I had always heard that Monarch butterflies laid their eggs on milkweed, but I never knew exactly what milkweed looked like. There are several different varieties, but here in the Caribbean they have one called Sodom Apple or Calotropis procera. It has large pale grey-green leaves, pretty purple flowers, and strange puffy fruits. There are many of these plants growing all over Bonaire. We recognized them from our visit to the butterfly farm back on Aruba and easily found Monarch butterflies and caterpillars on them.
Above you see one of the fruits covered with aphids. The fruits are soft and empty except for the brown seeds which are dispersed by the wind via their attached white tufts of hair.
It's easy to find the caterpillars on the leaves; just look for the leaves with holes in them.
The toxins in the milkweed plant make the caterpillars, and the mature butterflies too, taste bad to birds and provide a natural defense mechanism for the Monarch.