Effects of Rising Temperature on Interactions between Monarch Butterflies and their Parasites

Abstract

During the last few decades, global climate change has been one of the major drivers of species extinction, and has altered many ecological interactions. Monarch butterflies, which represent a cultural icon in North America, have been experiencing population declines due to several anthropogenic forces associated with environmental change. In addition, a monarch protozoan neogregarine parasite, Ophryocystis elektroscirrha (OE), aggravates their situation by decreasing the reproductive success and longevity of adult butterflies. Previous studies have shown that global warming can modify the interaction between parasites and their hosts. This study aimed to assess how a rise in temperature affects the virulence and transmission potential of OE. I hypothesized that an increase in temperature would result in shorter adult life spans and higher parasite loads. A total of 154 larvae were reared on artificial diet in incubators at three temperatures, 22°C, 25°C and 28°C until adult emergence or premature death. Approximately 30 larvae from each temperature were inoculated with 10 OE spores and 20 larvae were un-inoculated controls. Developmental time, pupal weight and spore loads were measured and data were analyzed using generalized linear models. Unexpected co-infection with an unknown pathogen resulted in high larval mortality in both OE-inoculated and control treatments, preventing an independent assessment of the effect of temperature on the monarch-OE interaction. As a result of co-infection, monarchs inoculated with OE were more likely to die during the larval stage. As expected, the total developmental time of butterflies declined at higher temperatures. Importantly, only 8 out of 42 OE-inoculated larvae that survived to adulthood produced OE spores, suggesting an antagonistic interaction between the unknown pathogen and OE. Overall, higher temperatures appeared to have a largely positive effect on monarch performance, since butterflies reared at warmer temperatures had shorter developmental times, resulting in a decrease in exposure time to the unknown pathogen. In conclusion, the unknown pathogen interacted in complex ways with OE, affecting monarch performance more than the increase in temperature itself. In this changing world, temperature sensitive host-parasite interactions could potentially be altered by global warming. However, they need to be studied within the context of how multiple pathogens will interact with their hosts under elevated temperature scenarios.