Milkweed is a vital food source for the milkweed tussock moth caterpillar, in addition to monarch caterpillars. The presence of these caterpillars, as well as other creatures, can lead to various issues in gardens, such as black mold on the milkweed. It is unclear whether the presence of tussock moth caterpillars and monarch caterpillars on milkweed leads to conflict between the two species.