Bad insects

There are many good insects, but there are also plenty of insects that are bad or harmful to people. On this page we will learn about bad insects and the different ways that insects can be harmful to people.

Insects in your house

We don’t like it when insects invade our house. Whenever we find an uninvited insect in our house, we will usually call it “bad” and try to kill it or chase it away.

Insects that are often bothersome in houses of people include mosquitoes, ants, termites, flies, cockroaches, and tiny moths that can destroy our clothes. Mosquitoes are a nuisance because they bite and keep us awake. Ants, flies and cockroaches can spoil our food. Termites can destroy wooden furniture and books.


The picture shows a biting mosquito. They like to bite near your ankles because they like the smell of sweaty feet.

Insect pests in agriculture

While several insects are useful for the farmer, many insects are classified as pests. Agricultural pests are insects that harm the crop or do damage to agricultural products.

There are many types of agricultural pests. Some insects feed directly on the plants, for example caterpillars eat leaves or damage fruits, and aphids suck juices from the plant with their beak-like mouthparts. Other insects do damage because they can transmit plant diseases, for example whiteflies and aphids can transmit virus diseases from one plant to another.

Also the harvested crop can still be attacked by insects. All kind of storage insects such as the rice weevil and the rice moth can cause big damage to stored rice and other grains. Every year millions of dollars are lost because of insect damage in agriculture.

Bollworm damage on tomato Scale insects

Caterpillar Aphids

The pictures show various types of insect damage. Holes in a tomato are caused by the bollworm (a caterpillar). Papaya fruits are completely infested by scale insects. A caterpillar eats leaves of cauliflower. Beans are infested by hundreds of aphids. In all cases there is serious damage to the crop and economic losses for the farmer.

Parasitic insects

Several types of insects are bad because they are parasitic. Some of these parasitic insects live on people, but others can be parasites of pet animals or farm animals.

Well-known parasitic insects include lice, bed bugs and fleas.

Biting and stinging insects

We don’t like biting and stinging insects. There are actually two reasons why an insect would bite or sting. One reason is to feed. For example a mosquito will bite us to suck our blood. Other insects sting to defend themselves. For example bees and wasps will sting when they are disturbed. They have a sting which is actually an ovipositor that has been modified into a venomous sting.

The best known insects that bite or string are mosquitoes, bees and wasps. Also some bugs can give a nasty bit with their needle-shaped mouthparts.

Wasp Bee on flower

The pictures show a wasp and a bee. Everybody knows they can give nasty stings. While a bee is “good” when it produces honey and wax for us, we call it “bad” when it gives us a painful sting.

Insects as vectors of diseases

Some insects are bad because they are vectors of diseases. There are several ways to transmit diseases. For example flies and cockroaches can pick up bacteria when they walk in dirty places. When they then walk on our food it becomes contaminated and can make us sick.

Other insects transmit diseases in a different way. For example a malaria mosquito will suck blood from a person who has the disease. If the same mosquito then bites a healthy person, the disease will be passed on and this person will also get sick.

Not only people, but also animals can suffer from diseases that are transmitted by insects. For example in Africa, many cattle get a disease that is transmitted by the Tsetse fly, which is a blood-sucking fly.

Mosquito Fly

The pictures show a biting mosquito and a fly on a beer glass. A mosquito bite is not only annoying but it can also be dangerous if the insect is transmitting a serious disease. Flies can transmit diseases by picking up germs and then walking on you food.

Are all insects bad?

On this page we have learned a lot about why insects can be bad for people. But of course not all insects are bad; some insects are good and very useful. Click here if you want to learn more about the various ways how insects can be good for people.

This page was designed for school children and school teachers. Any comments are welcome.

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