2013 student Kaela Caballero worked with Professor Sonia Altizer and graduate student Alexa Fritzsche to investigate the effect of temperature on immunity to parasites in monarch butterfly larvae.
Recent studies indicate that environmental factors, particularly temperature, can affect the outcome of host-parasite interactions, with implications for predicting pathogen responses and parasite development to future climate change. In this study, we experimentally tested how ambient temperature affects both immunity and parasite infection focusing on monarch butterflies (Danaus plexippus) and a common protozoan parasite, Ophryocystis elektroscirrha (OE). Monarchs become infected as larvae when they eat milkweed plants covered in OE. Each year, populations of monarchs in eastern North America migrate from as far north as Canada to overwintering colonies in Mexico. Monarchs encounter a wide range of temperatures over the course of migration. For this reason, monarchs are an exemplary system for identifying relationships between temperature and immune defense. In this experiment, we explore the response of two measures of innate immune defense – phenoloxidase (PO) activity and the concentration of haemocytes in monarch larvae reared across a gradient of four temperatures: 23o C, 26oC, 29oC, 32oC. These temperatures represent a range normally experienced by monarchs during the summer months. Both PO activity and haemocytes are relevant indicators of immune competence and pathogen defense in a number of insect species. At the second instar stage, we placed 25 larvae into each of the four temperature treatments. At the fifth instar stage, we weighed and bled larvae for immune assays. We also measured the outcome of parasite infection for monarch larvae experimentally inoculated with OE and reared under the same temperature treatments. Warmer temperatures are known to accelerate growth and development in a linear fashion for a variety of insect species, but previous studies have indicated that rising temperatures are not positively correlated with improved immune competence. We hypothesized that both haemocyte concentrations and PO activity would be highest at intermediate temperatures, and would decrease with both cool and very warm temperatures, as has been shown for some other insect species. We also predicted that the severity of OE infections would be lowest at intermediate temperatures, corresponding to greater resistance of the monarchs. Our initial results reveal that haemocytes were greatest at 29oC, but decrease at both warmer and cooler temperatures implicating that some components of immunity perform best at intermediate temperatures.