Summary: Journal of Animal Ecology 2008, 77, 120126 doi: 10.1111/j.1365-2656.2007.01305.x
© 2007 The Authors. Journal compilation © 2007 British Ecological Society
Blackwell Publishing Ltd
Host plant species affects virulence in monarch butterfly
Jacobus C. de Roode*, Amy B. Pedersen*, Mark D. Hunter and Sonia Altizer*
*OdumSchoolof Ecology,Universityof Georgia,AthensGA306022202,USA;andUniversityof Michigan,Department
of EcologyandEvolutionaryBiologyandSchoolof NaturalResourcesandEnvironment,1141NaturalSciencesBuilding,
830 North University, Ann Arbor, MI 481091048, USA
1. Studies have considered how intrinsic host and parasite properties determine parasite virulence,
but have largely ignored the role of extrinsic ecological factors in its expression.
2. We studied how parasite genotype and host plant species interact to determine virulence of
the protozoan parasite Ophryocystis elektroscirrha (McLaughlin & Myers 1970) in the monarch
butterfly Danaus plexippus L. We infected monarch larvae with one of four parasite genotypes and
reared them on two milkweed species that differed in their levels of cardenolides: toxic chemicals
involved in predator defence.
3. Parasite infection, replication and virulence were affected strongly by host plant species. While
uninfected monarchs lived equally long on both plant species, infected monarchs suffered a greater
reduction in their life spans (55% vs. 30%) on the low-cardenolide vs. the high-cardenolide host plant.