The worst pest is undoubtedly wasps, which eat the larvae (caterpillars) and eggs until late summer, when the wasps’ diet changes from protein to nectar. Some wasps have been deliberately introduced to control the White Butterfly.
Also, praying mantises will eat caterpillars, as will the Predatory Shield Bug (Cermatulus nasalis). Ants will also take eggs.
Disease can also be transmitted from caterpillars and will badly infect pupae – so if you are breeding Monarchs ensure that you keep their food supply in small, separate ‘islands’ in your garden rather than a concentrated area. In this way, any disease cannot spread as rapidly. If you keep caterpillars in containers try not to have them all in one container, and clean each container with a bleach solution before reusing.
If you have too many caterpillars, take some of them to where there is milkweed with no caterpillars; don’t let your plants get inundated with larvae.
