Helping the Monarchs

Monarch Butterfly

Monarch Butterfly

The population of Monarch Butterflies overwintering in Mexico has been in decline over the past decades. Reasons for the decline are mulitple: degradation of the overwintering site, changes in climate including freezing temperatures that kill overwintering butterflies and droughts that reduce host plant availability in summer, changes in agricultural practices that have reduced populations of milkweed disease, predators and parasites and other factors.

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
DOI: 10.1098/rspb.2014.1734

About jjneal

Jonathan Neal is an Associate Professor of Entomology at Purdue University and author of the textbook, Living With Insects (2010). This blog is a forum to communicate about the intersection of insects with people and policy. This is a personal blog. The opinions and materials posted here are those of the author and are in no way connected with those of my employer.
This entry was posted in behavior, by jjneal, Endangered Species, Environment. Bookmark the permalink.

2 Responses to Helping the Monarchs

  1. Gary Shackelford says:

    Jonathan,

    Does your recommendation to remove tropical milkweed in late fall apply to South Florida (Miami area), where Monarchs are year-round residents? I assume that it would apply if a migrant population of Monarchs passes through South Florida, but I would appreciate a clarification.

  2. jjneal says:

    It is too early to make a recommendation for Florida. Florida is an interesting location as Monarchs populations breed through the winter and have some migrants from North along the East coast but are separate from populations in the Midwest. Asclepias curassavica is not native to Florida so the effect on Monarch populations is uncertain. Florida Monarchs do have higher parasite loads than populations that migrate long distances. Florida Monarchs also have better resistance to the parasite than migratory populations. For these reasons, effects in the Southern Midwest could differ from Florida. Florida populations may not be harmed by Asclepias curassavica. There is not enough information. However, in the Midwest, it has potential to disrupt the parasite cleansing process, and that should be considered seriously in the Midwest. We are moving species around the globe to new locations, often with little knowledge of the potential ecological effects.

    I will update as information becomes available.

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