Your landlord does have a duty to exterminate insects, but depending upon what is happening the landlord may be able to charge you for the cost of extermination. If you have cats that you allow into your yard, and your cats are bringing fleas into the house, that's an infestation you are causing through your tenancy even if fleas are coming into your yard as a result of feral cats (or other wild animals or birds). If you don't have pets, it would be odd for the fleas to be infesting the house simply because they're present in the yard.