Development of color sticky traps for monitoring Cacopsylla pyri inspired by research on color preferences

Several species of phloem-feeding pear psyllids are the main vectors of the bacterial disease pear decline caused by the bacterium Candidatus Phytoplasma pyri. Furthermore, large psyllid infestations have been reported as the most important pest management problem of pear-growing regions in Europe (Jenser et al., 2010). For instance, the pear psylla Cacopsylla pyri (L., 1758) caused severe economic damage in pear orchards since the beginning of commercial pear production and it is considered the main pear pest in most parts of Europe (Berrada et al., 1996; Civolani and Pasqualini, 2003; Erler and Cetin, 2007). Psyllids are responsible for direct damage cause honeydew production by nymphs, leading to the growth of black sooty mould and overall nutrient removal and the transmitions of Pear decline (PD), induced by the phytoplasma 'Candidatus Phytoplasma pyri', one of the most devastating diseases on Pyrus communis in Europe and North America (Seemüller et al., 2011). Understanding the cues used by insects to select their hosts is essential to developing sustainable control strategies, particularly for plant disease vectors (Gross, 2013). However, color vision in insects is a factor in the biology of invertebrates that seems be neglected, while most of recent publications are exclusively related to the chemical communication that mediate host plant – phloem-feeding vector interactions (Gross et al., 2021) Plants reflect a diversity of visual cues; all of them are limited to the narrower variety of foliar pigments, essentially in the "green-yellow-orange" part of the light spectrum visible to insects. The spectral sensitivity of insect compound eyes ranges from 350 to 650 nm (near-ultraviolet to red) and thus includes shorter wavelengths than that of the human eye (Schoonhoven et al., 2005). Generally, species belonging to the Hemiptera are not attracted to visual targets that reflect or emit light < 400 nm or > 600 nm (Döring, 2014; Döring and Chittka, 2007). However, attraction to longer wavelength red light occurs in eucalyptus psyllids, Anoeconeossa bundoorensis, and Glycaspis brimblecombei, which specialize in red anthocyanic leaves (Farnier et al., 2014). Prokopy and Owens (1978) speculated that monophagous-oligophagous insects are more likely to be visual specialists than polyphagous insects. They further suggested that visual traps which mimic specific host-plant visual stimuli should help monitor monophagous-oligophagous insect pest populations on crops. If so, the monophagous pear psyllids should be well suited for monitoring with a visual trap (Westigard, 1979). Yellow sticky traps have been used as monitoring tool for pear psylla for more than 60 years. But these traps are very unspecific and trap a huge amount of unwanted bycatch, which includes high numbers of pollinators (Beers, 2012; Roubos and Liburd, 2008). In order to investigate the importance of color vision for pear psyllids and that way improving visual traps, the presented research aimed to investigate the color preferences of C. pyri based in the evaluation of specific wavelengths that could be attractive for this pest insect. To screen the insect color preferences, choice assays with different LED colors were performed in a small-scale choice arena. Over the six LEDs tested, a strong significant preference of C. pyri for dark green (532 nm) followed by light green (549 nm) could be observed. The other colors orange, yellow, light red and dark red attracted less psyllids than dark green. To mimic the LEDs tested in a field situation, newly developed traps were equipped with transparent-colored PVC sheets with the respective wavelength of the preferred LEDs. Red and completely transparent traps were also included as control. The experiment was conducted in a pear orchard cv. Williams Christ in Dossenheim, Germany. Field trials revealed that green traps attracted significantly more pear psyllids than red and transparent ones. In conclusion, we could show that the pear psylla C. pyri was attracted to green color, with a narrow and specific wavelength range, which was confirmed in both laboratory and field trials. The results are promising for improving psyllid monitoring, even at low population densities. Thus, visual cues should be considered and integrated into psyllid monitoring in future as part of integrated pest management strategies.