Title Thesis: Arriving at the right time: A temporal perspective on above-belowground herbivore interactions
Aboveground (AG) and Belowground (BG) herbivores are spatially separated, but can interact via their shared host plant. The outcome of AG-BG herbivore interactions can be mediated by many factors, including the relative timing of herbivory and the interaction of plants with other non-herbivorous organisms, such as mycorrhizal fungi.
In this thesis, I exposed plants to AG and BG herbivory at variable times and examined the responses of later arriving herbivores at both the individual and the population level. I also determined how the responses of AG herbivores can be mediated by the colonization of plant roots by symbiotic arbuscular mycorrhizal fungi (AMF). By using a perennial plant species Plantago lanceolate, I investigated the effects of sequence of AG (Spodoptera exigua) and BG (Agriotes lineatus) herbivore arrival at plants on the level of induced plant defense and on the performance of these herbivores (chapter 2). An intriguing finding was that AG herbivores when added alone reduced the food consumption and weight gain of their later arriving conspecifics, but the reduction disappeared if BG herbivores had been simultaneously introduced with the inducing AG herbivores.
In chapter 3, I investigated how the AMF species Funneliformis mosseae influenced the induction of plant defense compounds by an AG herbivore at different time points following exposure to the herbivore. I found that growth rates of the later arriving herbivore decreased as time progressed between initial induction and the second herbivory event by the later herbivore (a period of 0 to 8 days in my study). The decrease of herbivore growth corresponded with a gradual increase of defense compound catalpol over this period, but this was only observed in non-mycorrhizal plants. Hence, mycorrhizae may have suppressed further production of defense compounds in the plant because mycorrhizal plants had already higher levels of defenses prior to induction.
In chapter 4 and 5 I examined the impacts of timing of removing shoots, as a proxy for aboveground herbivory, on plant growth and defense as well as population dynamics of root-feeding nematodes below ground. Holcus lanatus plants were subjected to aboveground clipping and belowground nematode exposure at variable ages. I found plants regrew more shoots shortly after defoliation, and had lower concentrations of total phenolics and higher concentrations of N in the regrown foliage, whereas which did not depend on plant age at defoliation. However, plant root growth after defoliation showed an age-specific response. In response to these age-specific responses in root growth, Nematode species P. penetrans remaining in roots rather than in soil was higher in young plants than in intermediate and old aged plants, suggesting that root quality available to nematodes other than concentration of N and total phenolics tended to decrease as plants aged. However, these interactions may not operate via induced changes in plant N content and concentration of total phenolics that did not change with plant age or defoliation.
In my thesis I used a temporal approach to linking dynamics of AG-BG herbivore interactions with induced plant defense and tolerance. My thesis demonstrates that plant and herbivore performances can be altered by the timing of herbivory and this was particularly obvious for responses at the individual herbivore level. I conclude that the outcome of AG-BG herbivore interactions depends on the arrival time of the herbivores that modulate plant defense and tolerance but also on the presence of other organisms such as AMF that prime these plants. My thesis highlights the importance of timing of herbivory in assessing herbivore interactions and plant adaptations to these interactions in an AG-BG context.