Behavioural and olfactory responses of pest insects to endophyte-infected (Epichloë festucae variant lolii) perennial ryegrass (Lolium perenne):

Abstract

Asexual fungal endophytes (Epichloë) colonise agricultural grasses (Poaceae) in an interaction which provides host plants with protection against herbivorous insects. Despite 40 years of research there is still much we do not know about these complex interactions. A major gap in our knowledge is an understanding of the mechanisms involved in perception of endophyte by host-searching insects. It has, perhaps, been assumed that perception of endophyte is mediated by ingestion of endophyte-derived alkaloids, resulting in a malaise and an avoidance response. Although a post-ingestional malaise is one theory it is also feasible that insects detect endophyte via sensory perception before ingesting plant material. Sensory perception involves olfactory and/or contact (gustatory) chemoreception and is often referred to as an insect’s ‘sense of smell and taste’. I explored the mechanisms involved in insect perception of the endophyte, Epichloё festucae variant lolii (Latch, M. J. Chr. & Samuels, Hypocreales: Clavicipitaceae), in perennial ryegrass (Lolium perenne Linnaeus, Poales: Poaceae) hosts.

When presented with a choice a root aphid, Aploneura lentisci, was not deterred from endophyte-infected (AR37 strain) ryegrass in a host-preference assay and thus appeared to be unable to initially (24 h) perceive endophyte, demonstrating that negative effects of endophyte are not always associated with initial perception and avoidance responses. Further olfactometer experiments demonstrated that under the experimental conditions I used, apterous nymphs were unable to utilise olfaction during host-searching, suggesting that this morph cannot perceive host plants before contact is made with the plant. In contrast to this, olfaction was an important sensory mechanism for Argentine stem weevil adults (ASW, Listronotus bonariensis) which employed olfaction to locate host plants and distinguish between undamaged and herbivore damaged hosts. Olfaction is mediated by plant volatiles and both endophyte and herbivory were shown to alter the blend emitted by perennial ryegrass in this study.

ASW perceive certain endophyte strains and I propose a role of contact chemoreception in perception. Four lines of evidence were presented that support this theory. In the olfactometer bioassays; (1) there was no evidence that ASW avoided the odour blend released by endophyte-infected ryegrass before (AR1 or common-toxic) or after (AR1) plants had been damaged by conspecific insects. In the whole plant choice experiment; (2) there was no evidence that ASW utilised precontact cues (olfaction and vision) to orient away from endophyte-infected (AR1) plants from the outset; (3) ASW showed a strong aversion to endophyte-infected plants with just eight of 45 weevils observed feeding on AR1-infected plants and only one weevil feeding on both hosts during the observational period. In comparison, 32 weevils were observed feeding on endophyte-free plants (4 did not feed); (4) grooming of chemosensory appendages was only observed in weevils enclosed with endophyte-infected plants (AR1 or common-toxic) in the no-choice experiment.

This thesis has established a framework, based on investigations of pre- and post- contact behaviour, for investigating mechanisms of insect perception of endophyte and this can be utilised in future studies. Furthermore, this study has identified effects of endophyte on behaviours that have not previously been reported in the endophyte literature and this has provided an exciting area for future research.