How is the harvester ant harmful?

Harvester ants can sting and some sensitive individuals may be harmed. Otherwise, they are not harmful.

Besides ferocious bites and venonmous stings--which can be quite dangerous to allergic people--in the northern United States, the carpenter ant is another common intruder in and around buildings and dwellings constructed mostly of wood, e.g., cabins, barns, outhouses and corrals. They love wood and can cause damage similar to damage caused by termites. Trees are another favorite of the carpenter ant. Colonies of this structural bugger usually gnaw out the galleries and chambers of their nests in wood initially infested by beetles. Most beetle damage is done to pine trees.

On the other hand, carpenter ants may actually contribute to forest economy by hastening the breakdown and recycling of timber previously infested by other insects, usually beetles. It's simply natures way of eliminating old-growth trees, creating room for new-growth trees