Some articles on webworms:

Crambus - Damage - Management
... To be considered a serious infestation of sod webworms, 12 larvae must be found in a 0.1 square metres (1.1 sq ft) area ... cannot show 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 ... heliothidis can also infect sod webworms ...