INSTAR SUSCEPTIBILITY OF THE MONARCH BUTTERFLY (DANAUS-PLEXIPPUS) TO THE NEOGREGARINE PARASITE, OPHRYOCYSTIS-ELEKTROSCIRRHA

The susceptibility of the monarch butterfly (Danaus plexippus) larvae to the neogregarine parasite, Ophryocystis elektroscirrha, was tested in the laboratory. Spore loads recovered from infected monarch butterf lies were directly related to the inoculum level, larval stage of the host, and spore age. There was a linear relationship between spores in gested by first instar larvae and spore concentration. Larvae feeding on leaves treated with 0, 50, 500, 5000, or 50,000 spores averaged 0, 0, 193, 457, or 1,255 spores, respectively, on the abdomens of the adu lt butterflies. When first, third, and fifth instar larvae were given 14.5 spores/mg of body weight, there was no significant difference in the spore load of the adults resulting from the first and third instar s. However, there were significant differences in the spore load from adults resulting from the first and third instars versus the fifth ins tar. In addition, 1-year-old spores were not as infectious as fresh sp ores. Our findings indicate that under field conditions, the first ins tar is most likely to become infected because one spore appears suffic ient to produce a detectable spore load in the adult. Older instars ar e less susceptible and have fewer opportunities to encounter sufficien t viable spores for infection to occur. Thus, vertical transmission ap pears to be the primary mode of parasite maintenance in natural popula tions of monarch butterflies. (C) 1997 Academic Press.