Substrate is where your isopods live, eat, breed, moult, and hide. It is not just bedding. Get the mix wrong and the colony will struggle regardless of everything else you do right. Get it right and the colony largely takes care of itself.
The base recipe
There are plenty of variations, but a mix that works well for most commonly kept species is:
- 60% organic topsoil (peat-free, no added fertilisers or pesticides)
- 20% sphagnum moss (chopped)
- 10% fine sand or crushed limestone
- 10% leaf litter (crumbled into smaller pieces and mixed through)
On top of this, add a thick layer of whole dried leaves. Oak and beech are the standard. These serve as both food and cover. The isopods will eat them from the underside over weeks, and you top them up as they disappear.
Why each ingredient matters
Topsoil
The topsoil provides structure and holds moisture. It needs to be organic and free of chemical additives. Garden centre topsoil labelled "organic" usually works. Avoid anything that says "enriched" or lists fertiliser content on the bag. Pesticide residues in soil will kill isopods. If you are not sure about a product, leave it.
Sphagnum moss
Sphagnum holds moisture extremely well and releases it slowly. It also resists mould better than most organic materials. Chop it up and mix it through the topsoil rather than layering it on top. This creates pockets of moisture throughout the substrate rather than just at the surface.
Sand or limestone
A bit of sand improves drainage and prevents the substrate from compacting into a solid block. Crushed limestone does the same job while also providing calcium as it breaks down. Either works. Horticultural sand is fine. Avoid builder's sand, which may contain contaminants.
Leaf litter
Oak and beech are the go-to choices. Both are safe, break down at a reasonable rate, and isopods eat them readily. You can collect them yourself in autumn and let them dry out, or buy them from hobby suppliers. Avoid leaves from roadsides (vehicle pollution) or treated gardens (pesticide risk).
Other safe leaves include hazel and birch. Avoid walnut leaves, which contain juglone, a compound toxic to many invertebrates. Avoid anything from conifers. Pine needles, cedar, and spruce all contain phenols and terpenes that are toxic to isopods and most other terrestrial invertebrates.
Cork bark and rotting wood
Cork bark is a surface hide, not a substrate ingredient, but it belongs in every isopod enclosure. Lay pieces on the substrate surface. The isopods cluster underneath for shelter and moisture, and they will slowly eat the bark over time.
Adding chunks of rotting hardwood (white rot, ideally) gives the isopods another food source and creates microhabitats within the substrate. The wood should be soft enough to crumble when you squeeze it. Hard, freshly cut wood is useless to them. Again, hardwood only. Oak, beech, birch, and ash are all safe. No softwoods.
Depth
Aim for 5-8cm minimum. Deeper is fine, especially for larger species. The isopods need enough depth to burrow, and gravid females retreat below the surface to brood their mancae in the marsupium. If the substrate is only a centimetre or two deep, there is nowhere for them to go and the moisture evaporates far too quickly.
The moisture gradient
This matters more than almost anything else about the substrate. One side of the enclosure should be damp (not soaked), the other side drier. When you mist, only mist the damp side. The isopods then move between the two zones as they need to regulate their moisture level.
If you squeeze a handful of substrate from the damp side and water drips out, it is too wet. If it crumbles and does not hold together at all, it is too dry. You want it to hold its shape when squeezed but not drip. On the dry side, the surface should feel dry to the touch but there should still be some moisture a couple of centimetres down.
What to avoid
- Conifer-based products (pine bark, cedar chips, spruce shavings) are toxic
- Peat moss from non-sustainable sources, though the bigger issue is that pure peat is too acidic for most species
- Compost with added fertiliser or wetting agents
- Soil from areas treated with herbicides or pesticides
- Vermiculite on its own (it holds water but provides no nutrition)
- Coco coir on its own (fine as a component but lacks the nutrients of topsoil)
Adjustments for different species
The recipe above works for most Porcellio and Armadillidium species. For tropical species that need higher humidity, increase the sphagnum moss to 30% and reduce the sand. For species that prefer drier conditions (some Spanish Porcellio), increase the sand and reduce the moss.
Cubaris species generally prefer a chunkier substrate with more limestone and rotting hardwood pieces. Some keepers use a primarily limestone-based mix for Cubaris, which provides both calcium and the slightly alkaline conditions these species seem to prefer.
When to replace the substrate
You do not need to do full substrate changes. The isopod colony processes the substrate as part of its diet, and their frass (droppings) becomes part of the soil ecosystem. Top up leaf litter and cork bark as they get consumed. Add fresh substrate on top or to one side when the level drops. A full teardown stresses the colony and destroys the micro-ecosystem that has built up over months.
The exception is if something has gone properly wrong: a severe mite infestation, toxic contamination, or persistent mould from waterlogging. In those cases, move the isopods to a temporary container and start fresh. Otherwise, let the substrate mature. An aged, established substrate with a healthy microbial community is better for the colony than fresh mix.