SIGNIFICANCE OF INSECTS

 

Commercial Significance

Certain insects provide sources of commercially important products such as honey, silk, wax, dyes, or pigments, all of which can be of direct benefit to humans. Because they feed on many types of organic matter, insects can cause considerable agricultural damage. Insect pests devour crops of food or timber, either in the field or in storage, and convey infective microorganisms to crops, farm animals, and humans. The technology for combating such pests constitute the applied sciences of agricultural and forest entomology, stored product entomology, medical and veterinary entomology, and urban entomology.

 

Insects as a Source of Raw Materials

For primitive peoples who gathered food, insects were a significant food source. Grasshopper plagues, termite swarms, large palm weevil grubs, and other insects are still sources of protein in some countries.

The dry scaly excreta of coccids (Homoptera) on tamarisk or larc trees is the source of manna in the Sinai Desert. Coccids were once the source of the crimson dye kermes.

The cohineal, or carmine, from Dactylopius scale insects found on Mexican cacti, was used for dying cloth by the Aztees and is used today as a dye in foods, cosmitics,  drugs, and textiles. Several insect waxes are used commercially, especially beewax and lac wax. The resinous product of the lac insect Kerria lacca (Homoptera), which is cultured for this purpose, is the source of commercial shellac.

 

Insect Damage to Commercial Products

When insects that break down dead trees invade structural timbers in buildings, they become pests. This is true of insects such as demistides beetles and various tineid moths that ecologically are latecomers to carcasses and are capable of breaking down the keratin in hair and feathers. When these insects invade skins, furs, and wool garments or carpets, they can become problems for humans.

 

Agricultural Significances

Ecological factors

Many insects are plant feeders, and, when the plants are of agricultural importance, humans are often forced to compete with these insects. Populations of insects are limited by such factors as unfavourable weather, predators and parasites, and viral, bacterial, and fungal diseases, as well as many other factors that operate to make insect populations stable. Agricultural methods that encourage the planting of ever larger areas to single crops, which provides virtually unlimited food resources, has removed some of these regulating factors and allowed the rate of population growth of insects that attack those crops to increase. This increases the probability of great infestations of certain insect pests. Many natural forests, which form similar giant monocultures, always seem to have been subject to periodic outbreaks of destructive insects.

In some agricultural monocultures, nonnative insect pests have been accidentally introduced along with a crop but without also bringing along its full range of natural enemies. This has occurred in the United States with the oystershell (Lepidosaphes ulmi) of apple and other fruit trees, the cottony cushion scale (Icerya purchasi) of citrus, the European corn borer (Pyrausta nubilalis; also called Ostrinia nubilalis), and others. The Colorado potato beetle (Leptinotarsa decemlineata), which caused appalling destruction to the cultivated potato in the United States beginning about 1840, was a native insect of semi-desert country. The beetle, which fed on the buffalo burr plant, adapted itself to a newly introduced and abundant diet of potatoes and thus escaped from all previous controlling factors.

Similar situations often have been controlled by determining the major predators or parasites of an alien insect pest in its country of origin and introducing them as control agents. A classic example is the cottony-cushion scale, which threatened the California citrus industry in 1886.

A predatory lady bird beetle, the Vedalia beetle (Rodolia cardinalis), was introduced from Australia, and within a year or two the scale insects had virtually disappeared.  The success was repeated in every country where the scale insect had become established without its predators. In eastern Canada in the early 1940s the European spruce sawfly (Gilpinia hercyniae), which had caused immense damage, was completely controlled by the spontaneous appearance of a viral disease, perhaps unknowingly introduced from Europe. This event led to increased interest in using insect diseases as potential means of managing pest populations.

 

Insect Damage to Growing Crops

Insects are responsible for two major kinds of damage to growing crops. First is direct injury done to the plant by the feeding insect, which eats leaves or burrows in stems, fruits, or roots. There are hundreds of pest species of this type, both in larvae and adults, among orthopterans,

homopterans, heteropterans, coleopterans, lepidopterans, and dipterans. The second type is indirect damage in which the insect itself does little or no harm but transmits a bacterial, viral, or fungal infection into a crop. Examples include the viral diseases of sugar beets and potatoes, carried from plant to plant by aphids. Although most insects grow and multiply in the crop they damage, certain grasshoppers are well-known exceptions. They can exist in a relatively harmless solitary phase for a number of years, during which time their numbers may increase. They then enter a gregarious phase, forming gigantic migratory swarms, which are transported by winds or flight for hundreds or thousands of miles.  These swarms may completely destroy crops in an invaded region. The desert locust (Schistocerca gregaria) and migratory locust (Locusta migratoria) are two examples of this type of life cycle.

Medical Significance

Insect damage to humans and livestock's also may be direct or indirect. Direct human injury by  insect stings and bites is of relatively minor importance, although swarms of biting flies and mosquitoes often make life almost intolerable, as do biting midges (Sand Flies) and salt-marsh mosquitoes. Persistent irritation by biting flies can cause deterioration in the health of cattle's. Some blowflies, in addition to depositing their eggs in carcasses, also invade the tissue of living animals including humans, a condition known as myiasis. An example of an insect that causes this condition is the screwworm fly (Cochliomyia) of the southern United States and central America. In many parts of the world, various blowflies infest the fleece and skin of sheep. This infestation, called sheep-strike, causes severe economic damage. Many major human diseases are produced by microorganisms conveyed by insects, which serve as victors of pathogens. 

Malaria is caused by the protozoan Plasmodium, which spends part of its developmental cycle in Anopheles mosquitoes. Epidemic relapsing fever, caused by spirochetes, is transmitted by the louse  Pediculus.  Leismaniasis, caused by the protozoan Leishmania, is carried by the Sand fly Phlebotomus. Sleeping sickness in humans and a group of cattle diseases that are widespread in Africa and known as nagana are caused by protozoan trypanosomes transmitted by the bites of tsetseflies (Glossina).

Under non-sanitary conditions the common houseflies Musca can play an incidental role in the spread of human intestinal infections (e.g., typhoid, bacillary and amebic dysentary) by contamination of food. The tularemia bacillus can be spread by deerfly bites, the bubonic plague bacillus by fleas, and the epidemic typus rickettsia by the louse Pediculus. Various mosquitoes spread viral diseases (e.g., several encephalitis diseases; dengue and yellow fever in humans and other animals). The relationships among the various organisms are complex.  Malaria, for example, has a different epidemiology in almost every country in which it occurs, with different Anopheles species responsible for its spread. These same complexities affect the spread of sleeping sickness. Some relationships are indirect. Plague, a disease of rodents  transmitted  by flea bites, is dangerous to humans only when heavy mortality among domestic ratis forces their infected fleas to attack people, thereby causing an outbreak of plague. Typhus, tularemia, encephalitis, and yellow fever also are maintained in animals reservoirs and spread occasionally to humans.

The relationships among the various organisms are complex. Malaria, for example, has a different epidemiology in almost every country in which it occurs, with different Anopheles species responsible for its spread. These same complexities affect the spread of sleeping sickness. Some relationships are indirect. Plague, a disease of rodents transmitted by flea bites, is dangerous to humans only when heavy mortality among domestic rats forces their infected fleas to attack people, thereby causing an outbreak of plague. Typhus, tularemia, encephalitis, and yellow fever also are maintained in animal reservoirs and spread occasionally to humans.

Control of Insect Damage

The historical objective of the entomologist was primarily to develop and introduce modifications into the environment in such ways that diseases will not be spread by insects and crops will not be damaged by them. This objective has been achieved in numerous cases. For example, in many cities flies no longer play a major role in spreading intestinal infections, and land drainage, improved housing, and insecticides use have eliminated malaria in many parts of the world.

Massive outbreaks of the Colorado potato beetle in the 1860s led to the first large-scale use of insecticides in agriculture. These highly poisonous chemicals (e.g., Paris green, lead arsenate, concentrated nicotine) were used in large quantities. The continued search for effective  syenthetic compounds led in the early 1940s to the production of DDT, a remarkable compounds that is highly toxic to most insects, nontoxic to humans in small quantities (although cumulative effects may be severe), and long-lasting in effect. Widely used in agriculture for many years, DDT was not the perfect insecticide. It often killed parasites as effectively as the pests themselves, creating ecological imbalances that permitted new pests to develop large populations.

Furthermore, resistant strains of pests appeared. The environmental longevity of many early insecticides was also found to cause significant ecological problems. Similar difficulties were encountered with many successors to DDT, such as Dieldrin and Endrinn the course of developing effective insecticides, the primary emphases have been to reduce their potential to cause human health problems and their impact on the environment. Biological methods of pest management have become increasingly important as the use of undesirable insecticides decreases.

Biological methods include introducing pest strains that carry lethal genes, flooding an area with sterile males (as was successfully done for the control of the screwworm fly), or developing new kinds of insecticide based on modifications of insects’ growth hormones. The sugar industry in Hawaii and the California citrus industry rely on biological control methods. Although these methods are not consistently effective, they are considered to be less harmful to the environment than are some chemicals.

Reference-https://www.britannica.com/animal/insect/Natural-history