By and large we consider earthworms to be our friends. We can find many different species of earthworm, such as night crawlers, red earthworms, or the Alabama jumper worm, and they all consume decomposing organic material and tunnel through the soil. These feeding habits lead to nutrients being returned to soil and their tunnels can increase water flow and improve soil tilth.
As we all know though, too much of a good thing can be bad and the same is true of earthworms. When populations get high enough earthworms can create extensive damage in our lawns or other turf areas. This damage is created when individual earthworms make their way to the surface of the turf or soil and leave behind a mound of soil and excrement known as an earthworm cast. As these casts accumulate, they can dull mower blades and smother half dollar sized patches of turf. Damage of this nature is of particular note on golf courses where earthworm casts will interfere with putting. Earthworms may also attract predators such as birds who can damage turf when pecking for food and will decorate your yard and home with fecal material.
If you notice you are dealing with abnormally high earthworm populations and you need to control them, you may have a difficult time hiring someone to help you. Since earthworms are so beneficial they generally are protected and there are no products labelled for use against them and advertising as such is against the law. There have been several university based studies that indicate certain products can have negative side effects on earthworm populations and earthworm health. One such product would be Early Bird fertilizer, a chicken manure based fertilizer that also contains tea seed oil. Tea seed oil contains natural soaps that when they contact a worm will strip its protective mucus coating, driving the worm to the surface to die. There are several insecticides with documented negative effects on earthworms as well, including Sevin (carbaryl), bifenthrin, and other pyrethroids, as well as combination pyrethroid-neonicotinoid products. Again, none of these are labelled for use against earthworms, they merely have documented negative effects on earthworm health.