All good points!
What I've found is the best way to prevent the spread of disease is collecting the caterpillars at the egg stage and floating the leaves/eggs in water with a tiny amount of bleach. You can even wash all your leaves this way but it's often not necessary.
I agree that cleanliness and being mindful of overcrowding are very important. The cages/containers should be cleaned at least once a day (this often only requires changing out a paper towel and adding fresh milkweed). Also good to sterilize the containers/cages periodically. Keep the eggs separate and the caterpillars separated by size etc. Keeping them in a covered/shaded spot outside is also good practice so they're exposed to the outdoor temps.
There's a very helpful video series on YouTube for anyone wishing to learn more about monarch rearing.🌿🦋 I often refer back to it myself. 😊
