"It is very rapid", Sánchez-Bayo told The Guardian.
The researchers said the intensification of agriculture over the past six decades was "the root cause of the problem" and that the widespread use of pesticides was having a major impact.
Scientists believe that intensive agriculture and the heavy use of pesticides are the main drivers of the massive insect loss which may finally lead to many birds, reptiles, amphibians and fish that eat insects starving to death.
But according to the new review, the proportion of insects in decline is now twice as high as that of vertebrates and the insect extinction rate is eight times faster than that of mammals, birds and reptiles.
It reviewed 73 existing studies published around the world in the last 13 years.
Insects are also being hit by biological factors, such as pathogens and introduced species, and by climate change, where rising temperatures could affect the range of places where they can live, it says.
Matt Shardlow, chief executive of wildlife charity Buglife, added: "It is gravely sobering to see this collation of evidence that demonstrates the pitiful state of the world's insect populations".
They do everything from providing food for small animals, pollinating 75 percent of the world's crops and replenishing soil, to limiting pest numbers.
"It is becoming increasingly obvious our planet's ecology is breaking and there is a need for an intense and global effort to halt and reverse these awful trends - allowing the slow eradication of insect life to continue is not a rational option".
What we need to do is to change the way we make food, or we can say our goodbyes to the insects.
While some of our most important insect species are in retreat, the review also finds that a small number of species are likely to be able to adapt to changing conditions and do well.
But where many die, others are expected to thrive, as "plagues of pests" may arise out of the loss of butterflies, bees, and dung beetles. Sands said an immediate danger of the insect decline was the loss of insectivorous birds, and the risk of larger birds turning from eating insects to eating each other.
Meanwhile, the planet is said to be undergoing its sixth mass extinction due to the "biological annihilation" of wildlife in recent decades, while the insect population collapses that have already been reported in Germany and Puerto Rico are now thought to be related to a crisis that's global.
"So give it a million years and I've no doubt there will be a whole diversity of new creatures that will have popped up to replace the ones wiped out in the 20th and 21st centuries".