Ground-nesting solitary bees: Why you must leave holes in your garden

Key points at a glance

• Around three-quarters of our native ground-nesting solitary bees nest directly in the soil, not in insect hotels.
• Small holes in the earth can be entrances to highly complex brood chambers that are used throughout the summer.
• Interventions such as mulching, hoeing or piling up soil will permanently destroy the underground brood and the nesting site.
• Observe rather than act: Often, these are nesting aggregations (colonies of multiple females of the same species), which indicate a valuable microclimate.

Why the soil is the heart of your garden

If you discover small, mostly round holes in sunny, sparsely vegetated areas of your garden, the first instinct for many gardeners is to fill them in. From an ecological perspective, however, this is a mistake. About 75 percent of native ground-nesting solitary bee species, including representatives of the genera Andrena (mining bees), Halictinae (furrow bees), Lasioglossum (sweat bees), Colletidae (plasterer bees) and Dasypoda (furrow bees), rely on suitable soil structures as nesting sites.

These insects are specialised to a deep-seated ecological niche. They do not require artificial nesting aids; instead, they need specific soil conditions such as sand, loam or loess layers. A successfully colonised nesting site shows you that your garden provides an ideal microclimate for these animals—a combination of warmth, dryness and accessible sources of pollen and nectar in the immediate vicinity.

The biology of underground brood care

Females of ground-nesting species dig tunnels, sometimes with deep branches, into the soil. Inside, they create several brood chambers, into which they place a mixture of pollen and nectar as provisions for the larvae. After the egg is laid, the chamber is sealed, and the larva develops in complete safety underground.

Since the larvae often remain in the soil for long periods, the stability of the soil layer is crucial. Every mechanical disturbance not only affects the individual animal but also endangers the entire brood. The following table provides an overview of which sites are particularly worth protecting.

How to protect solitary bees in your garden

If you observe solitary bees colonising your soil, restraint is the most effective protection measure. Follow these guidelines to preserve the brood:

1. No bark mulch or landscape fabric: These materials deprive the soil of direct sunlight and cool it down. Furthermore, they prevent bees from accessing their nest entrances.
2. Avoid mechanical soil cultivation: Under no circumstances should you hoe, till or dig up areas where you notice nesting activity. Any digging will destroy the nesting tunnels and kill the larvae inside.
3. No chemical agents: The use of pesticides or herbicides (chemical weedkillers) directly harms both the bees' food supply and their health.
4. Keep areas open: If wild bees have chosen a spot themselves, this is the best indicator of a correct site. Only remove emerging vegetation sporadically and by hand if it completely shades the nest entrances.
5. Observe: Learn to distinguish between species. Ants or digger wasps also create holes. Take time to observe the flight activity before taking any action.

Remember: An ecologically valuable garden is not defined by the absence of holes in the ground, but by the presence of a diverse fauna that requires exactly these structures.

Frequently asked questions

Do holes in the ground always belong to solitary bees?

No, digger wasps, ants and beetles also dig holes. Observe the flight activity on sunny days to check whether they are solitary bees.

Am I allowed to hoe the soil near solitary bee nests?

No, hoeing or digging will destroy the underground nesting tunnels and permanently kill the brood and larvae inside.