Monarch butterflies are protected from predation by birds due to the toxic chemicals they sequester from their milkweed host plants. The interactions between birds, monarchs and chemicals in milkweed are well known. Lesser known are interactions with the protozoan parasite, Ophryocystis elektroscirrha, that can infect monarch butterflies and shorten their lifespan. The parasites are passed from mother to egg. Studies by Lefèvre and colleagues* have shown that the parasites produce fewer spores and cause less harm to larvae feeding on milkweed species with high cardenolide content, such as the tropical milkweed Asclepias curassavica, than on species with low cardenolide content such as the swamp milkweed, Asclepias incarnata.
Parasites can have an important role in host plant choice. Butterflies that choose a suitable host plant can survive the parasite. On plants that are otherwise suitable for monarch growth and development, monarchs may succumb to the protozoan parasite. These interactions further limit host plant range for many insects.
*(Lefèvre, T., Oliver, L., Hunter, M.D. & de Roode, J.C. (2010) Evidence for trans-generational medication in nature. Ecology Letters, 13, 1485–1493)