The hardest pest to get rid of are roaches including German roaches.
Roaches and German roaches are very hard to get rid of as they rapidly reproduce and are high adaptive and are also resilient to many chemicals and the roaches can also live almost anywhere.
Roaches also multiply rapidly, especially in kitchens and bathrooms.
Although freezing temperatures will kill roaches.
Temperatures below 15 F will kill roaches as roaches are tropical, cold blooded insects that seek warm shelter.
Sustained freezing temperatures of 32 F for several hours can kill roaches, although roaches survive winter most often when they seek shelter in heated buildings and homes, RV's etc.
Temperatures under 45 F also stops the roaches from breeding.
Roaches can die within 10 hours at temperatures that are below F and within 1 hour below 14 F.
Roaches cannot regulate their own body temperature, so roaches seek warmth in pipes, walls, homes and appliances or anywhere else they can stay warm during the winter.
Adult eggs may die in the cold temperatures although roaches egg cases also called the oothecae can withstand colder temperatures and lead to a new roach infestation when it warms up.
Other pests that are hardest to get rid of are bed bugs, termites, rodents lie mice and rats, ants and fruit flies.
Fruit flies can be very difficult to eliminate if their breeding source, like moist waste, drains etc is not found and removed.
Ants, especially carpenter ants can build complex ants nests in wood and require professional identification of the ants colony.
Mice and rats are also skilled at finding entry points and evading traditional traps.
Termites also cause severe and silent structural damage to homes and they can live in large and hidden colonies and so getting rid of termites often require specialized long term treatments such as the Sentricon System.
Bed bugs are masters of concealment and hide in furniture, mattresses and behind wall outlets.
Bed bugs also reproduce very quickly and many bed bug populations are also resistant to over the counter pesticides and require professional and multi layered treatment.