Abstract
A very large number of insects and fungi are specialised on using plants as a source of nutrients. Therefore, it is not seldom that herbivorous insects and
pathogenic fungi will attempt to exploit the same individual host plant. Direct but also indirect, plant-mediated interactions between both types of attackers
may take place in such a case. Indirect interactions may be the result of alterations in plant metabolism caused by insect infestation or pathogen infection.
Thus, previous attack by either type of antagonist may have a beneficial or detrimental impact on the other antagonist type.
The present study investigated such tripartite interactions between the host plant Chinese cabbage (Brassica rapa ssp. pekinensis) and the two crucifer
specialists Phaedon cochleariae (Coleoptera: Chrysomelidae) and Alternaria brassicae (Fungi imperfecti: Deuteromycetes). Both antagonist species attack
the leaves of their host. Interactions between the phytopathogenic fungus and the herbivore were investigated on a local (both antagonists on the same leaf)
and systemic scale (antagonists on different leaves).
The phytopathogenic fungus was found to have a negative effect on the herbivore´s performance and host selection behaviour. The effect was evident on a
local but not on a systemic scale. Larvae fed with leaves infected by the plant pathogen were significantly more susceptible to infection by the
entomopathogenic fungus Metarhizium anisopliae. Vice versa, herbivory did not result in measurable effects on the growth of Alternaria brassicae.
The ecological studies were supplemented by physiological assays to elucidate the mechanisms of the plant's response to insect attack and pathogen
infection. Concentrations of components of plant primary (water, C/N ratio, sucrose, total protein) and secondary metabolism (glucosinolates, anthocyanins,
peroxidase) were assessed. These studies showed that apart from other, yet unknown, factors the observed increase in peroxidase activity may be held
responsible for the adverse effect of fungus infected leaves on leaf beetle performance.
In addition to the studies on tripartite interactions between Chinese cabbage, A. brassicae, and P. cochleariae, dual plant-herbivore relationships were
investigated. The following questions were addressed: a) Does feeding by second-instar P. cochleariae induce resistance in Chinese cabbage leaves against
conspecific adult females and larvae? and b) Are there insect-derived factors, such as the larval defence secretion, larval faeces or regurgitant which may
directly or indirectly (by induction of plant responses) influence the feeding and oviposition behaviour of gravid P. cochleariae? The experiments showed
that herbivory by conspecific larvae had no effect on subsequent leaf beetle larvae feeding on the same leaves. However, adult beetles fed less and laid
fewer eggs on herbivore-damaged leaves compared with healthy leaves. Neither the larval defence secretion, which was found to adsorb to the leaf surface
in small quantities, nor the larval faeces or the regurgitant could be made responsible for the beetle's rejection of damaged leaves. Therefore, it may be
concluded that the observed preference of healthy leaves was the result of physiological plant changes induced by larval feeding activity.
The last chapter of this thesis reviews the progress made in the field of plant-fungus-herbivore interactions. Special attention is paid to studies that linked
ecological and physiological studies and to the spatial and temporal aspects of such interactions. |
