|Photo by David R. Tribble
Monarchs are commonly parasitised by the protozoan Ophryocystis ktroscirrba. The spores of this parasite are ingested by the Monarch caterpillars and asexually reproduce within the host's intestinal tract. When ingested in high numbers, these parasites have been shown to have considerable detrimental effects on the fitness and migration ability of the Monarchs. A pair of researchers set out to explored how monarchs infected by parasites exhibited different patterns in their flight endurance, speed, deceleration ability, and loss of body mass over their relative migration distances.
They raised 100 Monarch caterpillars in captivity and infected them with parasitic O. ktroscirrba. When they metamorphosed into adult butterflies, they were placed on an automated flight mill apparatus which was used to calculate the above mentioned parameters. The flight trials found that parasitised monarchs flew 14% shorter distances, at 16% slower speeds, and lost almost twice as much body mass as unparasitised Monarchs undertaking the same journey.
Just like a viral infection may sap our energy, O. ktroscirrba has a similar resource-consuming effect on Monarchs. The parasites inhibit the host’s ability to absorb nutrients and utilise stored energy for powered flight. Along with parasite-induced damage to tissues, muscles and membranes, this makes powered flight a much more effort-demanding activity. The parasites live in clusters inside the host’s intestinal walls, leading to water loss and faster dehydration. This is thought to account for the greater loss in body mass with each kilometre flown, as compared to unparasitised monarchs. These constraints contribute to overall reduced larval survival rates, smaller adult body size, shorter lifespans, and therefore the inability to migrate efficiently or survive long enough to migrate or reproduce. It becomes a sheer battle of survival – the host throwing every defence at the rapidly reproducing parasites living inside it.
|Photo by Dwight Sipler
This insight into host-pathogen interactions also gives rise to possible areas of further research. Throw the effects of climate change and human activities into the mix, and we have the potential to develop a deeper understanding of the mighty Monarch, and its risk of parasitism. But let us not forget the importance of continuing research into the Monarch itself – its physiology and its behaviour. After all, we cannot truly study a parasite without first understanding its host!
Bradley, C. A. & Altizer, S. (2005). Parasites hinder monarch butterfly flight: implications for disease spread in migratory hosts. Ecology Letters 8, 290-300.
This post was written by Kate Ives