I am primarily interested in the influence of plant defenses on interactions with herbivores and their natural enemies within three trophic levels – essentially the interaction between defense and foraging in both herbivory and predation as indicated here:
I’m intrigued by how plants allocate resources to chemical defenses and how these defenses influence the performance of herbivores and their natural enemies. Of particular interest is the problem that plants face when they are exploited by specialist insect herbivores that appear to benefit from plant chemical defenses through sequestration and deployment against their own natural enemies. This is what I call the “lethal plant defense paradox.”
I work with a system based on milkweeds that invest in a range of toxic steroids known as cardenolides that act as sodium-potassium ATPase inhibitors in animals. Cardenolides are sequestered by a range of specialist insect herbivores of milkweeds (see images of some of the insects) and these sequestered defenses can be effective against many invertebrate and vertebrate predators. Milkweed cardenolides are inducible by damage and I am interested in the implications of constitutive and induced levels of both total and individual cardenolides on the physiology, behavior and ecology of specialist insect herbivores. In turn I am also interested in how shifts in physiology, behavior and ecology might impact natural enemies.
Figures of cardenolides and system (click here for images of the system)
3rd trophic level Cheilomenes lunata predator of A. nerii
2nd trophic level Aphis nerii sucking herbivore of milkweeds
1st trophic level Asclepias syriaca, the common milkweed.
Techniques that I use include analytical chemistry in the laboratory with HPLC, HPTLC, GC, spectrophotometry and pattern recognition for chemical fingerprinting. Laboratory experiments incorporate artificial diets, growth physiology and behavioral analyses. In the field and greenhouse I use a range of experimental manipulations of plant attributes to measure their impact on herbivore performance and natural enemy activity. Most recently, I am also trying to use plant molecular techniques to identify plant modules and follow the dynamics of variable chemical defenses through time and local space in relation to herbivore feeding behavior. Although my research is motivated by ecological questions, I like interdisciplinary approaches and try to use techniques and collaborations from a range of disciplines.
Current and potential research projects in my lab include the following:
a) Danaus plexippus – female and larval responses to cardenolides and latex
b) Impact of sequestered cardenolides on parasitoids and predators
c) Aphis nerii – life history, numerical, functional and behavioral responses to the distribution of cardenolides within milkweeds
d) Comparisons of responses by milkweeds and herbivores in different feeding guilds (chewing, sucking, mining and boring).
2. Bottom-up versus top down defenses in milkweeds
a) Induction of volatiles and steroids in response to herbivory in different feeding guilds and the role of natural enemies.
b) Induction and reduction of milkweed steroids and volatiles by Aphis nerii and the response of natural enemies: modeling and experimental analysis of dynamics in a tritrophic system with included and peripheral natural enemies.
3. Does the southern monarch butterfly migrate?
a) Use of steroid fingerprinting and field work to determine the seasonal dynamics of spatial movement in Danaus erippus in Bolivia and Argentina.
a) Comparison of steroid sequestration in Danaus plexippus, D. erippus and D. cleophile in North & South America and the Caribbean.
a) Effects of time on plant response to a constant herbivore sink for different sized sinks.
b) Effects of distance on plant response to a constant herbivore sink for different sized sinks.
a) Bt corn and unintended effects on monarch butterflies and white-tailed deer.