The harmful effects of insects vary based on the type of insect and its location. However, a few general categories of harmful effects of insects are pervasive and occur with a variety of insects. These effects include things insects do to people, animals and plants.
Kill Plant Growth
Several types of insects have harmful effects on the growth and development of new and budding plants. They puncture the soft tissues and suck out the juices. The website Roses-Roses.com points out that a small bug called the macrosiphum rosae, or more commonly plant lice aphids, spends much of the spring damaging new plant growth by sucking on plant juice and destroying soft tissue, which harms or kills the plants.
Bites -- Irritations
Insect bites generally have a couple harmful effects -- irritation and illness. The less severe effect is the simple irritation, swelling and pain that sometimes come from bites by certain insects such as mosquitoes, bees and some spiders. These bites can become irritated, and the locations of the bites may become red and swollen in some instances. Over-the-counter medicines are sometimes used to provide temporary relief.
Bites -- Illness
The more serious side of insect bites is the illness that can result in extreme cases. Poisonous spiders such as the black widow and brown recluse can cause serious illness, skin defects and even death in rare instances. Certain ticks are notorious carriers of the very serious lyme disease or Rocky Mountain spotted fever, which can cause serious health issues for people.
An indirect harmful effect caused by insects is the use of pesticides and other chemical controls to prevent them from damaging crops and gardens. Many fields are sprayed with pesticides on a regular basis. These pesticides can harm small animals and even people who are regularly exposed to the chemicals in them. Food growers are sometimes criticized for heavy use of pesticides as well. Environmental groups monitor use of pesticides and try to influence alternatives to controlling harmful effects insects have on fields and plant growth.