Question

What will happen if insects become extinct ?

Answer 1

Most of the flowers get pollinated through insects like bees and butterflies.
If these extinct, plants pollination will be disturbed.
it leads to extinction of plant spieces.
the results effect is on other organisms and biodiversity will be imbalanced

Answer 2

there is a lot that could go wrong 
firstly there'll be an imbalance in the eco-system. and due to this imbalance there'll be a major change in the food web process.all the insect eating animals and plants have to change their food source or else this extinction will result in their extinction too.

secondly, there are many plants across the globe which are insect pollinating.so if these insects vanish, there'll be no one to help them in the
process of pollination.if there's no  pollination , there wont be fertilization and so there wont be any new fruit or plants occurring from that process