Induced defense in tomato damaged by Spodoptera littoralis larvae

Abstract: Plants defend themselves against feeding insects via both a constitutive; always existing defense, and/or via an induced defense that is activated in response to the feeding damage. The aim of this study was to investigate the induced defense in tomato after damage by insect larvae. The hypothesis was that if larvae chose to feed on undamaged plants over damaged plants, they would do so due to an induced defense in the damaged plants. Moreover, the aim was to examine temporal aspects of induced defense, the difference in constitutive defense between genetically improved cultivars and wild tomato plants, the possibility of interplant communication and if induced defense compounds reduce the growth of larvae. Food preference bioassays and a larval growth experiment were made to test the hypotheseses. In feeding choice experiments, the larvae of the moth Spodoptera littoralis fed more on leaves from undamaged plants over leaves from damaged plants, which suggest that there was an induced defense in the damaged tomato plants that affected the larval behavior. The strongest effect of induced defense was found two days after the initial damage, but it was present until at least eight days after the initiation of the damage. Also in wild tomato plants, the induced defense was strong in recently wounded plants. There were no significant differences in constitutive defense between wild tomato and genetically improved tomato cultivars. When interplant communication was tested by larval feeding choice experiments, no difference was found between damaged or undamaged plants neither when these had grown together nor separately, which indicate that both airborne and root borne signaling can occur. Finally, when the larvae were forced to feed on either damaged or undamaged plants in a larval growth experiment, the larvae feeding on leaves from damaged plants showed reduced growth rate. This result indicated an induced defense of the damaged plants that affected the larval physiology.