Because most of us don't eat insects, we tend to think they aren't that important except in negative ways, but in fact it could be argued that without insects we wouldn't even be here. Insects are of major importance in terrestrial ecosystems both as food sources for higher animals and as pollinators for plants. Insects and flowering plants evolved together and are dependent on one another. Without insects we would still be living in a world of ferns and conifers without the vibrant colours we associate with plant life today. But then without insects, it is possible that the higher primates would not have evolved, as the evidence is that our ancestors were insectivores.
Numerous bird species depend on insects for food as well, so without insects there would be far fewer birds to brighten our lives. No flowers, no birds. Yes insects are important. But they affect our lives in many other ways as well. They can certainly be annoying: mosquitoes biting us when we are trying to enjoy a summer's evening on the verandah, moths and silverfish attacking our clothes, fleas making our pets itch, our children brlnging head lice home from school. Of course it goes way beyond this. It is estimated that up to half of the world's food crops are lost each year to insect predations. They eat the crops in the field and they attack them in the storage sheds. Much of the money spent on pesticides goes to insect control and of course, the side effects of pesticides have been huge, having negative effects on ecosystems as well as human health. We almost lost peregrine falcons because their egg shells were damaged by DDT. My brother lives in an agricultural region and he has noticed the loss of butterfly species because of the heavy insecticide use on the crops. Overuse and misuses of pesticides have also been implicated in human health problems.
Insects don't usually cause disease but they are very good carriers and vectors. One of the major killers in the world today is Malaria, caused by a protozoan but spread by mosquitoes. The disastrous use of DDT began as an attempt to eliminate mosquitoes and reduce deaths by malaria. Other diseases are also spread by insects. Sleeping sickness in Africa, Ross River fever in Australia, even the Black Plague: spread by insect vectors. The Black Plague brings up another aspect of insect importance: the ways in which they have influenced human history. The fleas on the rats that spread the Plague throughout Europe in the Middle Ages changed history. It has been suggested that more wars have been lost because of disease decimating troops than have been lost in actual battle (see the book "Rats, Lice and HIstory" by Hans Zinsser).
The question "Are insects important" is very anthropocentric. Insects are mainly important to themselves. They are born with a life force within, that drives them to do everything they can to survive and reproduce. They are, as a group, more successful than the vertebrates, if numbers and diversity count as success. For size, we win, but we have been in a battle for survival with insects for millions of years. Our distant ancestors used them as important food sources before we took up the tools and weapons that allowed us to focus on bigger prey. Australian Aboriginals were eating Witchetty grubs when the Europeans arrived and some still enjoy their nutty flavour and protein-rich food value. Many agricultural societies have used insect food resources in times of need. When the locusts come through and devour the year's crops, its a good survival strategy to eat the locusts. I saw one estimate years ago, that the beetle larvae infesting bags of rice were actually a significant protein source in the diets of many Indians who were otherwise vegetarians.
Our ancestors became strong enough to protect themselves and their loved ones from the major mammalian predators of the times: tigers, lions, bears, wolves, but they suffered from insect born diseases and insect pests that could not be controlled until modern times. Early humans were infested with head lice, pubic lice and fleas. Was our loss of hair cover sparked by two things? Wearing animal skins and huddling by fires at night eliminated the need for thick fur coats of our own. Did lessening body hair help us lower our ectoparasite load as well? Fewer lice and fleas? Was that how head lice and pubic lice diverged into two separate species? It is all just interesting conjecture at this point.
Whatever the answers there, the fact is, we cannot ignore the insect world, so it must be important. If we wipe them out with our pesticides, the conservationists from Rachel Carson (Silent Spring) on have shown us that we will wipe ourselves out too. Without bees and other pollinators, the silence of the springtime will be followed by the dearth of food crops in the autumn. We are living in a great web of life made up of millions and millions of species, millions of which are insects. If we were to wipe them out, we would wipe ourselves out too. Se we have to learn to live with them and that makes them important.
Not that we aren't also important to the insects. We already kill them by the millions each year, usually for pretty good reasons such as human health and welfare. But we are also killing off whole species on a daily basis, many of which we haven't even named yet, for no better reason than human greed. Each day, each hour, thousands of acres of rainforests and other ecosystems are being strip-mined of their trees and in the process there is a hideous 'bycatch' of all the other species that live there: beautiful beetles and butterflies, gorgeous flowers, lovely little mammals and birds: gone. And for some: gone forever. Extinction. No matter how many human lives have been lost to insects, it is nothing compared to the species lost to human greed. It's something to think about.