Oviposition-mediated interactions of tomato fruitworm moth Helicoverpa zea (lepidoptera: Noctuidae) with its host plant tomato Solanum lycopersicum and the egg parasitoid Trichogramma pretiosum (hymenoptera: Trichogrammatidae)

Open Access
Author:
Kim, Jinwon
Graduate Program:
Entomology
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
December 05, 2012
Committee Members:
  • Gary Felton, Dissertation Advisor
  • John Frazier Tooker, Committee Member
  • James Homer Tumlinson Iii, Committee Member
  • Dawn S Luthe, Committee Member
Keywords:
  • Plant-insect interactions
  • antiherbivore plant defenses
  • oviposition
  • defense priming
  • egg defense against egg parasitoids
Abstract:
An increasing number of reports document that, upon deposition of insect eggs, plants induce a variety of defenses to remove the eggs from plant tissue using plant toxic compounds or lending a hand of egg predators and egg parasitoids. In this research, I explored the interactions of tomato fruitworm Helicoverpa zea Boddie (Lepidoptera: Noctuidae) with its host plant tomato Solanum lycopersicum L. (Solanales: Solanaceae) and its egg parasitoid Trichogramma pretiosum (Hymenoptera: Trichogrammatidae) mediated via H. zea eggs laid on tomato plants. In Chapters 2 and 3, tomato’s defensive response to H. zea oviposition was investigated, and in Chapter 4, a novel defense mechanism of H. zea eggs against the egg parasitoid T. pretiosum was explored. The tomato fruitworm moth, H. zea, lays eggs on tomato plants and the larvae emerging from the eggs consume leaves first and then fruit to cause serious loss in the plant fitness. However, little is known about the oviposition-inducible defenses in tomato. When tomato plants were exposed to H. zea eggs, hydrogen peroxide (H2O2) was produced and pin2 expression was induced in the leaf tissue beneath H. zea eggs. H2O2 functions as a secondary messenger compound between early responses (e.g. activation of wound signaling pathway) and late responses (e.g. expression of defense traits such as protease inhibitors) in tomato, and pin2 is the gene encoding a well-studied induced defense trait of tomato, of which the expression indicates the level of induced defense in this plant. I also found that pin2 expression at the H. zea oviposition sites reached the highest right before the emergence of larvae from the eggs. More importantly, H. zea oviposition primed tomato antiherbivore defensive responses. Jasmonic acid (JA), the plant hormone responsible for the activation of defenses against insect herbivores, is quickly and transiently produced in plant tissue when plants are challenged by chewing insects or mechanical damage. The level of JA accumulation in plant tissue represents the level of antiherbivore defenses in the plant. Tomato plants previously exposed to H. zea oviposition induced higher levels of both pin2 expression and JA accumulation upon mechanical damage than when without oviposition pretreatment. These results suggest that tomato antiherbivore defenses are primed by H. zea oviposition in preparation for the future herbivory by neonates that emerge from the eggs. Unfertilized eggs of H. zea also elicited pin2 expression at the oviposition site, but did not prime the defensive gene expression, indicating that tomato is able to distinguish the real future threat (i.e. viable fertile eggs) from the false alarm (i.e. inviable infertile eggs). Tomato also showed a varietal variation in the oviposition priming. The tomato cultivar Better Boy that was used throughout this research primed pin2 expression by H. zea oviposition as stated above, while another tomato cultivar Castlemart failed to prime pin2 expression upon H. zea oviposition. More interestingly, in the JA-deficient mutant of Castlemart, def-1, H. zea eggs suppressed pin2 induction upon the following wound treatment. The effect of priming of defenses by H. zea oviposition on the performance of H. zea neonates was dynamic. In one experiment, H. zea showed decreased performance on tomato plants pretreated with H. zea oviposition, but in the other experiment, previous H. zea oviposition treatment did not influence the growth and survival of H. zea neonates. Interestingly, some neonates were found feeding inside of rachises of tomato plants, and they apparently grew faster than other leaf-eaters. This rachis-boring behavior of H. zea neonates might be one of the reasons of the inconsistent results and an adaptation of H. zea neonates to cope with the decreased quality of food plant by induced defense in tomato. In Chapter 4, I tested the hypothesis that H. zea unfertilized eggs may function as a lethal trap of T. pretiosum. H. zea virgin females laid significantly fewer unfertilized eggs than fertilized eggs laid by mated females in the absence of tomato plants. However, when tomato plants are present, H. zea virgin females laid as many unfertilized eggs on tomato plants as mated females lay fertilized eggs. It was also found that, when the population density is high, H. zea females may remain unmated in the presence of males and lay unfertilized eggs on the host plants, implying male mate choice. T. pretiosum egg parasitoids not only parasitized H. zea unfertilized eggs but also preferred them as the host to the fertilized eggs. Many of H. zea unfertilized eggs desiccated in a few days after parasitization by T. pretiosum, and the undesiccated eggs were almost completely parasitized, meaning the parasitization rate of T. pretiosum on H. zea unfertilized eggs is almost 100%. While T. pretiosum successfully emerged from 90% of H. zea fertilized eggs, only half of H. zea unfertilized eggs allowed successful development and emergence of T. pretiosum, mainly because of desiccation of the unfertilized eggs. These results demonstrate that H. zea unfertilized eggs can function as a lethal trap of T. pretiosum egg parasitoids. The results of this dissertation provide valuable insight into the nature of the interactions between tomato and H. zea and between H. zea and T. pretiosum mediated by H. zea eggs deposited on tomato plants.