Modes of host plant resistance to pear psylla: A review

Host resistance in Pyrus to the pear psylla (Cacopsylla sp.) can be characterized under natural conditions in terms of the size of the pest population at specific times. Population size is the result of the cumulative effects of several modes of resistance, which have been experimentally investigated under controlled conditions in growth chambers or greenhouses. This paper summarizes the results of several investigations and compares relationships among modes of resistance and the types of assays used for their measurement, and draws conclusions concerning behavioral cues. The modes of resistance are ovipositional antixenosis, feeding antixenosis, and antibiosis, measured in terms of mortality, developmental rate, nymphal weight gain, and fecundity. Feeding antixenosis has been established as a key mode of resistance, as reduced feeding frequency is associated with reduced oviposition as well as increased mortality and delayed development. The results of assays can vary, depending on the type of assay and host phenology. Significant differences among genotypes can be detected with dormant plants. On young, but fully expanded, leaves in growth chamber experiments, the rank correlation between nymphal feeding and no-choice oviposition was 0.5 in a group of 73 genotypes from central Europe. In a free-choice experiment, the rank correlation between dormant and green-tip oviposition assays was non-significant, while correlations between these first two stages and expanded leaves were moderate, but not highly significant. These results indicate that phenological stage affects ovipositional preference. Integrated or independent studies of each mode of resistance are needed to accurately characterize host resistance.