Second, place a white sheet of notebook paper under a branch or group of leaves and rap them sharply with a small stick. If mites are present, they will fall onto the paper where they can be easily seen.

If one or two mites are found on a leaf, immediate treatment is usually not necessary. However, continue to monitor the tree for any changes in the mite population. If more than six to 10 mites appear on the sheet of paper, consider treatment with a miticide such as bifenthrin (Talstar). While sap-sucking insects may have a common method of feeding, keep in mind that each should be considered individually when it comes to selecting the most appropriate control strategies.

Control

Early inspection is clearly the first step in controlling leaf-feeding insects. Application of foliar insecticide products in an attempt to control mature, fully grown leaf feeders can be less than effective for two reasons. One, lack of efficacy. Big insects are usually more resistant to applications than those that have recently gone through a metamorphosis. Two, damage prevention. Later applications can kill the insects, however, the opportunity to keep the leaves on the tree is lost. In some cases they have already consumed a large number of leaves. At that point, the product is just a revenge application for the customer. Applying residual insecticides such as the synthetic pyrethroids (e.g., bifenthrin, cyfluthrin, cypermethrin, deltamethrin, lambda-cyhalothrin and permethrin) or systemic treatments (chlorantraniliprole, clothianidin, disulfoton, imidacloprid and thiamethoxam) while infestations are just beginning to develop and insects are still small is a much better option.

Trunk injections offer the potential for treatment that rescues the tree from damage and are especially useful when spray applications can cause injury or damage targets — automobiles, houses, mailboxes, etc. — that surround the tree. On the other hand, some injection technologies cause injury to the cambium, which should be kept to a minimum.

Physical removal may sound labor-intensive on the surface, and it is, but when the tree is in close proximity to a water source or picky neighbor (or the customer themselves), this approach should be considered. In fact, depending on the product label, it may be illegal to apply the insecticide where there are concerns of surface or groundwater contamination. When picking insects off a tree simply isn’t an option, consider strong water-blasting as an option. For some leaf-feeding pests, such as mites and sawflies, it can be effective without pesticide exposure to the environment.