Koller, Christina

Abstract The pear psyllid Cacopsylla pyri is the main vector of ‘ Candidatus Phytoplasma pyri’, the causal agent of pear decline, a disease associated with severe yield losses in pear production. Phytoplasmas are further known to interfere with their vectors metabolism; therefore, we investigated whether an infection with ‘ Ca. P. pyri’ is associated with changes in nutrient storage of the pear psyllids. Additionally, we measured whether the phytoplasma infection is associated with differences in flight performance in C. pyri , as the flight activity of the vector is a key component of phytopathogen spread. Using a flight mill, we quantified flight distance, duration, and speed, and combined qPCR-based infection detection with colorimetric assays to determine protein, carbohydrate, and lipid content from the same individual. Infected psyllids contained significantly higher levels of soluble carbohydrates than uninfected individuals. Lipid content was not affected by the phytoplasma infections but was found to be consistently higher in males than in females, while protein levels did not vary with sex or infection status. Despite infection-related differences in carbohydrate levels, phytoplasma infection did not significantly affect flight duration, distance, or speed. These results indicate that infection-associated shifts in bulk carbohydrate storage do not translate into altered flight motivation or performance, suggesting a functional decoupling between metabolic state and realised dispersal capacity under standardised laboratory conditions.

Abstract Comparing laboratory and field experiments is crucial to understand the ecological relevance of insect behaviour. In particular the use of reared and wild insects in behavioural experiments should be carefully considered and decided upon. In this study, we investigated colour preferences of the pear psyllid Cacopsylla pyri for different green colours, using both reared and wild-caught individuals. C. pyri is the main vector for ‘ Candidatus Phytoplasma pyri’, which is a phloem-limited bacterium that causes pear decline, a disease that damages the fruit and negatively affects the tree’s fitness. To investigate, whether the colour of non-symptomatic pear leaves may differ in relation to infection, spectral reflectance measurements were conducted, revealing no consistent differences. Based on these measurements, sticky traps were designed to match the natural reflectance of leaves in three different green colours. Laboratory experiments with reared C. pyri showed no clear preference for any of the three colours, though males exhibited a slight tendency towards light green (532 nm). In contrast, wild-caught psyllids tested under identical laboratory conditions strongly preferred light green traps, regardless of sex. Field experiments did not confirm these preferences. Non-target arthropods exhibited different responses: medium green (541 nm) and dark green (551 nm) traps captured higher numbers, highlighting that trap colour influences bycatch too. Over time, seasonal dynamics showed a moderate shift in psyllid preference towards medium green over time, whereas other arthropods increasingly preferred light green. Our findings demonstrate that the visual behaviour of psyllids is finely tuned to specific green colours, but that individuals reared in a laboratory may not fully represent behaviour of wild animals. The strong agreement between wild-caught laboratory assays and field experiments underscores the ecological validity of a green colour with a wavelength peak of 532 nm as an attractive trap colour. This emphasises the importance of validating laboratory results under natural conditions.