Take a bite out of leaf-feeding insects
Grounds managers know that leaf-feeding insects can be a major problem for ornamentals. However, they may or may not be aware of how the damage they are seeing occurs. If you want to gain a greater understanding of such insects, the damage they can cause and effective control strategies, read on.
The damage a leaf-feeding insect can inflict on a tree or shrub is significant in many cases. A graphic example is that of the gypsy moth. Charles Osgood of the CBS Radio Network was so impressed by the voracious appetites of these pests that he focused a special report on gypsy moths. He described how "so many caterpillars fell on a certain set of railroad tracks that they caused the rails to be slippery, preventing trains from making it up the grade of a rather modest hill." In another part of the segment, Osgood described how "the noise of crunching tree leaves [kept] small children awake at night."
Just how damaging are leaf-feeding insects likely to be on the grounds you manage? Actually, this is quite variable. Some leaf feeders occur in small numbers, and their effect is hardly noticeable. Others, even when numerous, do not injure ornamentals, particularly if the feeding takes place early in the season or for only a short period. In these situations, the plants should releaf soon after the feeding damage occurs and not suffer serious injury. If a pattern of annual infestation develops, however, long-term damage may result.
Feeding habits Leaf-feeding insects damage plants by removing the parts of the plants-the leaves-that make necessary sugars and carbohydrates. Therefore, their feeding not only causes disfiguration, it also weakens the infested plant. The extent of the damage depends on the number of feeding insects, the plant's health and the number of insect generations per year. Leaf feeders more seriously affect plants suffering from any pre-existing malady than healthy specimens.
By contrast, non-leaf feeders cause damage to other vital plant structures. For example, borers invade and destroy the conductive vessels (cambium) of the plant, slowly causing a blockage of water and nutrient flow throughout the plant. When borers continue feeding in a plant for several months, they also destroy inner wood, causing branches and stems to break off in windstorms. Piercing-sucking insects such as aphids insert straw-like mouthparts into leaves and soft stems and extract cell sap containing water and nutrients. When such insects feed, the leaf stays intact, but the plant slowly loses vigor.
Life cycles Leaf feeders are primarily members of four insect orders-lepidoptera (butterflies and moths, whose larvae are caterpillars), coleoptera (beetles, including several leaf beetles), hymenoptera (wasps, whose larvae include sawflies) and orthoptera (grasshoppers).
* Lepidoptera. Caterpillars vary widely in their biology and behavior, but most follow a similar basic life-cycle pattern. 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 plant. Most caterpillars feed voraciously and grow rapidly, shedding their skins several times before becoming pupae. Some caterpillars spin a protective cocoon in which to pupate. Many species overwinter as pupae, with moths or butterflies emerging when conditions are favorable. The number of generations in a season depends on the species and the weather conditions to which the insect is exposed.
The forest tent caterpillar (Malacosoma disstria), a voracious feeder on ash, oak, boxelder and cottonwood trees, is an excellent example of a leaf-feeding caterpillar. These hairy larvae are pale blue with white keyhole-shaped markings on their backs. Fully grown larvae are nearly 2 inches long. Eggs hatch in the spring, when leaves are beginning to expand. Caterpillars feed in groups on leaves in early summer. Although larvae cluster around branches to rest and molt, they do not form tents, as do the closely related eastern tent caterpillar, M. americanum. In late summer, female moths appear and lay eggs that overwinter in shiny brown masses that encircle twigs. Infested trees are often completely defoliated, reducing their growth and vigor.
* Coleoptera. Most leaf-feeding beetles overwinter as adults in leaf-litter layers under trees, emerge in spring and begin feeding on new leaves of trees and shrubs. Some species have several generations each year. All beetles have complete metamorphosis (as do caterpillars), which means they go through four stages of growth and development: egg, larva, pupa and adult. Adult beetle females lay eggs, which hatch into larvae. After feeding and molting (shedding their skin) several times, the larvae mature and become pupae, or pupate. The pupae later emerge as adults; the length of time it takes to complete the life cycle varies quite a bit from species to species. The weather also can influence the life cycle. Adult beetles tend to have thick, hardened upper wings that form a protective shield over the body of the insect. When the wings are folded, they form a straight line down the middle of the beetle's back.
A good example of a leaf-feeding beetle is the elm leaf beetle (Pyrrhalta luteola). The adults are 0.25 inch long, yellowish green and have a black stripe on the outside of each wing cover. Three black spots are present behind the head. Elm leaf beetles overwinter as adults in sheltered dry areas, especially in and around buildings and in litter and bark crevices. They emerge from overwintering sites as the buds of elm trees begin to expand in the spring and feed on leaves before laying clusters of yellow eggs on foliage in the late spring. Larvae skeletonize (consume soft leaf tissue, leaving a "skeleton" of veins) leaves for 2 to 3 weeks before pupating. Two generations occur each year. Elm leaf beetles often heavily defoliate trees, leaving them unsightly and weakened.
* Hymenoptera. Sawflies feed on a variety of evergreens and broadleaf trees and therefore are quite variable as a group in behavior, feeding habit and life cycle. Sawflies are an interesting group in that the adults are actually primitive wasps and tend to resemble small bees. Sawfly larvae resemble caterpillars.
Pine sawflies (Neodiprion spp.) are no exception. 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. Most species overwinter as pupae, but a few spend the winter as eggs inserted in the needles. In spring, larvae feed in groups on needles, starting at the needle tip. After feeding voraciously for a few weeks, the larvae of most species drop to the ground, spin cocoons and pupate in the soil. All larvae rear up in a characteristic defensive "S" shape when disturbed. Feeding habits are variable among species. Those that feed only on either old or young needles weaken trees or slow their growth, while species that feed on both young and old needles can kill or severely injure trees.
* Orthoptera. In most cases, grasshoppers are not major pests of ornamental trees and shrubs, unless ornamental plantings are adjacent to agricultural crops.
* Leaf miners are small larvae that feed between the upper and lower surfaces of leaves, "mining" the tissue but leaving the surface layers. Many types of leaf miners occur, including larvae of wasps, caterpillars, flies and even beetles. Leaf-miner damage occasionally is visually conspicuous, but these pests seldom require treatment. Their numbers can vary widely from year to year, and natural predators usually keep levels in check.
Control of leaf feeders * Identification. The first and most crucial step in the control of any insect pest is correct identification. Many novice grounds managers tend to skip over this step, anxious to get on with some sort of spray program. In many cases, a previously unidentified insect turns out to be a beneficial or non-injurious species that you should leave alone. Accidentally spraying a beneficial insect with an insecticide can cause other pest problems to increase.
* Insect control can be specific, but you must know with which pest you're dealing. Proper tools will help grounds managers in their quest for accurate identification. Every person ona grounds crew should carry a 10x hand magnifier. This will allow them to distinguish markings and other body features-the number of legs, etc. A small probe and collection vials will facilitate returning samples of insects back to the shop where you can take a closer look. Identification books are also helpful, especially ones with good color pictures. A standard volume for the landscape industry is Insects that feed on trees and shrubs (ISBN 0-8014-2108-X) by Johnson and Lyon. It is readily available through most bookstores.
* Monitoring. Inspect the trees and shrubs on the grounds you maintain on a regular basis. Early detection of common defoliators is important because most control measures work best when larvae are still small. As you travel the grounds mowing, fertilizing or picking up trash, pause for a moment to inspect each specimen on the property. If you find an infestation, consider how much damage it might cause to the tree or shrub. A small infestation may not cause much injury. It is difficult to control a caterpillar outbreak on a large shade tree, and often unnecessary if the tree is in good health. Such specimens can tolerate a considerable amount of defoliation without serious long-term harm. Monitor beneficial species as well as pests to gain a sense of how prevalent biological control agents may be.
* Threshold. A small number of insect pests feeding on a healthy tree usually poses little or no immediate threat to the tree's health and vigor. However, certain trees can tolerate considerable feeding, while others cannot. Therefore, the pest threshold is different for every tree and every insect pest. For example, fall webworms feed on crabapple leaves in large numbers in the autumn months. However, in most cases, healthy crabapples can tolerate almost complete defoliation and remain unaffected the following spring. This is because a large share of the food production has already taken place for the year by the time the feeding takes place. On the other hand, a pine tree on a Christmas-tree plantation can tolerate very few pine sawflies feeding on its needles. In a short time, the tree can become unsalable. In this case, the pest threshold is very low.
Often, clients have less tolerance for defoliators than the plants themselves. If informing the client that an infestation poses little danger to the plant doesn't dissuade them, you may need to perform control measures even though the health of the plant is not in jeopardy. After all, the value of an ornamental largely lies with its appearance. Further, some sites with high standards of appearance warrant treatment for relatively light infestations. In any case, it's better to treat while larvae are small and damage is minimal. Too often, treatment takes place when defoliation is too noticeable to be ignored. Because most trees, after a late-season defoliation, will not grow new leaves until the following spring, you've largely wasted the treatment.
* Control methods. Physical control of insects is possible in some cases, especially on low-growing trees and shrubs. Handpicking and forceful blasts of water work quite well to control small populations of tent caterpillars, bagworms and pine sawflies from ornamental plants. Physical methods of control are especially useful in "pesticide -restricted" areas, such as zoos or children's playgrounds.
Certain biopesticides have gained in popularity and effectiveness over the past decade. For example, Bacillus thuringiensis formulations cause insects pests such as caterpillars and beetles to die slowly from gastrointestinal disruption. Horticultural oils and insecticidal soaps also are effective for controlling leaf-feeding insects. Of course, many traditional pesticides also are available for defoliating insects (see tables, page 32 and facing page). Regardless of the product you use, control methods are usually more successful when you target younger stages. Knowing the life cycle of the pest should tell you when to monitor more closely, allowing you to catch and treat infestations in the early stages.
All foliar-applied pesticides are potentially limited because thorough coverage of leaf surfaces is essential for satisfactory control. Although the right equipment makes this possible in most cases, the issue of pesticide drift comes into consideration when spraying large trees.
An alternative is pesticide trunk injection. This limits drift because you place the insecticide directly into the tree trunk. Plus, because injected insecticides translocate within the plant's vascular system, they may be able to reach difficult-to-treat areas (such as high tree canopies) more effectively than foliar sprays. Many pesticide-injection systems are available (see table on facing page). Injection of these products requires drilling several holes into the tree trunk and moving the chemical into the tree by pressure or gravity, or passively (as with implants). Repeated treatments, however, can injure the tree, because you create wounds in the tree trunk each time you apply the insecticide. New microinjection techniques offer good pesticide-uptake potential, while limiting damage to the tree that results from drilling the injection holes. Regardless, continued reliance on injection as a sole method of insect control can be detrimental to trees in the long term.
John C. Fech is an extension educator, and Dr. Frederick P. Baxendale is professor of entomology, both at the University of Nebraska (Lincoln, Neb.).
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