Development and distribution of a pathogenic neogregarine (Ophryocystis elektroscirrha) in monarch butterflies

The pathogenic neogregarine Ophryocystis elektroscirrha infects the hypodermal tissues of monarchs (Danaus plexippus). However, the life cycles, development, distribution, and pathology of these parasites are poorly documented in monarch hosts. To evaluate this, we exposed 15 2nd-instar monarch butterfly caterpillars to 200 O. elektroscirrha oocysts obtained from monarchs collected in Stillwater, Oklahoma. Exposed 4th instar caterpillars and/or 8 and 11 day old chrysalises were fixed along with unexposed controls and processed for histology. Additionally, the abdomen of one infected and one unexposed control adult monarch butterfly was prepared for scanning electron microscopy (SEM). Our results indicate that we did not find any developmental stages of O. elektroscirrha in caterpillar stages. Still, we detected immature and mature oocysts of O. elektroscirrha in 8 and 11-day-old chrysalises and/or adult stages of monarch butterflies. Both immature and mature oocysts were distributed on the cuticle of chrysalises, and mature oocysts were distributed on the cuticle of adult butterflies. The majority of oocysts were located on the surface of the cuticle and below the scales, but also surrounding the cuticle of the reproductive structures, including the egg chamber in female chrysalises, the aedeagus in male chrysalises, and the adult monarch butterfly. These observations provide a plausible mechanism for maternal and sexual transmission of this parasite. Additionally, our SEM investigation of the adult monarch butterfly’s cuticle revealed numerous oocysts embedded in the cuticle, indicating host pathology (cuticle damage). Because previous studies suggest that infected monarch butterflies lose weight more quickly than uninfected individuals, damage to the cuticle in adult butterflies by the oocysts of this parasite might explain the drastic weight loss observed in infected adult monarchs. The implications of our hypotheses are discussed in the context of monarch butterflies’ unique biology and conservation efforts.