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
