Some articles on webworms:

Crambus - Damage - Management
... considered a serious infestation of sod webworms, 12 larvae must be found in a 0.1 square metres (1.1 sq ft) area ... any damage or serious infestations, predators of the sod webworms must be attracted, including birds and insect predators, such as ground beetles, robber flies, and predatory wasps ... carpocapsae and Heterorhabditis heliothidis can also infect sod webworms ...