Plants, under the attack of herbivore insects and pathogens can employ a wide variety of defense mechanisms, ranging from physical to chemical barriers. Plants directly defend themselves through physical barriers such as trichomes, spines, or chemical barriers such as alkaloids or cardenolides. Plant defense strategy, can be constitutive, i.e. always present, or induced, i.e., by deterring or reducing herbivore performance only after previous attack. Moreover, plants can indirectly defend themselves by producing induced volatile organic compounds, which can serve as cues for predators and parasites of the herbivore to locate their host (i.e. tritrophic interactions). Indirect defense, widely accepted for aboveground parts of the plants, has only recently being acknowledged for root systems.
Three major working hypotheses will be tested during the one year study:
1) Ascelpias syriaca (milkweed) plants, and its host-specific Tetraopes tetraophthalmus cerambycid beetle larvae feeding on the roots will be used to asses a possible new belowground tritrophic interaction. Soil around plant patches will be analysed to detect presence of entomopathogenic nematodes (EPNs) which can be susceptible of attacking the larval herbivore. Since it has been recently shown that EPNs can use root insect-induced terpene to locate their host in a maize system, this is likely to be encountered in another natural system.
2) Since T. tetraophthalmus larvae do not readily hatch on a host-plant root, but have to travel through the soil for a first feeding session. EPNs density in the soil, in concomitance with traveling distance of the larvae can result in strong mortality factors, thus influencing plant performance.
3) Since plant’s defensive strategy is never likely to be a single trait, it is assumed that attack by multiple herbivores, above- and belowground can result in convergence on suites of covarying defensive traits. This has been recently being acknowledged for direct defense trait, using aboveground guild of herbivores and belowground herbivory by T. tetraophthalmus larvae attacking A. syriaca plants. Hypothesis is that indirect defence mechanisms represented by EPNs can also be part of the covarying defensive traits a plant has.
Present work will help to disentangle the understanding the evolution of plant defense strategies, and understanding of how natural communities are fashioned. Result can be used as a general framework to ask future questions ranging form plant-insect evolutionary strategies of coexistence to the more applied field of pest control management.