Substrate is not just something you put in the bottom of the tub. For most invertebrates, it is where they live, where they moult, where they lay eggs, and in many cases what they eat. Get it wrong and you will poison your animals slowly without any obvious external cause. The beetle larva just stops growing. The isopod colony quietly crashes. The millipede dies mid-moult. Here is what to avoid and why.
Cedar and pine
This is the big one. Cedar and pine contain aromatic phenols, primarily plicatic acid (cedar) and abietic acid (pine), along with volatile terpenes that are directly toxic to invertebrates. These compounds damage respiratory tissues, interfere with moulting, and can kill animals over days to weeks of exposure.
Cedar shavings and pine shavings are widely sold for small mammal bedding. Hamsters and rabbits can tolerate them (though many vets advise against it even for mammals). Invertebrates cannot. The phenols that make cedar smell pleasant to us are biologically active compounds that disrupt invertebrate physiology.
This includes cedar bark, pine bark, and any decorative wood products made from softwoods. If it smells strongly resinous, do not use it. Some keepers have lost entire isopod colonies to a single piece of cedar bark added as a hide. The animals congregated around it, sat on it, and absorbed the toxins through their permeable cuticle and gill-like pleopods.
Other softwoods to avoid
Cedar and pine get the most attention, but the problem extends to softwoods generally. Spruce, fir, and yew all contain compounds that are problematic for invertebrates. The concentration varies by species of wood and how it has been processed, but as a general rule: if it is a conifer, do not put it in an invertebrate enclosure.
Stick to hardwoods. Oak, beech, birch, and ash are all safe when properly aged or dried. For beetle larvae specifically, the substrate needs to be fermented hardwood flake soil, not just any hardwood. But for decor, hides, and climbing surfaces, untreated hardwood is fine.
Treated and painted wood
Pressure-treated timber (the greenish stuff used for fences and decking) is impregnated with preservatives to prevent rot. Older treatments contained chromated copper arsenate. Newer ones use copper-based compounds. Neither is something you want your animals living on.
Painted, varnished, or stained wood is also off the table. The coatings can leach chemicals as they degrade, particularly in warm, humid conditions. This includes those decorative pieces you find in craft shops and garden centres. If it has any finish on it, skip it.
Driftwood sold for aquariums is usually safe because it has been waterlogged and any soluble compounds have leached out. Cork bark is universally safe and is the standard for invertebrate enclosures for good reason. It does not rot quickly, it provides good climbing and hiding surfaces, and it contains nothing harmful.
Commercially sold "reptile substrates"
Not all of them are suitable for invertebrates. Calcium sand, which is marketed for reptiles (particularly leopard geckos, where it is also a bad choice due to impaction risk), is not appropriate for any invertebrate. It is calcium carbonate ground to a fine powder, and while calcium itself is not toxic, the dust can clog spiracles and book lungs.
Reptile bark chips, if they are from pine or cedar, have the same problems described above. Check the label. If it says "pine bark" or does not specify the wood species, assume the worst.
Coconut fibre (coco coir) on its own is safe but is a poor sole substrate for most invertebrates. It dries out quickly from the top, compacts when wet, and has almost no nutritional value for species that eat their substrate (beetle larvae, millipedes). Use it as part of a mix, not the whole thing.
Garden soil and compost
Garden soil is risky because you do not know what is in it. If the garden has ever been treated with pesticides, herbicides, or slug pellets, residues persist in the soil for months or years. Metaldehyde (a common slug pellet ingredient) is lethal to isopods and millipedes. Neonicotinoid pesticide residues are toxic to virtually all invertebrates at very low concentrations.
Bagged organic topsoil from a garden centre is a safer option, but read the label. Some products marketed as "organic" still contain added fertilisers (particularly slow-release granules) that are not safe for invertebrates. Look for topsoil that lists ingredients rather than just saying "premium blend" on the bag. Ideally, it should contain nothing but screened soil.
Commercially sold compost is often worse than plain topsoil because it may contain green waste that was treated with persistent herbicides (aminopyralid, clopyralid) that survive the composting process. These herbicides are not directly targeted at invertebrates but can contaminate the plants and organic matter your animals eat.
Vermiculite and perlite
These are sometimes used for incubating eggs or as substrate additives. Vermiculite is generally safe when used appropriately, though it has no nutritional value and is not a substrate on its own. Perlite is inert and also safe in small amounts but can be dusty when dry, and the fine particles may irritate respiratory structures.
Neither should be the primary substrate for any species. They are additives at best, and for most invertebrate setups they are unnecessary. A proper soil-based substrate mix does the job better.
What to use instead
For isopods: organic topsoil, coco coir, and sphagnum moss in roughly equal parts, with a thick layer of dried hardwood leaf litter (oak and beech are standard) on top. Cuttlefish bone or crushed eggshell for calcium. This is the baseline that works for most species.
For millipedes: similar to isopods but with added rotting hardwood and deeper substrate (10-15cm minimum). Millipedes eat the decaying wood and need it as part of their diet. Calcium sources are equally important.
For beetle larvae: fermented hardwood flake soil. This is a specific product made by fermenting oak or beech sawdust until it breaks down into a dark, crumbly, nutritious material. You can buy it from specialist beetle suppliers. Homemade flake soil is possible but takes months of preparation. Potting soil, compost, and coco coir will not sustain beetle larvae regardless of what you mix in.
For jumping spiders: substrate matters less because they live above ground, but a thin layer of coco coir or organic topsoil at the bottom helps maintain humidity. Cork bark pieces and fake or live plants for climbing are more important than the substrate itself.
The common thread across all these groups is: no softwoods, no treated materials, no unknown soil with potential pesticide contamination, and no fertiliser additives. When in doubt, stick to the tested substrates that the hobby has been using successfully for years. There is a reason everyone recommends the same things.