Abstract/Summary

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.