Here is a list of some of the more common insect problems you may encounter.
You should look for them before you start considering the possibility of your 
problem being fungal, as damage and stress from insects can predispose your 
turf to a fungal attack. You turf may have lesions from a fungus and in fact the 
problem was started by an insect attack.
(please click one to see the its description and picture)

spittlebugs mole crickets cinch bugs grubs
mites army worms cutworms sod web worms