Immunity in the field - how microclimate affects the immune response of Lepidopteran herbivores in the Sierra Nevada

Understanding sources of variation in the insect immune response is a core tenet of ecoimmunology. Abiotic factors such as temperature, humidity, and solar radiation are all components of the environment that can vary between microhabitats. For insect herbivores, plants are microhabitats, and variation exists within and between host plant species. Here, we investigated the impact of variation in plant microhabitats on lepidopteran immunity. Prior data found that parasitism rates were higher in microclimates with low temperatures and high humidity. While this may be an outcome of parasitoid preference, it may also be caused by a lower immune response in certain microhabitats. To test this hypothesis, we investigated the effect of variation in plant microclimates on the immune response in several species of lepidopterans located in the western U.S. state of Nevada. To obtain field-relevant immune responses, we conducted immune assays in the field. These included behavioral avoidance, cuticular darkness, hemocyte counts, and phenoloxidase activity. Microclimates were characterized according to their leaf temperature, humidity, and solar radiation. Results from this study indicate that specific aspects of microclimate, including leaf temperature and humidity, had the strongest effects on immune responses. Plants that experienced cooler temperatures and higher humidities had lepidopteran larvae with decreased hemocyte counts and phenoloxidase activity. Depending on the microclimate of the plant, lepidopteran larvae may have decreased immune functions that make them more vulnerable to parasitism.