How could the demise of this seemingly unimportant insect cause severe damage to the rain forest in which it lives?

This particular insect might be a major pollinator of the flowering plants native to the area. If this pollinator disappeared, the majority of the said flora will also stop reproducing. Since everything is connected in a food chain and web, this disappearance of producer will eradicate the other consumers. Additionally, the said insect might also be a staple food for other species which will also affect the predation cycles in the environment.

the organisms are not adapted to live in some communities or habitats.