Monarch Menace

Abstract

Parasites are known to harm their hosts, although from an evolutionary perspective it is not intuitively obvious why an organism that depends on another for its survival and transmission would risk killing its partner. Virulence may be an unavoidable outcome of a parasite using host resources to replicate, which causes damage and provokes costly immune responses; parasites would thus be expected to limit their replication to a submaximal level. To test this hypothesis, de Roode et al. collected data on the migratory North American monarch butterfly and a spore-forming protozoan parasite. As expected, greater parasite replication and greater spore loads reduced the probability of a butterfly emerging successfully from a chrysalis and also reduced the subsequent mating success and life span of female butterflies. The tradeoff was that female monarchs with the lowest parasite loads transferred spores to only 20% of their eggs, even though the fecundity of the parasitized butterflies was unaltered by spore load. Significant differences in virulence were observed between eastern (less tolerant of virulence) and western monarchs. Because eastern migration is 10 times longer, butterflies carrying highly virulent genotypes of parasite could die of the effects of parasitism en route. — CA