Impact of Insects
Because they dominate all terrestrial environments that support human life, insects are usually our most important competitors for food, fiber, and other natural resources. They have a direct impact on agricultural food production by chewing the leaves of crop plants, sucking out plant juices, boring within the roots, stems or leaves, and spreading plant pathogens. They feed on natural fibers, destroy wooden building materials, ruin stored grain, and accelerate the process of decay. They also have a profound impact on the health of humans and domestic animals by causing annoyance, inflicting bites and stings, and transmitting disease.
The economic impact of insects is measured not only by the market value of products they destroy and the cost of damage they inflict but also by the money and resources expended on prevention and control of pest outbreaks. Although dollar values for these losses are nearly impossible to calculate, especially when they affect human health and welfare, economists generally agree that insects consume or destroy around 10% of gross national product in large, industrialized nations and up to 25% of gross national product in some developing countries.
These and other aspects of economic entomology will be our main emphasis in the final section of this course when we focus entirely on insect pests and the tactics used to control them. But despite the tremendous economic losses they may cause, it is not entirely fair to cast the members of Class Insecta as villains who rob us of our food and livelihoods. They are also cherished allies on whom we depend to keep the natural environment clean and productive. They have shaped human cultures and civilizations in countless ways, they supply unique natural products, they regulate the population densities of many potential pest species, they dispose of our wastes, bury the dead, and recycle organic nutrients. Indeed, we seldom stop to consider what life would be like without insects and how much we depend on them for our very survival. To paraphrase William Shakespeare, “The evil that insects do lives after them, the good is oft interred with their exoskeletons.”
Insects as Food
Silkworm pupae for human food in Thailand
Insects represent an important food source for a wide variety of other animal species. Freshwater game fish such as trout, bass, and bream feed extensively on aquatic insects like mayflies, stoneflies, or hellgrammites. Artificial “flies” used by anglers are often made to resemble a fish’s natural prey. Many toads, frogs, turtles, snakes, and lizards also consume insects as a major part of their diet. Insectivory is common among land-dwelling birds. Purple martins, barn swallows, vireos, warblers, flickers, whippoorwills, and swifts, for example, survive almost exclusively on insects. Other birds (such as egrets, quail, geese, plovers, snipes, and bluebirds) have a more varied diet, but they still derive a large percentage of their total nutrition from insects. There are even some insectivorous mammals: shrews, moles, bats, armadillos, and anteaters, for example. When other food is scarce, even foxes, racoons, skunks, and bears will turn to insects as a source of food.
Insects were undoubtedly an important source of nutrition for our early human ancestors. Even today, they are still collected and eaten by people of many cultures. In Mexico, dried grasshoppers are sold in village markets. High in protein and low in fat, they may be fried or ground into meal and mixed with flour to make tortillas. Sago grubs, the larvae of a wood-boring beetle, are considered a delicacy in Papua New Guinea. The islanders boil the larvae or roast them over an open fire. Ants, bees, termites, caterpillars, water bugs, beetle larvae, flies, crickets, katydids, cicadas, and dragonfly nymphs are among a long list of edible insects that provide nutrition for the people of Australia, Africa, South America, the Middle East, and the Far East. Indeed, Americans and other descendants of western European culture appear to be unique among peoples of the world in having such a strong cultural taboo against the use of insects as food.
As consumers, scavengers, and decomposers, insects play a vital role in the biogeochemical cycling of nutrients. Insects help aerate the soil, improve its retention of rainwater, and enhance its tilth. They turn more soil than earthworms and redistribute nutrients within the root zone as they burrow and nest in the ground. Flies and dung beetles prevent the build-up of manure from large animals and speed up its decomposition by fungi and bacteria. Without such scavengers, the gradual accumulation of waste products from large herds of cattle and other ungulates (mammals with hooves) would soon render much of the landscape unsuitable for agricultural purposes. 
But above all else, insects play an essential role as pollinators of flowering plants (angiosperms). These plants are dominant producers in many terrestrial ecosystems, but they cannot reproduce without insect intermediaries to carry pollen (the male gametophyte) from flower to flower. It is a close symbiotic relationship that has been shaped and polished by eons of co-evolution. The plants produce a wide array of colors, odors, rewards, and ruses to attract or seduce their insect accomplices. Each flower’s architecture is specially designed to insure that insect visitors do not leave without a thorough dusting of pollen — destined, perchance, for the stigma of another flower nearby. Various flower types are adapted for luring different groups of insects — not only bees and butterflies but also many species of wasps, moths, beetles, and flies. Wild ginger, for example, produces an unobtrusive brown flower that smells like a dead rat. It is pollinated by fungus gnats, flesh flies, and other saprophytic insects.
Since ancient times, honey bees (Apis mellifera) have been valued for the honey and beeswax they produce. In many cultures, these products, as well as the bees themselves, are viewed with mystical or religious significance. To ancient Greeks and Romans, honey was the “nectar of the gods” — endowed with healing properties and super-natural powers. People of Asia used it as a preservative for fruits and cakes, Egyptians employed it as an embalming fluid, and doctors in Europe and colonial America applied it as an antiseptic to treat burns and lacerations. In Europe, honey was often mixed with wine or beer and could be fermented to produce a popular alcoholic beverage known as mead. For many thousands of years, honey was the only sweetener most people ever used. The crystalline form of sugar sold today was not readily available until the early 1800’s when the first commercial refineries were built to extract sucrose from cane or beets.
A silkworm, Bombyx mori, is the source of a unique natural fiber used to make silk cloth. This species now exists only in captivity where it is reared to maturity on a diet of mulberry leaves. As each larva spins its cocoon, it produces a continuous fiber of silk that is about 0.075 mm (3/1000 inch) thick and 900 to 1500 m (3000-5000 ft) in length. This “domestic” silk is highly valued for its light color and lusterous finish. Silk is also harvested from the cocoons of several other moth species (mostly Antherea spp.) that live in the forests of India, China, and Japan. These “wild” silks (Eri, Yamamai, Muga, and Tussah are the most common types) tend to be heavier, darker, and less highly valued than that of Bombyx mori.
