One particular pest, the Chafer Grub has become considerably more destructive to lawns in recent years, and due to the tightening up of legislation regarding the use of herbicides there is currently no chemical treatment available to treat it.
Chafer grubs are the larvae of the Chafer beetle,and depending on the variety they can live in the lawn for up to 2 years growing to 3/4 inch in length.
The grubs feed on grass roots in your lawn, slowly weakening, and eventually killing it off. As the lawn starts to yellow it is a sign to insect eating birds such as magpies and woodpeckers that there is a good feed present, and if the population of grubs is high, the birds can quickly turn the lawn into a ploughed field in their search.
The only treatment available currently is a biological one that involves the introduction of a pathogenic nematode into the soil.
See the RHS Advice Website for more details