Polarization vision and the physiological basis for trichromatic vision in Philaenus spumarius: Understanding host‐seeking behaviour in insect vectors for Xylella fastidiosa control

Abstract In European outbreaks, the meadow spittlebug Philaenus spumarius is the primary vector of the xylem‐limited bacterium Xylella fastidiosa . The mechanisms underlying host plant location by spittlebugs—critical for the transmission of associated bacterial pathogens—remain poorly understood, particularly with respect to the potential role of visual cues. Here, we investigated the visual system of P. spumarius through an integrated anatomical, optical, physiological, and behavioural approach to explore the potential role of vision in host‐seeking behaviour. Using microscopy, 3D reconstructions, and single‐cell recordings, we examined the structure and function of the compound eyes. Optical mapping revealed relatively low spatial resolution, with interommatidial angles of 4°–8° and somewhat smaller angles and finer visual sampling in central and anterior‐ventral regions. Intracellular recordings showed that photoreceptors are maximally sensitive to ultraviolet (UV), blue (B), and green (G) light, suggesting the potential for trichromatic colour vision. UV and B‐sensitive photoreceptors exhibited high polarization sensitivity (PS), with UV and B photoreceptors maximally sensitive to vertically and horizontally or obliquely polarized light, respectively. The physiological evidence indicating that polarized light is detected primarily by UV and B photoreceptors was complemented by the observation of orthogonally arranged microvilli in anatomical cross‐sections of the retina, which might belong to polarization‐opponent photoreceptor pairs with orthogonal sensitivity maxima to polarized light. Behavioural tests in a Y‐maze demonstrated that starved spittlebugs preferred linearly polarized over a diffuse visual stimulus. This preference disappeared when a yellow filter blocked UV and blue light, implicating the necessity of UV and B photoreceptors for the detection of polarized reflections. Our findings demonstrate that P. spumarius uses visual cues, including polarized light, which may aid in the visual detection of reflections from host plants. Understanding these visual mechanisms provides new insight into the ecology of this key vector species and may inform strategies to disrupt its host‐finding behaviour.