Heat stress and host-parasitoid-endosymbiont interactions: Developmental timing and physiological mechanisms of thermal mismatch

Heat waves and other extreme climatic events are becoming increasingly common. Heat waves may disrupt the ecological interactions between insect hosts and their natural enemies, including parasitoids (insects that are parasites of other insects). The proposed studies use lab, greenhouse and field experiments to determine how heat waves alter the interactions between an insect agricultural pest, the Tobacco Hornworm Manduca sexta; its main natural enemy, the parasitoid Cotesia congregata; and a specialized symbiotic virus of the parasitoid that manipulates the physiology and development of the host. The studies test the hypothesis that the developmental timing of heat waves can generate a wide range of ecological outcomes, including successful parasitism and host death, parasitoid and viral death with host rescue and survival to adulthood, and suppression of the virus with production of abnormally large and long-lived caterpillars. The studies will also quantify how effects of heat waves on this host-parasitoid interaction alter hornworm consumption of its Tobacco hostplant, and the consequences for hostplant survival and seed production. These studies will provide a detailed experimental analysis of how heat waves can disrupt host-parasite interactions and result in a wide (but predictable) range of ecological outcomes. Because insect parasitoids are the major natural enemies of many insects, and parasitoids are widely used in biocontrol of important agricultural pests, understanding the consequences of heat waves for host-parasitoid interactions is critical for predicting effects of climate change on agricultural crops and their pests. The project continues outreach projects to both K-12 and adult learners, including hands-on science activities.

Extreme climatic events may be of particular importance for hosts, parasites and endosymbionts, because the developmental timing of their life stages is often intricately intertwined. How do physiological differences among hosts, parasites and symbionts affect their responses to thermal events throughout the life cycle, and determine the ecological outcomes of species interactions in complex thermal environments? The proposed studies address these questions using one model system, involving an important agricultural pest and its major control agent: the herbivorous insect host, Manduca sexta; its specialist Braconid wasp parasitoid, Cotesia congregata; and the parasitoid's endosymbiotic polydnavirus, CcBracovirus (CcBV). The proposed studies use an integrated set of lab, greenhouse and field experiments to explore the hypothesis that, depending on developmental timing, short sublethal heat stress events can produce a wide range of outcomes for parasitized caterpillars—from successful wasp emergence and host death, to complete wasp mortality, abnormal host phenotypes, and even successful host metamorphosis ('host rescue'). These studies will: A) determine how developmental timing of heat shocks following parasitization alters gene expression of the virus; growth, development and survival of the wasp; and immune responses, developmental physiology, and success of the host caterpillar; B) evaluate whether early heat shocks can rescue caterpillar hosts from parasitization; C) quantify how parasitization and heat shock alter herbivory by caterpillar hosts, and its consequences for hostplant success; D) use experimental field gardens to quantify thermal environmental variation and heat events, and document their consequences for thermal physiology, wasp emergence and host success.

This award was supported by the Integrative Ecological Physiology and the Symbiosis, Defense and Self-recognition programs in the division of Integrative Organismal Systems, Biology Directorate.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.