Flat-backed millipede

Flat-backed millipedes of the genus Polydesmus belong to the class Diplopoda. Adult individuals are characterized by a flattened body composed of 20 segments, typically measuring 15–30 mm in length.

Their coloration ranges from light tan to dark brown. The specific morphological trait of this genus is the presence of lateral extensions on each segment, which gives them a distinctive flattened appearance.

These organisms are nocturnal and highly sensitive to desiccation. During the day, they remain in dark, moist environments, such as deep in the soil, under rocks, or beneath piles of decaying organic matter.

Their life cycle involves a slow development process. Females lay eggs in clusters within the soil, and larvae undergo a series of molts, gradually adding body segments as they mature.

They thrive in habitats with consistently high humidity and rich soil organic content, making them frequent inhabitants of greenhouses, cold frames, and heavily mulched garden beds.

Polydesmus species are opportunistic polyphagous pests that feed on a variety of organic materials, including living plant tissues. They are particularly destructive to young, tender plants.

Major host crops include root vegetables like carrots, beets, and potatoes, as well as seedlings of cucumbers, tomatoes, and various cabbages, which they may consume or girdle at the soil line.

Greenhouse settings are especially prone to outbreaks, where millipedes can destroy germinating seeds and young sprouts, leading to significant stand losses and reduced crop yields.

Mature crops are also affected, as the millipedes create deep, irregular cavities or pits on the surface of tubers and roots, rendering them unmarketable and aesthetically damaged.

Beyond direct feeding damage, these wounds provide entry points for secondary pathogens, such as fungi and bacteria, leading to rot and the subsequent collapse of the affected plant.

The primary sign of a Polydesmus infestation is the presence of eaten patches or deep pitting on roots and tubers during harvest or inspection of the soil.

One may identify them by their rapid, coiling response when disturbed under damp debris or moist soil, which is a defensive mechanism against potential threats and desiccation.

In greenhouses, unexplained seedling death or stunted growth can often be linked to underground feeding, where the roots or stems are found to be partially eaten or severed.

The presence of millipedes in high densities under organic mulch or damp wooden boards in the garden indicates a favorable environment for their reproduction and feeding.

Yellowing or wilting of young plants during dry spells, despite adequate watering, may suggest damage to the root system caused by the feeding activity of these millipedes.

Effective management begins with soil moisture control. Reducing excessive irrigation and preventing water stagnation in garden beds can significantly limit the suitability of the habitat.

Sanitation is critical; gardeners should remove accumulated garden debris, rotting leaves, and old wood, which serve as essential daytime shelters and breeding sites for the pests.

Deep cultivation of the soil in late autumn exposes the millipedes and their eggs to cold air and surface predators, effectively reducing the overwintering population density.

Trapping techniques, such as burying pieces of potato or carrot near the base of susceptible crops, can be used to aggregate and remove millipedes manually from the infested area.

When populations reach economic threshold levels, use of soil-incorporated insecticides labeled for the specific crop may be necessary, though they should always be applied according to local regulations.