How Hot Is Too Hot for Isopods?

The old advice on this page was written on what it called "a rare warm day in the UK". That framing no longer holds. 2026 was the first year on record with temperatures above 35°C in May, June and July, and the summer has changed what heat means for an invertebrate room. This is a rewrite, and the headline change is this: the temperature itself can harm them, and the number on the forecast is not the number your animals are living in.

Quick answer: how hot is too hot for isopods?

In our experience keeping and shipping these animals in the UK, problems start above 30°C, and 32°C and above is where keepers start reporting real losses. Those are our own field figures rather than published ones, and they are worth stating as such, because the published literature stops short of them.

The most useful published measurement is narrower and more specific. Römbke et al. (2011, Soil Organisms 83(2)) moved Porcellio scaber from 20°C to 28°C and recorded mortality rising from 9.4% to 23.4%, with feeding up 86%. In the same work Porcellio pruinosus, a Mediterranean species, was unaffected. Ďurajková et al. (2022, ZooKeys 1101) found that at 27 to 31°C, P. scaber took 58 seconds to cross a maze it managed in 25 at lower temperatures.

So 28°C is not a cliff. It is the point at which a temperate woodlouse is already paying for the heat, and it is the sensible ceiling for most of what we sell. What happens between 28 and 32 is where our own observation takes over from the literature.

The forecast is not the temperature your animals are in

This is the part most heat advice gets wrong, and it is the single most useful thing on this page.

Monitored UK homes run substantially warmer than the outdoor air during a hot spell. Indoor temperatures in monitored flats have been recorded at 5 to 8°C above the outdoor mean, and outdoor temperature on its own explains less than a third of the variation in indoor heat stress. Modern, well-sealed housing is worse, not better: bedrooms in homes built after 1977 have been measured falling only 0.1 to 0.2°C overnight. The building holds the heat it gained during the day and hands it back after dark.

The practical consequence is that a 27°C afternoon on the forecast is not a 27°C enclosure. A tub on a shelf in a closed upstairs room can sit several degrees above that, and unlike the outdoors it does not cool down at night. Buy a thermometer and put it where the animals actually are. Not on the wall, not in the hallway, and not on your phone.

Check the microclimate before you panic

The old advice on this page was right about one thing and it survives the rewrite: check the temperature in the spot the animals occupy, not the air above it. Damp substrate under a layer of leaf litter runs cooler than the headspace, and a burrowing species can put several centimetres of soil between itself and the problem. If your reading is taken at the lid, you are measuring the worst part of the enclosure.

This is also the reason substrate depth is a heat strategy and not just a husbandry nicety. A 3 cm layer gives an animal nowhere to go. Ten to fifteen centimetres gives it a refuge that tracks the daily swing far more slowly than the air does.

Why "tropical" is the wrong word for a heat argument

A great deal of hobby advice reasons from the word tropical to the idea that a species wants to be hot. For the cave Cubaris that make up much of the premium end of the hobby, that inference is backwards.

McClure, Lim and Winn (1967, Pacific Insects 9(3)) recorded terrestrial isopods in Dark Cave at Batu Caves, Malaysia. The cave interior averaged 24.5°C with a monthly range of 23.0 to 25.0°C, while the surface outside reached mean daily maxima of 28.8 to 30.3°C. The cave sat four to six degrees below the surface peak, and its annual swing was smaller than the surface daily swing.

A limestone cave in Southeast Asia is not hot. It is stable and it is saturated. A keeper who reads "tropical" and reaches for a heat mat is recreating the wrong half of the habitat, and in a British July they may be adding heat to a room that already has too much.

Do not quote 40°C at anybody

You will sometimes see the figure 40.7°C given as an isopod's heat limit. That number is real and it is being misread. It is a critical thermal maximum from a ramping assay, where the animal is warmed at around a quarter of a degree per minute until it loses coordination. It describes what happens over minutes.

Husbandry happens over weeks. When Pallarés et al. (2019) measured seven-day lethal limits in cave arthropods rather than minutes-long ramps, the figures came out at 20 to 25°C. A short-exposure survival ceiling tells you almost nothing about a temperature an animal has to live at through a fortnight of hot weather.

What the animals actually prefer

Given a choice, isopods walk to the low twenties. Three studies, three species, three continents and four decades apart agree closely:

• Refinetti (1984, Crustaceana 47(1)) put Armadillidium vulgare on a thermal gradient and recorded a preference of 22 to 23°C
• Antoł et al. (2019, PLOS ONE 14(8)) recorded a preferred temperature of 20.93°C for P. scaber
• Castañeda et al. (2004, Evolutionary Ecology Research 6) found optima of 20.94 to 24.69°C across three Chilean populations of Porcellio laevis

That band sits below where most of the hobby keeps them. It is a useful corrective in both directions: it argues against panicking at 26°C, and it argues against treating 30°C as a target because a species came from somewhere warm.

Provenance beats any universal number

The Römbke work is the clearest demonstration. The same experiment that pushed P. scaber mortality from 9.4% to 23.4% left P. pruinosus untouched. Ďurajková found the same split on locomotion. A Mediterranean species and a temperate northern European one do not share a ceiling, and neither shares one with a Vietnamese cave animal.

This is why our care panels differ by species rather than carrying a house number, and why the honest answer to "how hot is too hot" begins with "for which animal".

A note on Dubrovnik isopods

We have kept Armadillidium klugii comfortably across 24 to 32°C, which is warmer than we would push almost anything else we stock. Both figures on this page are observations from the same shelves: the general caution above 30°C, and this species being an exception to it. A. klugii comes from the Dalmatian coast and it is genuinely more heat-tolerant than the tropical Cubaris people assume are the tough ones. Treat it as an exception with a reason, not as evidence that 32°C is broadly safe.

What to actually do in a hot spell

In rough order of how much good they do:

• Move the enclosure. Downstairs, north-facing, off the windowsill, out of the loft. This beats every cooling trick below and costs nothing.
• Close curtains in the morning. Most of the heat in a British house arrives as sunlight through glass. Shutting it out before the room warms is far more effective than cooling the room afterwards.
• Damp towel and a fan. Lay a damp towel over the enclosure where it is safe to do so and blow air across it. Evaporation pulls real heat out, and this is more effective than people expect. Watch the reading, because it can overshoot.
• Deepen the substrate and keep the damp end damp. Heat dries an enclosure fast, so in a hot spell you mist more often to hold the same moisture, not to make the tub wetter. Aggregation and survivorship are more sensitive to a 20 to 25% drop in humidity than to a 5 to 6°C rise in temperature (Dixie et al. 2015), so losing the moisture gradient is often the thing that actually costs you the colony.
• Increase airflow. Cross-ventilation through mesh on opposing walls, not a single vent. Still air in a hot tub is the worst combination.

Two things not to do

Do not add ice. It cools a small area sharply and then the temperature climbs straight back, and you have swapped a steady problem for an unstable one. Stability is worth more than a brief dip.

Do not add a standing water dish as a cooling measure. It does very little for air temperature, and isopods drown in open water. If you want moisture in the air, mist and keep a damp corner. If you already run a dish, put pebbles or a stick in it so anything that falls in can climb out.

Species that handle a British summer well

If your home genuinely runs hot, species choice does more than any intervention. Armadillidium beieri covers roughly 17 to 28°C, which spans almost any UK room across the year. Armadillo officinalis, including the Red Brick morph, comes from Spain and copes with both chilly winters and hot summers. Browse our Armadillidium collection for the temperate end of the hobby.

For the other direction, see our guide to how cold is too cold, and our seasonal care guide for the year as a whole.

Frequently asked questions

What temperature kills isopods? There is no single figure. Measured mortality in P. scaber rises from 9.4% to 23.4% between 20°C and 28°C, and in our experience losses become common above 32°C. Mediterranean and cave species differ, and short exposures are survivable at temperatures that would be fatal over a fortnight.

Is 30°C too hot for isopods? For most species we sell, yes, as a sustained temperature. It is above the ceiling on our care panels and above the preference recorded in every thermal-gradient study we know of. Brief peaks are less serious than a week of it.

My room is 24°C. Is my enclosure 24°C? Probably not. Indoor temperatures during hot spells have been recorded 5 to 8°C above the outdoor mean, and enclosures near glass or high on a shelf run warmer still. Measure at the animals.

Should I use a heat mat in summer? Almost never in a British July. Heat mats are a winter tool for tropical species. If one is on a thermostat it should simply not be triggering, and if it has no thermostat it should not be in use at all.