Winners and losers in toxic relationships affecting parasitoid wasps
Date
2020
Authors
Paul, Ryan Lee, author
Ode, Paul, advisor
Bjostad, Louis, committee member
Blumenthal, Dana, committee member
Naug, Dhruba, committee member
Journal Title
Journal ISSN
Volume Title
Abstract
Parasitoids are insects that develop on or inside another insect host, ultimately killing the host to complete their own development. Nearly every terrestrial plant-herbivore system has a suite of associated parasitoids. Success of parasitoids and their ability to regulate herbivore populations depends on interactions between multiple trophic levels. Of particular interest is the role of toxic chemicals that mediate interactions among species of plant-herbivore-parasitoid (tri-trophic) systems. My dissertation explores interactions involving toxins that affect parasitoids via multiple trophic levels. Plants produce a number of toxins to defend against herbivores that can also have consequences for parasitoids. In Chapter 2, I review the current published research on plant defense toxins and parasitoid interactions. The effects on parasitoids vary based on mechanism of interactions, study system, and life history of parasitoids. I first discuss the evidence of plant defense impacts on host immunity and the direct impacts of plant chemistry on parasitoid fitness. Then I explore the well-studied glucosinolate defenses found in the plant families Brassicaceae and Capparaceae as a case study on plant toxin and parasitoid interactions. I also review the current evidence for the commonly presented hypothesis that plant defense effects are reduced at higher trophic levels. Finally, I examine the recent advances in research on reciprocal influence of parasitoids on plant toxin expression. Despite the growing number of studies exploring the effects of plant toxins on parasitoids, the effects of variability in the expression of plant toxins on parasitoids has received little attention. Variability in plant toxins negatively affects herbivores but the influence of toxin variability on the ability of parasitoids to suppress herbivore populations is unclear. In Chapter 3, I studied the effects of variability in the plant defensive toxin, xanthotoxin, on development of a polyembryonic parasitoid of a generalist caterpillar. Parasitized caterpillars were fed artificial diets containing either different constant concentrations of xanthotoxin or multiple diets containing varying levels of xanthotoxin but with the same mean as the constant concentration treatment. Parasitoids performed worse on diets containing constant high levels of xanthotoxin. However, parasitoids were unaffected when herbivores fed on diets varying between high and low levels of xanthotoxin, compared to constant diets with the same mean. Herbivore suppression is therefore greatest when experiencing varying plant defense diets which strengthens bottom-up impacts and maintains equal top-down pressures. Parasitoids may also influence expression of plant defense toxins. Many solitary parasitoids reduce herbivore feeding by killing the host before it completes development. However, gregarious parasitoids often cause the host to feed more, removing the plant's advantage of attracting these parasitoids. Since toxins are costly to produce, plants whose herbivores are consistently attacked by gregarious parasitoids which increase herbivore damage are expected to increase toxin production. In Chapter 4, I compared the induction response of glucosinolate defenses in Brassica rapa plants to feeding by caterpillars parasitized by either solitary or gregarious parasitoids. Plants produced increased concentrations of defensive toxins when fed upon by caterpillars parasitized by gregarious parasitoids than when unparasitized or parasitized by solitary parasitoids. By using caterpillars at the same earlier developmental stages, which feed the similar amounts, I show that plants respond to parasitoid identity rather than feeding amount. This research demonstrates the unique response plants can have to herbivores attacked by parasitoids with different life histories. Toxins in tri-trophic systems are not only used by plants to defend against colonizing insects, but also by parasitoids in competition. A single herbivore species is often attacked by more than one parasitoid species (multiparasitism). When multiparasitism occurs, the larval parasitoids of the different species must compete for control of the host, usually to the death. In one such system, the solitary parasitoid Cotesia rubecula generally outcompetes the gregarious C. glomerata when they share a host. In the final chapter (Chapter 5), I explore the role of oviposition fluids as a source of toxins in larval competition between C. rubecula and C. glomerata. Oviposition fluids are injected along with eggs into a host by adult C. rubecula. These fluids are responsible for many physiological changes in the host, but their role in competition has received little attention. I injected caterpillars parasitized by C. glomerata with individual oviposition fluids from C. rubecula without a C. rubecula egg or larva. Many C. glomerata individuals were deformed in caterpillars injected by C. rubecula oviposition fluids. I demonstrate a physiological suppression mechanism that a C. rubecula larva uses to outcompete heterospecific larvae inside the host. Furthermore, I provide evidence for C. rubecula larvae using multiple mechanisms to suppress competitor development at several life stages. This study demonstrates a secondary competitive function beyond the physiological host changes for C. rubecula oviposition fluids and highlights the importance of competition in driving characteristics of parasitoids.
Description
Rights Access
Subject
parasitoid
plant-insect interactions
tritrophic interactions
plant defenses
intrinsic competition
secondary plant chemistry