Substrate is the most important aspect of millipede care, serving as food, a medium to maintain moisture and in which to lay eggs, as a "hide", and so on. Sadly, there still remains much misinformation on substrate online and in the literature. However, substrates composed primarily of coco coir, dirt, or a similar product with little to no forest detritus (i.e rotting wood and leaves) are not suitable and often result in specimens randomly dying.
A proper substrate for millipedes, which are detrivorous creatures, involves forest material such humus/mull, wood and leaves in various stages of decomposition, lichens, mosses, etc. A very common misconception is that millipedes do well on a diet of veggies and fruit alone, which is testament to some species' hardiness, but is not suitable for long-term health for the vast majority of species.
Good substrates include hardwood humus, the decomposed plant material found under the leaf layer in forests, mull (humus containing mineral material), and some types of potting soil (primary ingredients being decomposed peat moss, humus, and sometimes kelp). A variety of additives, such as shredded aspen (aspen bedding for snakes), sphagnum moss, or bamboo leaves are often added to help keep substrates loose and prevent compaction.
Specific substrate mixes can be found in the Library of Substrate, but a recommended mix for novices keeping juliform millipedes (spirobolids, spirostreptids, and julids) would be a mixture containing 30-40% forest humus, 30% crushed rotting wood, 30% crushed rotting leaves, and 10% sphagnum moss/aspen bedding. This mix can be a good starting point for keeping other millipede orders as well but its primary use is for the round millipedes.
Polydesmid millipedes are the most diverse of all Diplopoda, and this diversity is mirrored in the varying care required by different families and even different species. Many of the Paradoxosomatidae, do well on juliform millipede substrates with a little more rotting wood and/or fungus, while others, such as the Oxydesmidae and the Euryuridae that have been kept in captivity appreciate a substrate that is composed of 90% or more of shredded rotting wood (these two families tend to unduly influence the care outlook for the entire order). Still others, such as the Xystodesmidae, feed mainly on mull and decomposed peat moss. As such, careful attention must be paid to each species when feeding them.
The ball or pill millipedes, specifically the Glomerida, often do well on the standard juliform substrate but some species (specifically the European Glomeris pustulata and pulchra) require copious amounts of living sphagnum moss placed on the surface; the dirt that congeals under the moss is eaten by these particular species. Others, like Glomeris marginata, eat eroded calcium carbonate much more frequently than other millipedes, and as such should be provided with limestone or other such sources of calcium to ensure their needs are met.
The Sphaerotheriida are more enigmatic and have proven difficult to feed in captivity, likely consuming mosses, lichens, fungi, and other such growths native to their countries of origin; this being difficult to replicate in captivity, the majority of species rarely last more than a year (though this problem may also be caused by other stressors).
The Platydesmida, though vaguely similar to some Polydesmida, nevertheless are quite different from them in terms of food. The diet of the species kept in captivity is restricted largely to saprotrophic fungi, with some species/locales feeding only on certain kinds of molds or polypore fungus. As such, depending on the species in question, the substrate should be composed of rotting wood, with frequent additions of the fungus/mold as it is eaten.
While other orders of millipedes are kept in captivity infrequently, such as the Chordeumatida and Polyzoniida, little is known about their feeding habitats at this time.
It is critically important to remember these requirements when housing multiple species together. Most juliform millipedes can be kept together without issue provided their needs are met, they do not outcompete each other, and males do not unduly harass females (occasionally a problem even in single species tanks), but members of other orders have generally not done well in communal enclosures.
It is important to note that unlike most predatory invertebrates, since the substrate is a millipede's main food source, it will need to be changed or added to with more regularity. If the substrate is close to the point where it is entirely composed of frass (millipede poop), it is time to change it.
For many species a thick layer of rotting leaves on the surface of the substrate is also appreciated (and in the case of Archispirostreptus gigas and other species may be necessary for breeding), and should be periodically replenished as the leaves are eaten.
"Random" deaths can often occur when the substrate has not been replaced in a long time or is of a poor quality. If there are numerous offspring in the enclosure, or some of the specimens have burrowed away to molt, it may be a better idea to simply add more substrate to the surface, provided the enclosure is high enough, rather than changing the substrate.
Eventually one will need to replace the entirety of the substrate, in which case it is recommended to remove all specimens from the surface as they appear over the course of a few days, and then go through all the substrate to remove any other specimens (keep in mind some molting specimens may be crushed in doing so). If there are small offspring in the enclosure it is best to set aside the substrate in bags or another bin to see if any more eggs hatch or young appear. If in doubt, this is always the best practice.
Old substrate can be thrown into the garden or a compost pile after being sterilized (be sure there are no missed millipedes first to avoid cooking/freezing any specimens), or used as an alternative to coco coir/potting soil as the base substrate with the next batch. According to one contributor this is often preferable to using new base material.
The majority of millipede substrates are prepared with materials collected from the wild, which necessitates some knowledge of what, where, and how to forage for leaves, wood, fungi, etc.
When collecting materials such as wood and leaves, it is important to find material at the appropriate stages of decomposition. Rotting wood that has turned white and flakes/crumbles easily is the best, through brown and tan wood that also flakes and crumbles easily works well. Logs that are discolored by visible mold growths are best to be avoided.
Leaves should be in the stage where they are starting to break apart into smaller pieces; this is usually when they have been on the ground for 3-6 months. Leaves that bend easily, are green or freshly fallen, or that have turned black and rotted in anaerobic conditions are to be avoided as millipedes will generally not eat them, and they will attract unwanted pests such as grain mites and nematodes.
Other creatures that are living in, on, and around the wood and leaves should be removed as best as possible in order to preserve the local environment. The best materials are riddled with other invertebrates, including wild millipedes, and it is recommended to set collected logs outside for a day or two to allow whatever creatures in them to depart; splitting them open helps with this purpose.
Of course, destroying habitat, flipping logs and stones, and creating large holes in leaf litter is strongly discouraged. A best practice is to spread one's collecting over as wide an area as possible.
It is very, very important that collecting areas have not seen any recent pesticide or chemical applications, and is as free of trash as possible (one contributor found a sock in one of his collection bags after indiscriminately collecting leaves). Many enthusiasts have lost specimens due to careless material collection. Many online sellers offer clean leaves, wood, moss, entire substrate mixes, and even lichens and fungi for decent prices if collecting is not possible.
The vast majority of millipedes do best on the leaves and wood from deciduous hardwood trees such as oaks (Quercus sp.), beeches (Fagus sp.), alders (Alnus sp.), birches (Betula sp.), and so on. Softwoods and evergreens, such as pines, cypresses, etc. (Pinus sp., Cupressaceae spp.) can be dangerous to millipedes due to the toxic sap found in their branches, though a notable exception to this rule are the aspens, cottonwoods, and poplars (Populus sp.). For a select few millipede species, softwoods like pine should replace hardwoods in the substrate (though these species, such as Onomeris, are rare in culture).
Carya sp. (hickory and pecan) and Jugulans sp. (walnut) have natural insecticides in them and have been proven to kill millipedes, and as such are to be avoided despite being hardwoods. Bamboo and rose leaves have been used by some enthusiasts, though they are similar to aspen shavings in being consumed last as the substrate becomes depleted.
For those who live on the west coast of the USA where safe hardwood trees are not always plentiful, the oaks Quercus lobata, engelmanii, and wislezenii are recommended for collection, as their leaves do not usually have sharp points like most other Quercus encountered in that part of the world, which while harmless to millipedes, are often very painful to crush with the hands after they have dried (the coast live oak, agrifolia, is a nightmare in this regard). The edges of Q. douglasii leaves, depending on the season, are usually rounded but occasionally do have sharp points.
In order to avoid the addition of pests to the enclosure, it is important to sterilize, or perhaps more accurately "pasteurize," any and all collected material. On this many millipede enthusiasts are not in agreement; some argue that a low-temperature bake in the oven (200°F/93.3°C for 30 minutes up to 250-300°F/121.1°C-148.9°C for 2-4 hours) is sufficient, while others argue for freezing. Still others utilize boiling and/or microwaving, or say simply spreading the materials in full sun for a few hours is all that is necessary.
All these methods usually do the job of killing most/all pests (though freezing often fails to kill certain pests such as centipedes, nematodes, and dermapterans). Some special considerations must be taken when using heat to ensure it does not burn the leaves or wood, and that the temperature is not excessive. Often when using high heat and/or excessive amounts of time in the oven/microwave, it reduces the fungal and bacterial communities already present on the materials, so that when introduced to a moist environment, it acts as a blank canvas for undesirable fungal and bacterial growths. One of the most common problems after creating a new setup is rampant mold outbreaks, which can often be traced back to excessive pasteurization techniques.
Lastly, unless one uses the boiling method, it is important to make sure the materials are as dry as possible before being pasteurized; if moisture remains inside a log or on the bottom of a pan after pasteurization there is a good change a pest also survived, no matter the temperature used. Breaking up logs and crushing leaves helps prevent moist pockets from forming.
For a long time amongst American millipede keepers, substrate was divided into two major types, the first being food substrates, such as rotting leaves, rotting wood, and flake soil, and the second being base substrates (typically coco fiber, potting soil, or compost), which serve as a filler to support burrows and maintain moisture. This distinction was made most clearly by Orin McMonigle in his 2012 Millipeds in Captivity.
This distinction does not seem to have been ever made in Europe or elsewhere, where substrate mixes have typically been composed of collected forest humus without any filler substrate. Starting around 2020, American keepers began to emphasize using substrates that are completely edible, and the majority of breeders today have since stopped recommending the use of filler substrates such as coco fiber and certain potting soils due to their lack of nutritional value.
Historically there remains a split amongst enthusiasts over the efficacy of coco fiber, also known as coco coir or coco humus. While it is not disputed that a substrate consisting only of coco fiber is bad for millipedes, whether it can be used as part of the base substrate (similar to potting soil and peat moss) in low concentrations is often contested.
European enthusiasts have long argued that coco fiber is primarily indigestible to millipedes and is likely to lead to impaction, especially with younger specimens, citing a 2011 German experiment and Shura Sigling's recommendations in her book Millipedes. Americans, on the other hand have up until recently followed Orin McMonigle in arguing that coco fiber used in small amounts is safe because it is surrounded by food, so problems with indigestion of the occasional coco fiber particle are negligible.
TMED takes the position that coco fiber is best left out of millipede substrates, especially when raising immatures, due to higher rates of breeding success and better growth that the majority of our contributors have experienced when replacing it with humus or quality potting soils.
Alongside the substrate, supplemental foods include fruits, vegetables, dog/cat/fish/tortoise food, powdered milk, and watered-down maple syrup. While not altogether necessary, most round millipedes will heartily eat these supplemental foods when offered at intervals.
Poly-and-platydesmids will generally not eat any supplemental foods; information is lacking for glomerids and sphaerotheriids. Whatever is chosen, one should be careful to make sure all supplemental foods are free of pesticides and other chemicals, as well as from potentially harmful pests. Keep in mind that while a product may be labeled as "Organic", this does not mean the product is necessarily safe due to the prevalence of "organic" pesticides.
Uneaten food should be removed within a day or two to prevent mold and pest outbreaks as the food decomposes, though this is less of a concern with vegetables such as carrots and many types of squash. Supplemental foods can be offered once a week to once every few months, depending on how rapidly the food is consumed and the preferences of the keeper.
Many enthusiasts add calcium additives, ranging in form from reptile calcium powders to crushed egg shells to cuttle bones, to their substrate mixes. While many millipedes will chew on these items, there is no evidence they contribute noticeably to the health or growth of most millipedes in captivity.