Because insects are pollinators and at the pretty much beginning of the food chain.
Pollinator part:
Insects go around flowers and bring pollen from flower to flower so the flowers can grow into fruits/reproduce. If this doesn’t happen, a lot of plants will either go extinct or start reproducing way slower than before (why that is an issue I’ll explain in the food chain part)
Food chain part:
This is honestly a very interesting topic and you should look into it a bit, I won’tbe able to explain all the details.
So basically insects are the main food of a lot of bigger animals, like birds for example. If insects go extinct, certain birds won’t have anything to eat and will go extinct too, and then the animals and plants that either eat or benefit off of those birds will either go extict too or have issues with reproduction, this will spiral on and on until a *lot* of other species go extinct.
The same principle for plants; if they go extinct (they won’t actually go *extinct* but rather become more rare, reply if you want me to explain why plants becoming less would lead to extinction of other species rather than jut the shrinking of said species) because insects don’t pollinate them, so then e.g. cows won’t have to eat anything and will go extinct too, (this example would directly affect humans), and also things like lavines and floods will get more common because plants usually hold together the land so it’s not as easily moved
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