page_title: Monarch butterfly
section_title: Parasites
hierarchical_section_title: Monarch butterfly / Threats / Parasites
caption_attribution_description: English: Monarch chrysalis - Danaus plexippus and Pteromalid Wasp, Meadowood Farm SRMA, Mason Neck, Virginia. There are conflicting reports about whether the wasp can parasitize the chrysalis after it has hardened. I hope the monarch will be OK!
context_page_description: The monarch butterfly or simply monarch is a milkweed butterfly in the family Nymphalidae. Other common names, depending on region, include milkweed, common tiger, wanderer, and black veined brown. It may be the most familiar North American butterfly, and is considered an iconic pollinator species. Its wings feature an easily recognizable black, orange, and white pattern, with a wingspan of 8.9–10.2 cm A Müllerian mimic, the viceroy butterfly, is similar in color and pattern, but is markedly smaller and has an extra black stripe across each hindwing.
  The eastern North American monarch population is notable for its annual southward late-summer/autumn migration from the northern and central United States and southern Canada to Florida and Mexico. During the fall migration, monarchs cover thousands of miles, with a corresponding multi-generational return north. The western North American population of monarchs west of the Rocky Mountains often migrates to sites in southern California but has been found in overwintering Mexican sites as well. Monarchs have been bred on the International Space Station.
context_section_description: Parasites include the tachinid flies Sturmia convergens and Lespesia archippivora. Lesperia-parasitized butterfly larvae suspend, but die prior to pupation. The fly's maggot lowers itself to the ground, forms a brown puparium and then emerges as an adult.
  Monarch chrysalises are parasitized by pteromalid wasps, specifically Pteromalus cassotis. These wasps lay their eggs in the pupae while the chrysalis is still soft. Up to 400 adults emerge from the chrysalis after 14–20 days, killing the monarch.
  The bacterium Micrococcus flacidifex danai also infects larvae. Just before pupation, the larvae migrate to a horizontal surface and die a few hours later, attached only by one pair of prolegs, with the thorax and abdomen hanging limp. The body turns black shortly after. The bacterium Pseudomonas aeruginosa has no invasive powers, but causes secondary infections in weakened insects. It is a common cause of death in laboratory-reared insects.
  The protozoan Ophryocystis elektroscirrha is another parasite of the monarch. It infects the subcutaneous tissues and propagates by spores formed during the pupal stage. The spores are found over all of the body of infected butterflies, with the greatest number on the abdomen. These spores are passed, from female to caterpillar, when spores rub off during egg laying and are then ingested by caterpillars. Severely infected individuals are weak, unable to expand their wings, or unable to eclose, and have shortened lifespans, but parasite levels vary in populations. This is not the case in laboratory rearing, where after a few generations, all individuals can be infected. Infection with this parasite creates an effect known as culling whereby migrating monarchs that are infected are less likely to complete the migration. This results in overwintering populations with lower parasite loads. Owners of commercial butterfly breeding operations claim that they take steps to control this parasite in their practices, although this claim is doubted by many scientists who study monarchs.
caption: Pteromalus cassotis on monarch chrysalis
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