Impact of atmospheric CO₂ levels on host choice of the pear psyllid, Cacopsylla pyri

The pear psyllid Cacopsylla pyri is a major pest in pear growing regions all over Europe (Garcia-Chapa et al., 2005). Infested pear trees can suffer directly from feeding damage due to massive psyllid colonization. Additionally, trees can be indirectly damaged in the event of mass occurrence, when enormous amounts of honeydew, especially from nymphs, are excreted and promote the growth of sooty mold fungus (Burckhardt, 1994). Even more threatening for pear production is the infection of pear trees with the phytopathogen ‘Candidatus Phytoplasma pyri’, a specialized cell wall-less bacterium transmitted by pear psyllids during phloem-feeding (Carraro et al., 1998). ‘Ca. P. pyri’ induces so called pear decline, a severe disease in pears causing high economic losses. Additionally, climate change poses new challenges to growers, nowadays, but consequences on plant – insect interactions are still rarely investigated. Carbon dioxide (CO2) is one of the most significant climate change drivers (Feng et al., 2020; IPCC, 2014). At first glance a fertilizing effect of increased CO2 concentrations could be expected, as plants convert CO2 into primary plant metabolites during photosynthesis. Indeed promoting impacts of CO2 on plant growth and yield as consequence of increased photosynthetic metabolism have been reported (Alae-Carew et al., 2020; Bisbis et al., 2018). Increased photosynthetic rates were also detected in Pyrus pyrifolia and resulted in higher soluble sugar content in pear fruits (Han et al., 2012), demonstrating that elevated CO2 concentrations can be expected to affect plant chemistry. Primary plant metabolites are resources for secondary metabolites, such as volatile organic compounds (VOCs). An increase in primary production can therefore also lead to changes in volatile emission of plants. Plant borne volatile organic compounds (VOCs) are important for host finding in insects. VOCs are commonly used by insects to locate potential hosts over distance, while non-volatile plant metabolites are important cues for evaluation of suitability of plants for feeding and reproduction (Schoonhoven et al., 2005). An essential question is whether possible changes in pear plant chemistry affect insect pests such as C. pyri and might even lead to increased insect infestations. Therefore, in this work, we investigated the impact of elevated CO2 concentrations in the atmosphere on the interaction of pear psyllids and pear trees. In two consecutive years we collected and analysed volatiles released from pear trees grown for a certain period of time under two CO2 levels (ambient: ca. 400 ppm and elevated, according to the predicted concentration for the mid-21st century: ca. 450 ppm CO2, respectively) in a Free-Air Carbon dioxide Enrichment (FACE) facility at Geisenheim University in Germany. In both years, potted pear trees were transferred to the FACE facility before bud break (end of February/ beginning of March) and were cultivated under specified CO2 conditions for three months, until the second volatile collection. The growth under elevated CO2 concentrations did not induce a detectable change in overall VOC patterns of pear trees. Nonetheless, the release of single compounds changed in response to CO2 increase. These differences in VOC release were inconsistent over time (phenology stages) and between study years, indicating interactions with other climate parameters, such as temperature and water availability. Specific volatile compounds or mixtures can be important for insect – plant interactions (Bruce and Pickett, 2011), therefore we further investigated, whether the detected changes of the pear tree volatiles affect the host choice of C. pyri. The odours of P. communis plants cultivated under elevated or ambient CO2 levels were offered simultaneously to C. pyri females in olfactometer experiments at two different plant growth stages (at the beginning and the end of the shoot development). In general, C. pyri females showed a high motivation to walk in the olfactometer, but didn’t show a preference for any of the pear odours. This result indicates that the impact of CO2 on the VOC release of pear trees is not relevant for host choice of C. pyri. To ensure that no impact of atmospheric CO2 on psyllid population of pear orchards can be expected, we conducted no-choice and binary-choice oviposition experiments with C. pyri. In accordance with olfactometer results, C. pyri females laid as many eggs on pear trees grown under elevated as on ambient CO2 concentrations. In conclusion, atmospheric CO2 concentrations impact the volatile emission from P. communis plants under field conditions, but these changes don’t promote or impair the host finding and acceptance of C. pyri. It needs to be further investigated, whether CO2 impacts psyllid fitness directly or indirectly due to changes in non-volatile plant metabolites. This knowledge will specify the prediction of pest pressure of C. pyri on P. communis in future scenarios.