Ultraviolet polarized light and individual condition drive habitat selection in tropical damselflies and dragonflies

Abstract

Artificial objects can polarize ultraviolet light sources to a higher degree than natural objects like water bodies. This can induce a strong attraction response by insects that use such cues as proxies of habitat suitable for reproduction. Visible range polarized light (VRPL) can create evolutionary traps for aquatic insects, but it remains unclear whether insects can use ultraviolet polarized light (UVPL) as a habitat selection cue or if UVPL pollution can create evolutionary traps for aquatic insects like VRPL can. Odonate (dragonflies and damselflies) insects require an aquatic habitat to perform their mating and egg-laying behaviours yet they also perform such behaviours on artificial surfaces (i.e. metal pieces). We measured the preference for UVPL versus VRPL via exposing three species of odonates (Enallagma praevarum, Ischnura denticollis and Sympetrum illotum) to experimental test surfaces differing in these visual cues and assessing behavioural preference via differences in mating behaviour, body condition (i.e. lipid and protein content and body size) and visual acuity (based on eye width size). Ischnura denticollis performed more diverse mating behaviours in association with the VRPL treatment, while S. illotum preferentially exhibited these behaviours in association with the UVPL. Ischnura denticollis individuals associated with the preferred habitat had lower lipid reserves, smaller body size and larger eyes, while habitat preference was unrelated to individual condition and morphology in E. praevarum and S. illotum. These results suggest intra- and interspecific variation in trap preferences, which are related to individual condition. They also show that UVPL is a cue that odonates use in habitat selection that has the potential to create evolutionary traps, suggesting conservation problems for aquatic insects that rely upon it to locate water bodies.