You walk out to check on your bin and something's wrong. Grubs are climbing the walls, piling up in the corners, crawling out the entrance holes and heading across the patio like they've got somewhere better to be. A few is normal. A whole wave of them at once is your bin trying to tell you something.
Before you panic, there's one thing worth sorting out first: are they running from the substrate, or are they just growing up? Because those are two very different situations, and only one of them is a problem. Here are the five reasons the fat little grubs decide to leave, most common first.
1. The substrate went sour
This is the big one. It's behind more mass exoduses than everything else combined.
BSFL breathe. The whole bin does. When the substrate gets too wet, or packs down too tight, air stops moving through it and it goes anaerobic, meaning the good aerobic bacteria die off and the stinky ones take over. That's when you get ammonia. It builds up fast in a wet, airless layer, and ammonia is toxic to your larvae. So they do the only thing they can do, they climb out of it or they die in it.
You'll know it by your nose before your eyes. A healthy bin smells earthy, kind of like a compost pile or wet leaves. A soured bin smells sharp and rotten, like a porta-potty on a hot day. If you're getting that ammonia bite, the grubs aren't being dramatic, they're evacuating.
The fix: stop adding wet food for a bit and mix in dry carbon to soak up the excess and open the material back up. Shredded cardboard, coco coir, wood shavings, chopped straw, spent brewery grain, all good. You're aiming for a substrate that clumps when you squeeze it but doesn't drip. If it drips, it's too wet.
2. You're overfeeding
This one usually causes number one, but it's worth its own spot because the fix is different.
It's easy to assume more food means bigger grubs, so you keep piling it on. But your larvae can only process so much at a time. Whatever they can't get to just sits there and rots, and rotting food in a pile is exactly how you get the wet, anaerobic, ammonia mess from above. Overfeeding also pulls in the pests you don't want, like houseflies and their maggots.
Here's how to tell: if the food you added is gone within about 12 hours, your colony is hungry and you can feed more. If there's still a bunch of it sitting there after a day or two, you're overfeeding. Back off and let them catch up. A rough starting point is feeding around 10 to 15 percent of the larvae's weight per day, but honestly the "is it gone yet" test will steer you better than any number.
3. It's too hot in there
Your grubs like it warm, but there's a ceiling. They're happiest somewhere around the upper 70s to lower 90s Fahrenheit. Push much past that and they start looking for the exits.
The tricky part is that a colony makes its own heat. A dense mass of feeding larvae runs a good bit hotter than the air around it, sometimes the core of the pile can climb past 100°F on its own. Add a hot afternoon on top of that and the middle of your bin can hit a temperature that never felt that hot to you standing next to it. This is also why your bin lives in the shade and never in the sun. Direct sun on a bin is just cranking the heat on a colony that's already generating plenty of its own.
The fix: get it out of any sun it's catching, give it some airflow, and if the bin is packed wall to wall with grubs, that ties right into the next one.
4. Too many grubs, not enough room
A bin can only hold so many larvae before it gets crowded, and crowding stacks up every other problem on this list. More grubs means more metabolic heat, more moisture, more food demand, and more competition for the same space. At some point the ones on the losing end just leave to go find somewhere less packed.
You'll see it as a bin that seems almost solid with larvae, wet even though you haven't overwatered, and warm to the touch. If that's what you've got, you've got a good problem, you're growing more than that bin can hold.
The fix: harvest some, or split the colony into a second bin. Giving them more room brings the heat and moisture back down on its own.
5. They might not be running from anything
This is the reason I said to check first before you panic.
When a larva is done growing, it doesn't stay in the muck. It empties its gut, its body darkens to a deep brown or almost black, it stops eating, and it goes looking for somewhere cool, dark and dry to become a fly. In the wild that means crawling away from the food. In your bin that means climbing the ramp and marching out the exit, exactly like they're supposed to. This is self-harvesting, and it's the whole point of a good bin design. It's a feature, not an emergency.
So how do you tell this apart from a real exodus? Look at the grubs that are leaving. If they're dark, firm and not really interested in food, those are prepupae doing their thing, let them go collect themselves in the bucket. If the ones leaving are still pale and cream-colored and clearly should still be down in the substrate eating, that's when you go back through reasons one through four, because those grubs are fleeing, not graduating.
If you're getting a real exodus, nine times out of ten it comes back to a wet, overfed bin that went anaerobic. Get some dry carbon in there, ease up on the food, keep it in the shade, and give them room. Do me a favor and check the smell first though, your nose will usually tell you what's wrong before anything else does.
