As far as I know, they can go up to 1-2 days without food, but they do need water. Mist the sides of the tent and ground. Do you have the bottom secured? Wasps, etc. can get in and eat them if you don't.
Also, I've never seen a butterfly use begonia as a nectar source. http://www.monarchwatch.org/rear/index.htm
[Scroll to near the bottom of the page]
"If you keep the adults, they should have a cage large enough to allow flight. Hanging or wooden frame cages are examples of cages that work well for adults. Adults do not need to be fed until the day after they emerge. After this they should be fed daily. They can be fed in a variety of ways. Fresh cut flowers can be placed in containers and put in the cage, a small dish or jar lid containing a sponge saturated with a 20% honey/water solution can be set in the cage, or fresh fruit such as watermelons, honeydew or cantaloupe melons can be cut and set in the cage. These should be changed daily to prevent fermentation. "Juicy-Juice" purchased from a grocery store can substitute for the honey water solution. We also have a non-fermenting artificial nectar mix available. To encourage feeding in any of the above methods, place the front feet into the solution and the butterfly will sometimes unwind the proboscis and start feeding. If the butterfly does not unwind its proboscis after several tries, place a probe or pin in the loop of the proboscis and pull the pin away from the head so the proboscis is extended and touches the honey solution. Once the proboscis is in the nectar solution, the butterfly is feeding."