Most cosmopolitan polyphagous insect is

The most cosmopolitan polyphagous insect is Locust.
Locust, is any of a group of insects that are distributed worldwide, the common name of which generally refers to the group of short-horned grasshoppers that often increase greatly in numbers and migrate long distances in destructive swarms.
Polyphagous pests are primarily agricultural pests that feed on economically important agricultural and horticultural crops of wide taxonomic diversity across the globe.
A highly polyphagous insect, desert locust eats flowers, fruits, seeds, leaves, stems, shoots as well as bark of a large number of species of plants including food crops, cash crops, trees, etc.