Injury to Living Plants
Of all the insect species now living on earth, at least half of them (400,000 - 500,000) feed directly on the tissues of living plants. It is probably no exaggeration to claim that every plant species on earth serves as food for at least one species of insect. No parts of a plant are immune from attack: insect herbivores chew the leaves, suck the sap, mine the cambium, gather the pollen, invade the buds, destroy the flowers, and devour the fruit. Even plants that manufacture potent insecticides (such as the nicotine in tobacco, pyrethrum in chrysanthemums, or rotenone in tropical legumes) have insect pests that are especially adapted to feed on their tissues and detoxify their chemical defenses.
Herbivores with chewing mouthparts consume a plant directly. They use mandibles to masticate (knead), triturate (grind), or abrade (scrape) plant tissue into manageable bites. Most species consume entire roots, stems, leaves, buds, flowers, fruit, and/or seeds. All members of the orders Orthoptera and Phasmida feed in this manner, as do nearly all larvae of Lepidoptera. Chewing herbivores are also found in the order Diptera (e.g. larvae of Tephritid fruit flies), in the Coleoptera (e.g. larvae and adults of weevils, scarab beetles, leaf beetles, and many others), and in the Hymenoptera (ants, some bees, and the larvae of all sawflies). Immatures of aquatic orders, such as Trichoptera, Plecoptera, and Ephemeroptera, also feed by chewing plant tissues but they are seldom regarded as major pests.
Not all chewing herbivores consume entire portions of their host plant. Leaf miners (represented by several families in the Diptera and Lepidoptera) are specialized herbivores that excavate galleries in mesophyll, the inner layer of cells between a leaf's upper and lower epidermis. Other species concentrate on epidermal cells. The pear slug (an immature sawfly) skeletonizes the leaves of its host plant by abrading only cells from the lower epidermal surface. Many bark beetles feed only in cambium, the layer of cells just beneath the bark of a tree or shrub.
Plant tissue is also damaged by herbivores with piercing-sucking mouthparts. Some species (e.g., stink bugs) stab their feeding stylets into individual plant cells, inject digestive enzymes to liquify the substrate, and then suck out the partially digested food. Other species (e.g., aphids) thread their stylets delicately through intercellular spaces, tap into the plant's vascular system, and feed by withdrawing sap from the phloem. Most pest species with piercing-sucking mouthparts belong to the orders Hemiptera (Heteroptera and Homoptera) and Thysanoptera. Damage from these insects is often less visible than that of chewing insects, but it may still result in severe injury to the plant.
Gall-making insects are herbivores that live within the tissues of a plant. They secrete a hormone-like substance that stimulates growth in surrounding plant tissue and induces the host plant to produce a specialized shelter (gall) that nourishes and protects the herbivore inside. Gall-making insects (sometimes called cecidozoa) include species in the orders Hemiptera, Hymenoptera, Lepidoptera, and Diptera. Gall formation is also induced by certain species of mites, fungi, and bacteria. Gall makers are highly specialized herbivores; each species is narrowly adapted to its host plant and produces a distinctively shaped gall.
Herbivores may also injure their host plants during oviposition; females often cut holes or slits in plant tissues as they deposit their eggs. Oviposition scars from cicadas, crickets, and katydids may be severe enough to kill twigs or stems. This problem is most troublesome in nurseries and greenhouses where ornamental plants with obvious injury are less marketable.
Many insects that feed on plants also serve as vectors of plant diseases. All major taxa of plant pathogens are spread by insects, including viruses, mycoplasmas, bacteria, protozoa, fungi, and nematodes.
Plant pathogens may be carried externally on a vector's feet, mouthparts, or ovipositors. This mechanical transmission has been well-documented in vectors representing at least eight arthropod orders [Hemiptera (both suborders), Thysanoptera, Orthoptera, Diptera, Coleoptera, Hymenoptera, Lepidoptera, plus Acarina (mites)].
Some pathogens even provide symbiotic advantages for their insect vectors. Bark beetles, for example, infect trees with a pathogenic fungus whenever they establish new colonies. As the fungus becomes established in each new tree, it changes the micro-environment of the wood and improves reproductive success for the bark beetles. A blue-stain fungus carried by Ips spp. bark beetles and Dutch elm disease spread by Scolytus spp. bark beetles are examples of this type of symbiotic relationship between pathogen and vector.
Insect Vectors of Plant Pathogens
Plant pathogens may also be carried internally in the salivary glands, digestive tract, or reproductive system of insect vectors (biological transmission). These pathogens are usually inoculated into new hosts during feeding or oviposition, but they may also be spread through the insect's feces. Several families of vectors show strong affinity for biological transmission of pathogens in specific taxonomic groups. Thrips, aphids, and whiteflies, for example, tend to be vectors of virus pathogens, whereas leafhoppers are more likely to transmit mycoplasma-like organisms (MLOs).
Damage to Stored Products and Structural Materials
Virtually any product or commodity derived or manufactured from plant or animal matter may be damaged by insects. Silverfish, ants, cockroaches, and psocids (e.g., booklice) will feed on starch sizing, wallpaper paste, bookbindings, and old photographs. Ants, beetles, and moths will destroy animal products such as furs, leather, shoes, stuffed museum specimens, and even pinned insects! Some beetles will chew through inedible packaging just to reach the food inside. One species, known as the "short-circuit" beetle, became notorious for chewing through lead sheathing on telegraph cables in order to eat the fiber insulator around the copper wires. Drugstore beetles, Stegobium paniceum, have been reported feeding on at least 45 different types of drugs in addition to nuts, ginger, chocolate, cork, and pepper.
Stored agricultural products such as seeds and grains, cheese, cured meats, dried fruits, grain products (e.g. flour and corn meal), hay and other forages, and even cured tobacco serve as food for a variety of beetles, moths, cockroaches, ants, and silverfish. These insects inhabit grain bins, silos, elevators, warehouses, grocery stores, and home pantries wherever food is stored. In addition to damage caused by feeding, these pests also accelerate spoilage of agricultural commodities by introducing bacteria, fungi, and fecal matter.
The U. S. Department of Agriculture estimates that losses incurred from pests of stored food total nearly $2 billion a year.
Textiles and natural fibers, including wool, cotton, silk, hemp, kapok, raffia, jute, flax, and sisal serve as food for carpet beetles and clothes moths. These insects destroy upholstery, carpeting, draperies, and clothing by consuming the natural fibers. Before the invention of nylon, clothes moths were a major cause of damage to hemp and sisal rope used in shipyards.
Lumber, wood products and wooden structures are subject to attack by a variety of insects including termites, wood-boring beetles, carpenter ants, and carpenter bees. At least 10 species of termites are found in the United States, mostly in southern or coastal regions. The eastern subterranean termite, Reticulitermes flavipes, is the most destructive species. It is distributed throughout the South and Southeast from Texas to Pennsylvania where it causes more than $800 million in damage each year. The National Pest Control Association estimates that nearly 700,000 infested structures are treated for termites every year.
Wood-boring beetles (including members of the families Anobiidae, Bostrichidae, Buprestidae, Cerambycidae, Curculionidae, Lyctidae, and Scolytidae) are responsible for extensive damage to lumber both before and after it is harvested. The lumber industry estimates that about 10 percent of its annual saw timber production sustains enough insect injury to affect its grade. Loss of value may be only about 10 cents per board foot, but with annual timber production at nearly 45 billion board feet, the total loss is close to $450 million each year.
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