Insect biodiversity accounts for a large proportion of all biodiversity on the planet—over half of the estimated 1.5 million organism species described are classified as insects.^[1][2]

Estimates of the total number of insect species or those within specific orders are often highly variable. Globally, averages of these predictions estimate there are around 1.5 million beetle species and 5.5 million insect species, with around 1 million insect species currently found and described.^[3] Between 950,000–1,000,000 of all described animal species are considered insects, so over 50% of all described eukaryotes (1.8 million species) are insects (see illustration). With only 950,000 known non-insects, if the actual total number of insects is 5.5 million, they may represent over 80% of the total, and with only about 20,000 new species of all organisms being described each year, most insect species likely will remain undescribed, unless species descriptions greatly increase in rate.

Of the 24 identified orders of insects, five dominate in terms of numbers of described species, namely Coleoptera (beetles), Lepidoptera (butterflies and moths), Diptera (flies and mosquitoes), Hymenoptera (ants, bees, wasps and sawflies) and Hemiptera (true bugs, e.g. cicadas, aphids, leafhoppers, bed bugs and assassin bugs). At least 900,000 described species — about 90% of all known insects — belong to the five aforementioned orders, each of which has over 100,000 species, while the next (sixth) most diverse order, Orthoptera (locusts, grasshoppers and crickets), has just under 24,000 species.

The fossil record concerning insects stretches back for hundreds of millions of years. It suggests there are ongoing background levels of both new species appearing and extinctions. Very occasionally, the record also appears to show mass extinctions of insects. The Permian–Triassic extinction event saw the greatest level of insect extinction, with the Cretaceous–Paleogene being the second highest. Insect diversity has recovered after past mass extinctions, due to periods where new species originate with increased frequency, though the recovery can take millions of years.^[4]

While biodiversity loss is a global problem, conserving habitat for species of insects is uncommon and generally of low priority, although there are exceptions. More commonly insect conservation occurs indirectly, either through the setting aside of large portions of land using "wilderness preservation" as the motive, or through protection of "charismatic vertebrates". Some studies estimate that global insect populations are in rapid decline, perhaps by as much as 80% in recent decades. The windshield phenomenon describes people noticing vastly fewer insects flying into the path of their cars after long drives, and this may reflect worldwide loss of insect abundance.

Single-species insect conservation can preserve other species indirectly; this preservation-by-default is referred to as the umbrella effect. Showy insects such as butterflies or large, colourful beetles serve as flagship species, and can expand public awareness and financial contributions for conservation efforts. Wealthy nations such as the United States do list species of concern, and occasionally insects are placed on its Endangered Species List. In 2017 this list had classified over 80 insects as endangered species, the majority of them beetles or butterflies; a significant percentage of these listed insects are native only to the Hawaiian Islands.^[19] Migratory species, such as the well-known monarch butterfly (Danaus plexippus), are in need of special conservation methods. One species may require several habitat locations, even across international boundaries, for the different periods of their migratory patterns.

Insect conservation has been labelled in the past as a concern only for the affluent. The developing country of Papua New Guinea has a "happily ever after" ending in their attempts to preserve the world's largest butterfly, Queen Alexandra's birdwing (Ornithoptera alexandrae). This species is restricted to a very small range of habitat due to specificity in their diet. In the international market of insect collecting, the butterfly can retrieve up to US$2000. In 1978, the government of Papua New Guinea set up the Insect Farming and Trading Agency (IFTA) to regulate the exploitation and conservation of Queen Alexandra's birdwing and other valuable butterflies.

In agricultural ecosystems, biodiversity is important for the production of food and for ecological services such as the recycling of nutrients, regulation of microclimate and local hydrological processes, and biological control of pests.^{[citation needed]}

In the United States alone, pollination by bees accounts for over US$9 billion of economic revenue.^[20] According to some estimates, over ⅓ of the human diet can be traced directly or indirectly to bee pollination.^[21] Losses of key pollinators have been reported in at least one region or country on every continent except Antarctica, which has no pollinators. The Millennium Ecosystem Assessment ^[22] concluded that with the global decline in the amount of pollinators, there is not a complete loss of fruit or seeds, but a significant decrease in quantity and viability in fruits, and a lower number of seeds.

• Insect ecology
• Defaunation