Avoiding soil compaction: Protecting habitats for wild bees

Key points at a glance

• About 75 percent of native wild bee species nest in the ground and require loose, unsealed patches of earth to do so.
• Soil compaction caused by foot traffic or heavy garden machinery seals nest entrances and prevents the digging of new brood burrows.
• Open, vegetation-free or sparsely vegetated soil patches are essential for the survival of these species.

The physical barrier: Why ground-nesting is under pressure

In July, the activity of many wild bee species is at its peak. While we usually focus on the floral offerings in our gardens, the essential substrate is often neglected. For ground-nesting species such as the vernal mining bee (Colletes cunicularius) or the grey-backed mining bee (Andrena vaga), soil composition is a limiting factor. These insects require a specific physical soil structure: the substrate must be diggable, yet stable enough to prevent the brood tunnels from collapsing.

Soil compaction is primarily caused by mechanical impact – whether through walking on the edges of flower beds, placing down heavy pots, or using rollers on lawns. When soil is physically compacted, the pore volume decreases. The result is a lack of aeration and, more seriously for the bees, an impassable barrier for the burrowing females. A firm crust formed by soil compaction blocks access to previously constructed larval chambers.

Comparison: Soil structures and suitability

Practical tips to avoid soil compaction

1. Define pathways: Create fixed paths in your garden. Anything not explicitly designated as a path should not be walked on during the growing season. This provides lasting protection for the soil structure.
2. Avoid heavy loads: Refrain from placing heavy plant tubs or stone slabs on unplanted soil areas, as their weight causes extreme, localised soil compaction.
3. No mechanical tilling: Avoid digging over or heavily rolling soil surfaces. Especially in spring and summer, you could destroy the deep-lying brood tunnels of species like the hairy-footed flower bee (Anthophora plumipes).
4. Allow for open spaces: Consciously create small, unplanted areas in sunny, dry locations. These serve not only as nesting sites but also allow the insects to reach their operating temperature through solar radiation.

Maintaining nesting sites is just as important as planting food sources. If nesting sites are destroyed, a rich supply of flowers for the red mason bee (Osmia bicornis) or other species will not help, as the reproductive cycle will be interrupted.

Frequently Asked Questions

Why is soil compaction harmful to wild bees?

Compacted soil is too hard to dig. Furthermore, existing brood tunnels are mechanically destroyed and the bees' access for egg-laying is blocked.

What kind of soil condition do wild bees prefer?

Many species prefer sunny, sparsely vegetated or patchy areas with a loose structure where they can easily dig their nesting tunnels.

Am I allowed to compact the soil under trees?

No. Specialised bees also nest under trees. Furthermore, compaction damages the root zone and causes lasting harm to the soil ecosystem.

Main article: Ground-nesting wild bees in the garden: Protecting their habitat