The most common diggers in mulch are voles, moles, earthworms, and beetle larvae
Holes in mulch usually come from one of four sources: small rodents tunneling underneath, moles hunting for grubs, earthworms working through the soil, or beetle larvae feeding in the mulch itself. Each leaves different signs—voles make small surface runs and quarter-sized holes, moles create raised ridges and deeper tunnels, earthworms leave tiny holes with castings around them, and beetle larvae bore irregular tunnels through the mulch material itself. The digger matters because the solution changes depending on what you're dealing with.
The depth and pattern of holes tell you what's happening. Surface-level disturbance with visible tunneling just under the mulch layer points to voles. Deep, raised ridges with no visible entry holes suggest moles. Tiny holes scattered across the mulch surface with small mounds of soil nearby indicate earthworms. Crumbly, broken-down mulch with irregular tunnels means beetle larvae are already decomposing the material—which is actually normal and not harmful.
Key Takeaways
- Voles create small surface tunnels and quarter-sized holes in mulch as they move between feeding areas, and they damage plant roots and bulbs.
- Moles dig deep tunnels while hunting earthworms and grubs, leaving raised ridges but usually not eating plants directly.
- Earthworms create tiny holes with small soil castings and actually improve soil structure, so they do not require removal.
- Beetle larvae breaking down mulch is a normal part of decomposition and does not harm plants or require treatment.
- Removing or replacing mulch, installing barriers, and reducing moisture can discourage voles and moles without chemicals.
Voles: Small rodents that tunnel through mulch and damage roots
Voles are the most destructive diggers in mulch. These mouse-sized rodents create visible surface tunnels and small holes (about the size of a quarter) as they move between feeding areas. Unlike moles, voles eat plant material—they gnaw on roots, bulbs, and the bark of young trees and shrubs. You'll often see their damage in spring when mulch is pulled back and reveals chewed roots or girdled stems at the soil line.
Vole tunnels run just under the mulch surface, creating a network of runs they travel repeatedly. They prefer thick mulch because it gives them cover while they feed. To reduce vole activity, pull mulch back 3 to 4 inches from tree trunks and shrub bases—this removes their cover near plants they might damage. Keep mulch depth to 2 to 3 inches rather than piling it thick. Remove fallen leaves and debris where voles hide. If voles are already present, trapping is more effective than repellents; snap traps or electronic traps placed along their visible runs work best in spring and fall when vole populations are highest.
Moles: Deep tunnelers hunting for grubs and earthworms
Moles create the most visible damage—raised ridges and tunnels that run through mulch and soil. Unlike voles, moles don't eat plants. They're hunting earthworms and grubs, and they tunnel constantly because they need to eat every few hours. A single mole can create dozens of tunnels in a small area in just a few days. The tunnels themselves are the problem: they disturb soil, expose roots, and make mulch look torn up.
Moles are harder to control than voles because they're solitary and move through large territories. Trapping is the most reliable method—use scissor traps or harpoon traps placed directly in active tunnels. To find active tunnels, flatten a ridge with your foot and check it the next day; if it's raised again, the mole is using that tunnel. Reducing grubs in your lawn (which are mole food) can make your yard less attractive, but this takes time and doesn't may provide moles will leave. Castor oil-based repellents have mixed results and require repeated process. Physical barriers like underground fencing around garden beds can keep moles out of specific areas, though installation is labor-intensive.
Earthworms: Beneficial diggers that improve soil
Earthworms create tiny holes in mulch with small mounds of soil (called castings) around them. These holes are not a problem—earthworms are working for you. They break down organic matter, improve soil structure, and increase water infiltration. The presence of earthworms in your mulch is a sign of healthy soil. Their tunneling aerates the ground, which helps plant roots grow.
You should not try to remove or discourage earthworms. If you see many earthworm castings on the surface of your mulch, it straightforward means the mulch is decomposing and the soil underneath is active. This is exactly what you want. The only reason to be concerned about earthworm activity is if you're trying to maintain a perfectly smooth mulch surface for appearance, but the trade-off isn't worth it—healthy soil with earthworms will grow better plants.
Beetle larvae and other insects: Normal decomposition in mulch
Beetle larvae, millipedes, and other insects that live in mulch are decomposers. They tunnel through the material as they feed on decaying wood and organic matter, which is their job. You might see irregular holes, crumbly sections, or the insects themselves when you dig into the mulch. This is not damage—it's mulch doing what it's supposed to do: breaking down and returning nutrients to the soil.
These insects do not harm plants or roots. They do not eat living plant material. If you see them, it means your mulch is actively decomposing, which means it's feeding your soil. Replacing mulch frequently to avoid these insects defeats the purpose of using mulch in the first place. Leave them alone and let them work.
How to tell which digger you have
Start by looking at the pattern and depth of the holes. Vole damage appears as small surface tunnels and quarter-sized holes, often with visible runs connecting them. You may also see chewed roots or bark damage at the soil line. Mole damage shows as raised ridges running through mulch and soil, with no obvious entry holes—the ridges themselves are the tunnels pushed up from below.
Earthworm activity looks like tiny holes (smaller than a pencil tip) with small piles of soil around them, scattered across the mulch surface. Beetle larvae and other decomposers leave irregular, crumbly sections of mulch with visible insects or tunnels when you dig into the material. If you're unsure, dig gently into the mulch with a hand trowel and look for the animal itself—you'll find voles or moles in their tunnels, earthworms in the soil, and beetle larvae in the mulch material.
Practical steps to reduce digging without chemicals
Start with mulch management. Reduce mulch depth to 2 to 3 inches—thick mulch provides cover for voles. Pull mulch back 3 to 4 inches from tree and shrub trunks to remove cover near plants. Remove fallen leaves, branches, and debris where voles and moles hide. Keep the area around your plants clear and open.
For voles, trapping is the most effective control. Place snap traps or electronic traps along visible surface runs, especially in spring and fall. Check traps daily. For moles, use scissor or harpoon traps in active tunnels—flatten a ridge and check it the next day to confirm activity before setting a trap. If you have a persistent mole problem, consider installing underground barriers (hardware cloth or metal mesh) around garden beds, though this is a larger project.
Reduce moisture if possible. Voles and moles prefer damp soil and mulch. Improve drainage, avoid overwatering, and allow mulch to dry out between waterings. This makes your beds less attractive to these diggers. Do not use poison baits unless you're certain you have a severe infestation—they can harm pets, wildlife, and beneficial predators like hawks and owls that eat voles and moles.
Frequently Asked Questions
Are the holes in my mulch dangerous to my plants?
It depends on what's digging. Vole tunnels expose roots and their feeding damages plants, so vole activity is a real problem. Mole tunnels disturb soil but don't directly harm plants. Earthworm and beetle larvae holes are harmless and actually beneficial. Identify the digger first—look for chewed roots (voles), raised ridges (moles), or tiny holes with soil castings (earthworms).
Can I use poison to kill moles and voles in mulch?
Poison baits work but carry risks. They can poison pets that eat poisoned rodents, and they kill beneficial predators like hawks and owls that naturally control vole and mole populations. Trapping is safer and more effective because you control exactly where and when the trap is set. If you choose poison, keep it away from areas where pets play and follow all label instructions.
Will replacing my mulch stop the digging?
Replacing mulch temporarily removes the diggers' tunnels, but new animals will move in if conditions remain attractive. Focus on making your mulch less appealing instead: reduce depth, pull it back from plant bases, remove debris, and reduce moisture. These changes discourage new tunneling without the cost and waste of replacing mulch repeatedly.
Do I need to remove earthworms from my mulch?
No. Earthworms are beneficial and improve soil structure, water infiltration, and nutrient cycling. Their presence is a sign of healthy soil. The small holes and castings they leave are not damage—they're evidence that your mulch is decomposing and feeding your plants.
How do I know if a mole tunnel is still active?
Flatten a raised ridge with your foot or hand, mark the spot, and check it the next day. If the mole has raised it again, that tunnel is active and a good place to set a trap. Moles use the same tunnels repeatedly, so active ridges are your best targets for trapping.