Though mounds of earth left behind by wild visitors at least temporarily scar lawns, not every type presents a serious threat to the well-kept landscape. Some could even be beneficial--the small mounds deposited overnight by large earthworms fertilize plants and their small tunnels aerate soil. Other types of mounds indicate that destructive pests are at work. The special features of mounds reveal what animal or insect created them--identify the builders before deciding on a response.
Two of the most destructive lawn visitors leave distinctive mounds of earth at the entrances of shallow burrows. The mole builds a symmetrical volcano of soil and seals the central entrance when done. The mound of earth comes from the tunnel the mole digs as it searches for grubs and earthworms. When the feeding tunnel collapses a shallow groove forms in the lawn above. Lethal mole traps are the usual human response, but long term solutions include treatment of the lawn with milky spore. Milky spore attacks the root-eating grubs in the lawn, killing one of the mole's favorite foods and making the area less attractive to the foraging animals.
Pocket gophers build crescent-shaped mounds with an open burrow at the lower side. Gophers eat roots and may even pull entire plants down into the burrow for undisturbed feeding. Tubers of flowering plants, garden vegetables and roots of lawn grasses are at risk. Some repellents may be effective, as well as poisoned food pellets injected into the tunnels. Local laws determine exactly what remedies may be used. Arsenic-laced grain eliminates gophers quickly but endangers other animals as well.
Symmetrical fine-grained conical mounds mark ant colonies. Most are harmless and small. The mounds of fire ants multiply as the colony expands--fire ants range widely when feeding and bite and sting when disturbed. Destroy the colonies by placing ant baits near the mounds. Workers carry the poisoned food back to the colony, where the bait kills grubs and queen. Poisoned baits could create problems for pets and wild animals if left in the open.
Mounds of pellets indicate that either earthworms or crayfish live beneath the lawn. Random low piles of small earth pellets surrounding a hole of pencil diameter or less mark an earthworm's tunnel. Night crawlers may build tunnels several feet deep, coming out after dark to forage for lawn debris. The castings combine earth with digested vegetable matter and fertilize surrounding plants. Scattering eliminates the mound and benefits the lawn. Crayfish build more disruptive structures--towers of mud pellets surround open burrows and create obstacles for mowing machines. Lobster-like crayfish reach several inches in length and eat green vegetation while feeding above ground at night. Reducing irrigation reduces the activity of both night crawlers and crayfish. Water only enough to dampen the first three to six inches of soil.