ANTS!
Most ant species are highly developed social insects that live in permanent nests, which depending on the species, may be in the soil, in timber, under pavers, in wall cavities or roof voids.
Ants may travel large distances in search of food. Even the cleanest of homes can provide a ready food source for ants which once found can invade in large numbers, such that professional help is required.
Some inflict severe bites but most are a nuisance pest when they infest kitchens and BBQ areas in large numbers.
Some do-it-yourself products may provide short term relief, but may in fact, make a problem worse. Ants are easily repelled by some common residual remedies and, without finding the colony, their repeated use can often cause a colony to move or even split into multiple colonies. And simply killing foraging ants will not eliminate the colony since the queen remains in the colony and her sole purpose is to lay more eggs to ensure the survivability of the colony.
Ants are now the number one pest problem for which homeowners rely on professional advice and remediation.