Katydids are not not known to be harmful to humans or other pets. They can damage young plants but generally won't cause serious damage to your garden. Certain types of katydids, mainly in the tropics, eat smaller insects and can help deter other creatures from invading your garden.
Katydidae mainly eat leaves and grass. Along with crickets and grasshoppers, they can be attracted to plants in your garden or to tall grasses on your property. Katydids are nocturnal and are also attracted to bright lights at night.
A type of katydid widely distributed in North America, the broad-winged katydid, likes eat the leaves of citrus trees and can be a pest for people with orchards.