Vision in the Western Flower Thrips: analysis of their behavioural response to colour and study of their compound eyes : A thesis submitted in partial fulfilment of the requirements for the Degree of Doctor of Philosophy at Lincoln University

Abstract

The western flower thrips (WFT), Frankliniella occidentalis (Pergande, 1895), (Thysanoptera) are tiny insects with a body length of around 1 mm and commonly known as a worldwide, plant virus-transmitting pest causing economic losses in many important crops. Extensive research has been done regarding WFT biology and behaviour, including on their response to coloured traps for monitoring and pest management. However, little effort has been given in that case to understand how their visual system uses colour to guide their behaviour. Consequently, the different levels of response that have been recorded amongst published studies for WFT to the most attractive colours of yellow (570 - 590 nm), blue (450 - 470 nm), and UV-light (< 400 nm) remains unexplained at the fundamental level affecting applied outcomes. Effectively, intra-specific variation is accepted at the expense of suboptimal trap systems that remain difficult to improve across the species. Ultimately, using colour to optimise the attractiveness of traps, to the extent that they could be used to reduce populations as a means of control, continues to be elusive. Therefore, the overall aims of the thesis were to have a better understanding of how colour vision guides WFT behaviour in the context of attractive colours, including their spatial and directional movement and also to develop relevant knowledge about WFT eye anatomy as a fundamental aspect that contributes to behaviour. The debate regarding WFT behavioural response to colour was addressed here by isolating key variables such as light wavelength and intensity with the use of LEDs, as these factors are confounded in studies utilising coloured sticky and water traps which rely on light reflection. Experiments producing end point data of WFT landing on a sticky LED surface were conducted in highly controlled conditions in a wind tunnel where thrips were free to fly. With the laboratory colony of WFT used for this work, the results confirmed that WFT landings varied with colour, with UV-A and yellow obtaining higher landings than any other evaluated colour, including blue. No significant effect was found when intensity of the LEDs was increased except for UV-A light which had a decrease in number of landings with higher light intensity. With a focus on WFT attraction to blue and yellow, it was shown here for the first time that WFT response to these colours can vary intraspecifically when evaluated under the same experimental conditions and methods. This suggests that population effects may also be the source of variation causing the different conclusions found in the literature as to the most attractive colour. Real-time 3D tracking and high-speed videography was used to examine in more detail the effect of UV-A light as well as paired colours adjacent to each other, allowing landing and directional flight behaviour of WFT in close proximity to the LEDs to be recorded. Results showed that a positive interaction with the UV-A and yellow combination elicited the highest number of landings and flight time. Conversely, a negative interaction was found when yellow and blue were paired together with decreased number of landings and flight time. Tracking data also suggest that WFT might use different colours at different distances with differences in the landing pattern found between UV-A light and colours of longer wavelength like green and yellow. In addition, the data suggest that in the specific set up used in the experiments, a ‘decision’ to land is made at around 70-80 mm from the LED. Finally, several microscopy and imaging methods such as electron, fluorescence and light microscopy together with X-ray microtomography were used to describe the anatomy and morphology of WFT miniaturised eyes and reveal previously unknown information about their visual system. Together the data infer that WFT eyes are of the apposition type with 60 ommatidia on each eye. In addition, the ultrastructure of their eyes indicate various anatomical and structural adaptations from which it is argued that they likely prioritize light sensitivity over visual acuity. Despite the size constraints of WFT as a micro insect (with body length of ~ 1 mm), their compound eyes present a functional visual system composed of cornea, crystalline cone and at least seven photoreceptor cells that form a fused rhabdom. In addition, WFT present an acute zone in the fronto-ventral part of their eyes and possess putative filter pigments that may help in spectral tuning of underlying photoreceptors. Overall, new knowledge from the combined aspects of this study is discussed in the context of the role that vision and colour play in the ecology of WFT and as a contribution to studies focussed on improved applied outcomes.