Hornworms are a great feeder insect but their cost does prohibit them being fed as a staple. They grow to quite a big size (often marketed as goliath worms) and can grow quickly even when bought small. They grow faster than silkworms. One inch horn worms can be 4 inches in a week, if the temps are good (room temperature of about 75-78F) and given plenty of food.
I haven't tried breeding these guys as i never seem to be able to get sufficient numbers for a good price. OR when the price is good, the vendor doesn't have many of them. The fact they need to eat so much to grow makes them more expensive to rear than silkworms. AT least with silkworms you can feed them natural foods (ie mulberry leaves) instead of the chow if you have the trees, with hornworms, as soon as they are fed their natural food, they are toxic. You have to feed them the chow.
If fed regularly your dragon will have softer stools, but this fine, as the stools are softer due to high water content instead of say, parasite loads. When mixed with greens and regular feeder insects, they provide a great variety and help with hydration.
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PHLdyPayne