thesis
Asymptomatic pathogen infection alters interactions at higher trophic levels
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Abstract
Plants and insects have intimate and complex relationships. However, other species, such as
plant pathogens, may change the way plants and their herbivores interact. Plant diseases
can change the host plant quality, alter physical and physiological traits and defensive
capability. These changes can have a consequential effect on insects associated with the
infected plant, thus altering interactions between species. However, the effects of these
interactions can quantitatively vary amongst individuals and may influenced by many factors
including the severity of plant disease (e.g. if the infection is symptomatic or asymptomatic).
To date, little is known on the effect of asymptomatic pathogen infection on insects at
higher trophic levels. My research aims to examine how patterns of interactions between
plants, microbes and insects change when plant pathogen infection is asymptomatic. I
examined the effect of asymptomatic plant pathogen infection on the life history and
behaviour of herbivorous insects, and its consequent effects on an insect community at
higher trophic levels. In the laboratory, I found that asymptomatic pathogen infection
altered plant traits and changed the life history and behaviour of an herbivorous insect and
its parasitoid. In the field, the effect of asymptomatic pathogen infection differed between
plant varieties and species of insect involved. Asymptomatic pathogen infection greatly
affects the assemblage and population dynamics of aphids on one variety of lettuce (Tom
Thumb), but no significant effects were found on Little Gem. However, both lettuce varieties
were less attractive to natural enemies when asymptomatically infected. The asymptomatic
pathogen infection also modulated the interaction of plants and insects on neighbouring
plant. More aphids were found on the uninfected plant neighbouring an asymptomatic
infected plant, and thus influencing the number of natural enemies observed. These findings
show the importance of asymptomatic pathogen infection to the ecological interactions at
higher trophic levels in the laboratory and field. Pathogen infection, even if asymptomatic,
can influence interactions between organisms. This is of considerable interest to both pure
and applied ecologists, and can affect the success of biological control programme in
agriculture systems