Abstract
Tabanid flies (Diptera: Tabanidae) are attracted to shiny black targets, prefer warmer hosts against colder ones and generally attack them in sunshine. Horizontally polarised light reflected from surfaces means water for water-seeking male and female tabanids. A shiny black target above the ground, reflecting light with high degrees and various directions of linear polarisation is recognised as a host animal by female tabanids seeking for blood. Since the body of host animals has differently oriented surface parts, the following question arises: How does the attractiveness of a tilted shiny black surface to male and female tabanids depend on the tilt angle δ? Another question relates to the reaction of horseflies to horizontal black test surfaces with respect to their surface temperature. Solar panels, for example, can induce horizontally polarised light and can reach temperatures above 55 °C. How long times would horseflies stay on such hot solar panels? The answer of these questions is important not only in tabanid control, but also in the reduction of polarised light pollution caused by solar panels. To study these questions, we performed field experiments in Hungary in the summer of 2019 with horseflies and black sticky and dry test surfaces. We found that the total number of trapped (male and female) tabanids is highest if the surface is horizontal (δ = 0°), and it is minimal at δ = 75°. The number of trapped males decreases monotonously to zero with increasing δ, while the female catch has a primary maximum and minimum at δ = 0° and δ = 75°, respectively, and a further secondary peak at δ = 90°. Both sexes are strongly attracted to nearly horizontal (0° ≤ δ ≤ 15°) surfaces, and the vertical surface is also very attractive but only for females. The numbers of touchdowns and landings of tabanids are practically independent of the surface temperature T. The time period of tabanids spent on the shiny black horizontal surface decreases with increasing T so that above 58 °C tabanids spent no longer than 1 s on the surface. The horizontally polarised light reflected from solar panels attracts aquatic insects. This attraction is adverse, if the lured insects lay their eggs onto the black surface and/or cannot escape from the polarised signal and perish due to dehydration. Using polarotactic horseflies as indicator insects in our field experiment, we determined the magnitude of polarised light pollution (being proportional to the visual attractiveness to tabanids) of smooth black oblique surfaces as functions of δ and T.
Highlights
Influence of surface tilt angle on tabanid attraction. Both degree d and angle α of linear polarisation of light reflected from a black test surface depend on the angle of reflection β determined by the tilt angle δ of the surface and the direction of view of the polarimeter relative to the normal vector of the surface as seen in Figs. 1 and 2: d is maximal when light is reflected under the Brewster’s angle θB (= arctan n, where n is the refractive index of the reflecting material, and θB is measured from the surface’s normal vector)
A polarisation-sensitive tabanid flying around a given tilted test surface perceives surface-reflected light with continuously changing degree and angle of linear polarisation as demonstrated in Figs. 1 and 2
The higher the degree of linear polarisation d of light reflected from a target, the larger is its attractiveness to tabanid flies (Kriska et al 2009; Egri et al 2013)
Summary
Tabanid flies (Diptera: Tabanidae) are visually attracted to shiny black targets (Bracken et al 1962; Thorsteinson et al 1966; Granger 1970; Roberts 1970; Browne and Bennett 1980; Allan and Stoffolano 1986; Sasaki 2001; Lehane 2005; Mihok and Mulye 2010; Krcmar 2013; Baldacchino et al 2014; Horváth et al 2014a). If the shiny black target lies above the ground and reflects light with high degrees and various directions of linear polarisation, the optical signal means host animal for female tabanids seeking a blood meal (Egri et al 2012a; Horváth et al 2017) Based on these two different kinds of polarotaxis in tabanids, efficient L-shaped polarisation horsefly traps have been developed (Horváth et al 2014b). One of these polarising traps consists of a horizontal and a vertical surface, both being shiny, black and sticky (Egri et al 2013): the horizontal component traps water-seeking male and female tabanids, while the vertical element exclusively captures host-seeking females. Since the body of host animals has differently oriented (horizontal, oblique, vertical) surface parts (Horváth et al 2010a), the same question arises: How does the attractiveness to male and female tabanids of a tilted surface depend on its tilt angle?
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