Host plant chemistry as a mediator of the interactions between plants, herbivores and the natural enemies of those herbivores is a key concept in evolutionary ecology. Recently, higher trophic levels (and therefore predation) have assumed importance in accounting for population dynamics and evolutionary relationships between plants and insects. Rigorous tests in model systems are still needed to understand the intricacies of tritrophic interactions and, in particular, to clarify the role of chemistry as a mediator of direct and indirect interactions between different trophic levels. This collaborative project will examine how the sequestration of plant chemicals mediates multi-trophic interactions with special reference to an understudied group of natural enemies, wasp and fly parasitoids. A model system of plants containing iridoid glycosides, three pairs of closely related caterpillar species - one species in each pair that does and one that does not sequester iridoid glycosides - and their wasp and fly parasitoids will be used to investigate the role of sequestered plant chemicals in herbivore-parasitoid interactions. Two sets of laboratory experiments will be initiated to examine the effects of chemical sequestration on parasitoid success. Chemical content of caterpillar diets will be manipulated by painting plant extracts onto leaves of caterpillar host plants.
Broader impacts: This will be one of only a few studies to examine the role of sequestered chemicals on parasitoid success. The investigators have extensive experience training undergraduate and graduate students in all the techniques and experimental methods described in the proposal and will continue their active involvement with important training programs such as Research Experience for Undergraduates (REU), the Ecological Society of America's SEEDS program for minorities, the Louisiana Alliance for Minority Participation, the University of Colorado's Undergraduate Research Opportunities program and Summer Minority Access to Science and Technology program, and Fulbright's LASPAU program for training faculty from other countries.
? Two major goals of ecology and evolutionary biology are: 1) to elucidate the roles of plant chemistry and predators as factors that control herbivore populations, and 2) to understand how these forces affect the evolution of specialized feeding by insects. Results from this project contribute to both of these goals via experiments with model systems of plants and herbivorous insects and developing novel research tools for studying plant-insect interactions. We developed methods to investigate several different systems including plants containing toxic chemical compounds (iridoid glycosides), several different caterpillar species feeding on those plants, and some of the predators that attack those herbivores. The predators that we examined are termed parasitoids – insects that lay their eggs on or in other host insects and whose larvae consume those hosts. Parasitoids rank high among the most important sources of insect mortality in many ecosystems, thus research on parasitoids is important for understanding phenomena such as forest insect outbreaks or major insect-driven losses of agricultural production. An example of a method we developed and utilized for the NSF funded research was a simple but powerful technique to simulate parasitoid attack (injection of dyed glass micro-beads) that has proven invaluable to our investigations as well as others who investigate parasitoid biology. This research combined traditional approaches to plant-insect interactions with new techniques in plant chemistry, careful experiments, a strong statistical approach, and thorough natural history. We trained two post-docs, eight graduate students, and many undergraduate students to successfully integrate these approaches. We have published or in press over 20 papers that acknowledge support from this grant and have presented results from the research funded by this project in over 25 different seminars and papers at professional meetings. The main results of this research demonstrated that the chemical make-up of the plants on which caterpillars feed can influence the immune response of those caterpillars to invading organisms such as pathogens and parasitoids. Specifically, the plant species on which a caterpillar feeds may affect the insect’s ability to combat invading pathogens and parasitoids. These effects may be due to the chemical compounds found in the plants as well as the ways in which the caterpillars process these compounds. We have also found that the ability of some caterpillar species to store those compounds, which makes them toxic to potential predators, may also have effects on the caterpillar’s immune response and ability to deal with pathogens and parasitoids. Results from this research also showed that increasing the amount of plant compounds stored by a host caterpillar can have a negative effect on that host’s immune response. In addition, our data from a natural system demonstrated that for some parasitoids, sequestering hosts may provide "enemy free space" for the parasitoids: parasitoids developing inside toxic caterpillars are protected from being eaten by predators of those caterpillars, who would also consume the parasitoids developing inside the caterpillars. These novel findings are important for theory on chemical interactions between plants, herbivores, parasites, and predators, which has been slowed by a dearth of experimental or observational studies. Contributions from this project have allowed us to produce more synthetic papers on chemical mediation of plant-insect-natural enemy interactions. Another ancillary result from the research is our findings that the natural products we studied, iridoid glycosides, act synergistically in their effects on herbivores and parasites. The effects on insects of plant compounds in mixtures were far greater than expected based on effects of individual compounds. There is a long standing interest in such chemical synergy from fields as different as pharmacology, organic chemistry, ecology, and evolutionary biology. However, our results contribute to a small group of studies that have demonstrated synergy for natural plant toxins against organisms that naturally consume plants containing these toxins. In the course of this research, we have used our results in presentations to the public, in the development of exhibits in a public museum, in the training of K – 12 teachers, and in working with volunteers recruited by Earthwatch Institute. We have worked with groups of pre-college and college students to train them in methods of scientific investigation and hypothesis testing.
