Most of a beetle's life is spent as a larva. An adult sun beetle might live three or four months; its larval stage lasted twice that. For rhinoceros beetles the ratio is even more extreme. If you're going to keep beetles, you need to get larval care right. Here's what that actually involves.
What beetle larvae are
Beetle larvae are C-shaped grubs with a brown head capsule, three pairs of small legs near the front, and a soft, pale body. They spend their time burrowed in substrate, eating. That's essentially it. They don't need UV light, they don't need a water dish, and they don't need enrichment. They need the right substrate, the right temperature, and to be left alone.
Substrate is food
This is the most important thing to understand. For beetle larvae, substrate isn't bedding. It's their entire diet. Most hobby species (flower beetles, rhinoceros beetles) need fermented hardwood flake soil: deciduous wood that has been colonised and broken down by white-rot fungi. The fermentation process makes the wood digestible for the larvae.
You can buy flake soil ready-made, or make it yourself (which takes several months of preparation). Either way, it needs to be made from hardwoods: oak, beech, or birch are standard. Never use softwoods. Pine and cedar contain phenols that are toxic to beetle larvae, and even small amounts mixed in can cause problems.
Some flower beetle larvae are more flexible and will accept a mix of well-rotted wood, leaf litter, and compost. Pachnoda larvae, for instance, can manage on less refined substrate than Mecynorrhina. But for rhinoceros beetles and stag beetles, quality flake soil makes the difference between healthy adults and dead larvae.
Instars: L1, L2, L3
Beetle larvae grow through three stages called instars. Each transition involves moulting, where the larva sheds its old skin to accommodate its larger body. The head capsule width is the easiest way to tell which instar you're looking at, since the body size can vary with feeding.
L1 larvae are tiny, often only a few millimetres long when they hatch. They're fragile. Most keepers leave them in the breeding substrate for the first few weeks and don't attempt to move or handle them. If you're separating larvae from a communal egg-laying container, wait until they're at least L2.
L2 larvae are larger, sturdier, and this is when you can safely transfer them to individual rearing containers if you want to track their development. Growth is rapid during this stage, and substrate consumption increases noticeably.
L3 is the final and longest instar. This is when the larva puts on the most weight and reaches its maximum size. For large species, the L3 stage is impressive. An L3 Trypoxylus dichotomus grub is the size of a grown person's thumb. An L3 Dynastes hercules larva can weigh over 100 grams.
Container setup
Individual rearing containers work well for most species. A 1-2 litre tub is fine for smaller flower beetle larvae. For large rhino beetle larvae, you'll want 3-5 litres or more per larva. The container should have a few small ventilation holes in the lid, but not so many that the substrate dries out quickly.
Fill the container with flake soil, lightly compressed, to a depth of at least 15cm. The substrate should be moist but not wet. Squeeze a handful: if it holds its shape without water dripping out, it's about right. If it crumbles apart, it's too dry. If water runs between your fingers, it's too wet.
Bury the larva gently just below the surface. It'll dig down on its own within minutes.
Topping up substrate
Larvae eat their substrate continuously, converting it into frass (droppings). Frass looks like dark, granular soil and accumulates at the surface. When about half the substrate volume has been converted to frass, it's time to top up with fresh flake soil. You can either add fresh substrate on top, or carefully transfer the larva to a container with entirely fresh substrate.
Some keepers do a full substrate change every four to six weeks for fast-growing species. Others just top up as needed. Either approach works, but the key thing is not to let larvae sit in mostly-consumed substrate for too long. They'll slow down, lose weight, and eventually produce undersized adults.
Temperature
Most tropical species do well at 22-26C. This is warm room temperature, achievable in a heated house for much of the year. In winter, you might need a heat mat on a thermostat to keep things stable. Place the heat mat on the side of the container rather than underneath, so larvae can move away from it if they want to.
Temperature affects final adult size. Larvae reared at the warmer end of the acceptable range tend to develop faster but produce slightly smaller adults. Larvae kept a bit cooler grow more slowly but can end up bigger. Experienced keepers sometimes cool their larvae down slightly during mid-L3 to maximise adult size, though this is species-dependent and not something to worry about as a beginner.
The pre-pupal stage
When an L3 larva has finished growing, it enters a pre-pupal phase. It stops eating, its body may develop a yellowish tinge, and it becomes less active. It begins constructing a pupal cell: a smooth, oval chamber made from compressed substrate and frass.
This is when you stop everything. Do not open the container. Do not check on the larva. Do not move the tub. The pupal cell is structurally fragile, and if it cracks or collapses, the pupa inside is almost certainly going to die or emerge deformed.
The pupal stage itself lasts anywhere from three weeks to two months depending on species and temperature. After that, the adult beetle breaks out of the pupal skin and digs its way to the surface. Newly eclosed beetles are soft and pale; their exoskeleton needs several days to harden before they're ready for adult life.
Common problems with larvae
Most larval deaths come down to substrate issues. Bad flake soil (wrong wood species, insufficient fermentation, contamination) is the number one killer. The second most common cause is disturbance during pupation.
If a larva isn't growing, check temperature first, then substrate quality. A healthy larva in good substrate at the right temperature will eat steadily and gain weight. A larva that sits near the surface and doesn't burrow down may be rejecting the substrate.
Mites on larvae are usually grain mites, which aren't directly harmful but indicate the conditions are too moist or there's decomposing food nearby. Improve ventilation and check moisture levels.