Gallinger, Jannicke

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.

AbstractApple proliferation disease is caused by the phloem-dwelling bacterium ‘CandidatusPhytoplasma mali’, inducing morphological changes in its host plant apple, such as witches’ broom formation. Furthermore, it triggers physiological alterations like emission of volatile organic compounds or phytohormone levels in the plant. In our study, we assessed phytoplasma-induced changes in the phloem by sampling phloem sap from infected and non-infected apple plants. In infected plants, the soluble sugar content increased and the composition of phloem metabolites differed significantly between non-infected and infected plants. Sugar and sugar alcohol levels increased in diseased plants, while organic and amino acid content remained constant. As ‘Ca. P. mali’ is vectored by the phloem-feeding insectCacopsylla picta(Foerster, 1848), we assessed whether the insect–plant interaction was affected by ‘Ca. P. mali’ infection of the common host plantMalus domesticaBorkh. Binary-choice oviposition bioassays between infected and non-infected apple leaves revealedC. picta’s preference for non-infected leaves. It is assumed and discussed that the changes in vector behavior are attributable to plant-mediated effects of the phytoplasma infection.

AbstractPhytoplasmas are specialized small bacteria restricted to the phloem tissue and spread by hemipterans feeding on plant sieve tube elements. As for many other plant pathogens, it is known that phytoplasmas alter the chemistry of their hosts. Most research on phytoplasma-plant interactions focused on the induction of plant volatiles and phytohormones. Little is known about the influence of phytoplasma infections on the nutritional composition of phloem and consequences on vector behavior and development. The plum psyllidCacopsylla prunitransmits ‘CandidatusPhytoplasma prunorum’, the causing agent of European Stone Fruit Yellows (ESFY). While severalPrunusspecies are susceptible for psyllid feeding, they show different responses to the pathogen. We studied the possible modulation of plant-insect interactions by bacteria-induced changes in phloem sap chemistry. Therefore, we sampled phloem sap from phytoplasma-infected and non-infectedPrunus persicaandPrunus insititiaplants, which differ in their susceptibility to ESFY and psyllid feeding. Furthermore, the feeding behavior and development ofC. pruninymphs was compared on infected and non-infectedP. persicaandP. insititiaplants. Phytoplasma infection did not affect phloem consumption byC. pruninymphs nor their development time. In contrast, the study revealed significant differences betweenP. insititiaandP. persicain terms of both phloem chemistry and feeding behavior ofC. pruninymphs.Phloem feeding phases were four times longer onP. insititiathan onP. persica, resulting in a decreased development time and higher mortality of vector insects onP. persicaplants. These findings explain the low infestation rates of peach cultivars with plum psyllids commonly found in field surveys.