Satterfield and colleagues* identify another factor in Monarch decline: adults that do not complete migration to Mexico, but survive and breed in the Southern US. They studied infection with the protozoan Ophryocystis elektroscirrha. Infection rate by this protozoan declines in the fall and is greatly reduced in the overwintering population in Mexico. Infected Monarchs often lack the energy to complete the flight and are more likely to die during migration. Warmer winter temperatures make it possible for Monarchs to survive most winters in the gulf states, however, lack of food prevents caterpillars from developing. Lack of food eliminates the non-migrating Monarch population and cleanses it of disease.
Well meaning butterfly enthusiasts, attempting the aid the butterfly population, have planted tropical milkweed, Asclepias curassavica, in the southern US. These plants, mostly confined to gardens, do not die in fall like other temperate milkweed species. They maintain foliage throughout the winter and provide a host for non-migrating butterflies to lay eggs. This encourages butterflies not to complete their migration and allows butterflies infected with the protozoan to mate and potentially transmit the disease to their offspring. Availability of tropical milkweed may allow disease to persist at higher rates.
The Monarchs need a process that selectively removes infected individuals from the population so the Spring generation can start relatively disease free. Thus, people should be discouraged from planting tropical milkweed or remove it from their gardens in late fall so they do not create a disease reservoir that can infect migrating Monarchs on their return.
*Dara A. Satterfield, John C. Maerz, Sonia Altizer. Loss of migratory behaviour increases infection risk for a butterfly host. Proc. R. Soc. B:2015282 20141734. Published 14 January 2015