Rearing Papilio rutulus
Having a nectar plant in the cage may not be enough. As insurance I manually fed the female butterflies at least every other day, sometimes once or twice a day, to stimulate more egg production. I figure they need a lot of extra energy to hold up under that hot lamp. I mixed up a 20% honey solution (4 parts water, 1 part honey) in a saucepan on a warm stove and dispensed it into vials. Then I froze the vials. When I needed to feed the butterflies I thawed out one vial at a time. Each thawed vial lasted for a few feedings when kept in the refrigerator. This way I don't have to keep mixing fresh batches of solution.
This technique is sometimes called force feeding. It doesn't look like I forced the butterflies to feed in this picture but in order to get them to feed on the honey-saturated cotton balls I had to hold them down and extend the proboscis with an insect pin until it touched the wet cotton. It's easier than it sounds: if they are hungry they will immediately become still when they taste the honey and you can let them loose to feed for awhile without them flying away. After feeding I wash off their face and feet so they won't be sticky, which really bothers them.