Millipedes (class Diplopoda) are common arthropods found in garden soil and moist outdoor environments. While their presence often raises questions about potential threats to cultivated plants, millipedes are generally considered beneficial inhabitants of the ecosystem, not primary pests. They are slow-moving organisms whose diet focuses almost exclusively on dead or decaying material. Exceptions occur only under specific environmental pressures that shift their feeding habits.
Millipede Primary Diet and Role
The millipede’s natural function is that of a detritivore, meaning their primary food source is decomposing organic matter. They actively consume materials like fallen leaves, rotting wood, grass clippings, and damp mulch. By breaking down this coarse debris, millipedes play a significant role in the initial stages of decomposition.
Their feeding activity transforms large organic waste into smaller fragments, making nutrients more readily available for microorganisms. This process accelerates the cycling of essential nutrients like carbon, nitrogen, and phosphorus back into the soil. Millipedes are natural recyclers that contribute to the formation of rich, fertile soil. They are not predators and lack the mouthparts necessary to consume other insects.
Circumstances Leading to Plant Damage
While millipedes prefer decaying organic matter, they become opportunistic feeders on living plants when specific environmental conditions are met. The primary trigger for this shift is a lack of sufficient moisture or a scarcity of their preferred food source. During extended dry spells, millipedes may seek out the moisture-rich tissues of live plants to survive.
Damage is typically localized, occurring on the softest parts of the plant, such as tender seedlings, roots, or fruit resting directly on the soil surface. Millipedes also target plants that are already weakened, diseased, or damaged, acting as secondary feeders. A population explosion can also force millipedes to look for alternative food sources, even if decaying matter is present.
The specific signs of millipede feeding are distinct from those left by other pests. They use scraping mouthparts, which often leave irregular, shallow marks or pits on the surface of stems and roots. Damage to seedlings often manifests as girdling around the base of the stem near the soil line, resulting in the collapse or death of the young plant. For crops like potatoes, sweet potatoes, or strawberries, millipedes create small, uneven holes in the underground or ground-contacting parts.
Identifying Millipedes and Controlling Infestations
Millipedes are often confused with centipedes, but they are easily distinguished by physical characteristics. Millipedes possess a rounded, cylindrical body shape and have two pairs of short legs attached to almost every body segment, resulting in a slow, wave-like movement. Centipedes, conversely, have a flattened body and only one pair of longer legs per segment, allowing them to move quickly.
Managing millipede populations that cause plant damage should focus on cultural controls that modify their habitat. Since they thrive in damp conditions, a primary strategy is to improve drainage and reduce excess moisture in the garden area. Watering plants only in the morning allows the soil surface to dry out before nightfall, when millipedes are most active.
Removing excessive layers of organic debris, such as mulch, leaf litter, and grass clippings, can reduce their numbers. If an infestation is localized, physical removal is an effective option, as millipedes are slow-moving and often coil up when disturbed. By eliminating the conditions that allow for high population density and ensuring a steady supply of decaying organic matter, millipedes revert to their beneficial role.