I imagine that the smaller caterpillars enjoy the new tip growth of my tropical milkweed because it is so tender. I keep a close eye on the plants and collect the caterpillars if I see them. I need the plants to flower and create pods, so I have seed for the following year. Of course this constant collecting is adding up... We get at least 20 to 25 new caterpillars everyday in our garden.
This photo shows a normal haul from a garden inspection. These new cats are placed on some picked common milkweed stems. Moist paper towel is placed around the stems to provide moisture and prevent fast drying of the stems. We also will wrap the stem of individuale leaves this way to keep them fresh longer. I read that MW does not actually suggest doing this, as the moisture could create mold/fungus in the container. It works for us, but everyone's conditions are different, and always be careful with trying new ideas. What will work for one person could cause trouble in another situation. We place leaves with eggs in plastic containers (about shoe box size) and when we run out of those we use standard plant trays with plastic domes. The trays with the drainage holes provide some air circulation.