harmful insects and how they become harmful?

Answer 1

Examples of harmful insects:

-insects that feed on the same food that we do (i.e. agricultural pests)
-those that physically harm us, via bites, stings, or infestation within our bodies
-insects that find harborage within our homes (cockroaches, stored product pests, etc.)
-those that can transmit diseases as either a mechanical or biological vector
-insects that destroy the structure of our homes (termites, carpenter ants)

To tell the truth, the concept of whether an insect is harmful or not is purely a human one. A weed is just a plant, until it finds its way into our garden and suffocates the flowers we find pretty. Likewise, a boll weevil was just an insect that had adapted to lay its eggs in cotton--no big deal, until we decided to cultivate the cotton. Then it was in competition with us, thus, a "pest."

So, in short, an insect becomes harmful when humans decide that it is harmful.

Answer 2

The harmful ones are the ones that like to eat what we eat. An example is the mediterranean fruit fly that grows its baby worms in apples or the plant lice that suck the leaves of tomatoes or the flour weevils that lay eggs in flour. They are just looking for a meal but the meal is something people value.

Answer 3

for humains, animals or vegetables... entomology is the most complicated science. There are billions of species... hmmm