Hornworms and silkworms get bought for the same reason: they are soft, they are wet, and animals that have gone off dry feeders will usually take them. They also get killed the same way, by being treated like mealworms and put in a tub with something to eat. Neither is a stored feeder. Both are caterpillars in the middle of growing, and holding them is a matter of slowing that down without letting the container turn against them.
The container is the cup they came in
Both species arrive in a purpose-built cup for a reason, and with both the usual mistake is upgrading them into something roomier.
Hornworms
A hornworm cup has food set in the bottom, a grid or mesh under the lid, and nothing else. The caterpillars hang from the grid and their frass drops clear of them. That separation is the entire design. Move them into a tub and they sit in their own waste, which in a humid container goes moldy fast.
Keep the cup upright, keep the lid on, and empty the frass rather than the caterpillars. Do not mist it. Hornworms get all the water they need from the food, and added moisture only pushes the cup toward condensation and mold. If the inside of the lid is beading, the cup needs more air, not less.
Silkworms
Silkworms are kept in a shallow ventilated container on their food, usually a mulberry-based chow, and the food is the substrate. The rule that matters is that everything in the container stays dry apart from the chow itself. Wet chow molds, mold kills the batch, and a silkworm container is unusually unforgiving about it.
- Ventilated lid, shallow container. Depth does nothing for either species.
- Take out old chow and frass regularly. This is the whole job with silkworms.
- Never mist either species. Their food carries the water.
- Do not crowd them. Both grow quickly, and a container that was roomy last week is not this week.
Temperature is a growth dial, and both animals are on a short clock
These are the two feeders where holding conditions and growth rate are the same question, because what ends a batch is size, not death.
Standard laboratory rearing of Manduca sexta, the tobacco hornworm, runs at 25 to 26 °C on an artificial wheat-germ diet. The larvae survive and grow across roughly 22 to 35 °C, but mortality climbs at the top of that range, with published figures of around 11% at 30 °C and around 50% at 35 °C. Suitable relative humidity is given as 30 to 60%; at 10 to 20% hatchlings and first instars die of desiccation.
Silkworms are reared cooler and wetter, and the published practice splits by age: young larvae at about 26 to 28 °C and 80 to 90% relative humidity, older larvae at about 24 to 26 °C and 70 to 80%. Normal growth is described across 20 to 28 °C, with the productive range 23 to 28 °C, and temperatures above 30 °C affecting the health of the worm directly.
Two things follow for a keeper. First, ordinary room temperature is inside the survivable range for both, and a cooler room slows both down, which is what you want if the batch is outgrowing the animal it is meant to feed. Second, those published humidity figures come from rearing rooms where the food is fresh mulberry, and reproducing 80% humidity in a small plastic cup of chow produces mold rather than healthy silkworms. Aim for the temperature, not the humidity.
Do not refrigerate either of them
The chilling trick that works on mealworms does not transfer. Neither species has a published cold-storage protocol, both are soft-bodied and actively growing, and keeper experience is consistent that a cold hornworm is a dead hornworm. If you need to slow a batch, use the coolest ordinary room in the house, not the fridge.
Never feed a hornworm you found in the garden
This is the one genuine safety point in the guide and it is worth stating plainly.
Wild hornworms feed on plants in the nightshade family: tomato, potato, pepper, eggplant and tobacco. Those plants carry defensive compounds, including nicotine in the case of tobacco. The caterpillar handles them perfectly well itself, and the cautious assumption is that some of that can pass to whatever eats it. Commercially produced hornworms are raised on an artificial diet instead, which is the reason they are sold as a feeder at all.
So: a hornworm on your tomato plants is not a free feeder. It is a different animal from the one in the cup, with a different diet history. The same logic applies to any wild-caught insect, and to leaving a bought hornworm on garden foliage to "top it up". If an animal has already eaten one and you are worried, that is a call to an exotics vet, not something a care guide can answer.
Feeding, and why the food is not interchangeable
Both species are specialists, and both are difficult to switch.
Hornworms eat the gel-style diet they arrive on. It goes in the bottom of the cup and it is topped up as they eat through it. They will take fresh nightshade foliage, which is exactly what you do not want for the reason above.
Silkworms eat mulberry leaf, or a mulberry-based chow, and very little else. A silkworm raised on chow does not reliably move to fresh leaf, and a silkworm on leaf does not reliably move to chow. Whichever your batch arrived on is the one to keep it on. Chow is normally prepared, cooled and then cut into strips, and it is offered fresh rather than left to sit; old chow is the mold problem again.
Gut-loading in the usual sense does not apply well to either. You are not adjusting the diet in a 24-hour window; you are feeding the only thing the animal will eat. What you can still do is dust immediately before offering. The Merck Veterinary Manual's position is that insects should be dusted with a high-calcium supplement immediately before being fed to a reptile, and that applies here as much as to a cricket. The nutrition table in our bearded dragon feeder guide lists silkworms, and explains why hornworms have no published figures to compare.
Hygiene is the whole silkworm game
Silkworms have a real disease literature behind them, because sericulture has been fighting the same pathogens for a very long time. Grasserie is caused by a nucleopolyhedrovirus, flacherie by a group of viruses and bacteria. Both are managed the same way, and the sericultural guidance is blunt about it: control is preventive, built on disinfection, rearing hygiene and removing and disposing of diseased larvae rather than treating them.
For a keeper holding a cup of feeders, that translates into three rules:
- Remove anything limp, discolored, shrunken or leaking as soon as you see it, and do not put it back.
- Do not return survivors from a failing container into a fresh one.
- Wash and dry anything reusable properly between batches.
Hornworms have their own version of this. A cup that has gone wet and moldy at the bottom takes the whole batch, and picking a few clean-looking caterpillars out of it into a new cup usually just moves the problem.
How long each keeps
| Hornworms | Silkworms | |
|---|---|---|
| What ends the batch | Growth past feeding size, then pupation | Growth, then spinning; mold and disease before that |
| How to slow it | Coolest ordinary room | Coolest ordinary room |
| Refrigerate? | No | No |
| Main daily job | Empty frass, top up food | Fresh chow, remove old chow and frass |
| Main failure | Condensation and mold in the cup | Mold on chow, then disease |
Neither is a feeder you stock up on. Both are bought close to the point of use, in a quantity the animal will get through, which makes them a poor fit for anyone trying to reduce how often they order. That is the trade for the thing they do well, which is getting food into an animal that has stopped eating dry feeders. Keepers of chameleons and bearded dragons generally buy them as a treat or an appetite lever rather than a staple.
When to write off a batch
Write off a hornworm cup when the food at the bottom has gone sour or fuzzy and the caterpillars are down in it rather than hanging from the grid. Write off a silkworm container when several larvae have gone limp or discolored in the same day, or when the chow is molding faster than you can replace it. In both cases the container goes in the bin rather than the cupboard.
A batch that arrives already collapsed, leaking or blackened is a different matter and is not a husbandry problem. Both species travel badly compared with roaches or worms, so open the box as soon as it arrives and check them straight away. Our guide on what to do when your feeders arrive covers the sequence, and our shipping page sets out the live arrival terms, including sending photos or video within 1 hour of delivery if bugs arrive dead.
Where to go next
Because both of these are short-window feeders, they work best alongside something you can hold for weeks rather than days. Setting up a feeder storage station covers running a short-window feeder and a long-window one in the same cupboard without one routine swallowing the other, and the wider feeder range is where to work out the pairing.