Pear psyllids are a serious insect pest of the cultivated European pear (Pyrus communis L.) in the production regions of North America and Europe. In North America, Cacopsylla pyricola (Förster) is the only psyllid pest of pears, whereas in Europe, C. pyri (L.) and C. pyrisuga (Förster) are also endemic. Cacopsylla bidens (Šulc) is endemic to parts of southern and eastern Europe as well as the Middle East. The insect has rapidly developed resistance to most of the pesticides used for control, and biological controls alone have rarely provided sufficient control for commercial production. The annual cost per hectare of chemical control is high. Cultivars with durable resistance would enhance the economic and environmental sustainability of pear production by reducing producer costs and pesticide use.
These species of pear psylla and their host apparently coevolved in Europe. Investigations of western European cultivars identified only a few moderately resistant cultivars, including ‘Spina Carpi’ (Quarta and Puggioni, 1985), ‘Triomphe de Ronchin’, ‘Joséphine des Malines’, ‘Doyenne du Poitiers’, and ‘D’Août Lamer’ (Robert et al., 2004, 2005). Higher levels of resistance have been identified in the small-fruited Asian pear species, P. betulifolia Bunge, P. calleryana Decne., P. fauriei Schneid., P. ussuriensis Maxim., and P. × bretschneideri Rehd. and in a few genotypes of the European “snow pear,” P. nivalis Jacq. (Quamme, 1984; Westigard et al., 1970), and in P. ussuriensis × P. communis hybrids (Harris, 1973; Harris and Lamb, 1973; Pasqualini et al., 2006; Quamme, 1984). Pomologists and breeders have also been collecting wild germplasm and landrace cultivars to identify diverse sources of resistance for use in the development of new psylla-resistant cultivars (van der Zwet et al., 1987, 1989).
The behavioral and physiological responses of arthropod pests to resistant host genotypes are termed modes of host plant resistance as distinct from the underlying mechanisms of resistance. Major modes of host plant resistance are 1) antixenosis, which can be observed in terms of disruptions to insect orientation and settling (i.e., host finding) and inhibition of feeding and oviposition; 2) antibiosis, observed as delayed development, increased mortality, and reduced fecundity and egg hatchability; and 3) tolerance or the ability to withstand infestation without damage (Painter, 1951). The term antixenosis was introduced by Kogan and Ortman (1978) to define the inhibitory effects of the host plant, whereas nonpreference refers to behavior exhibited by the pest. Feeding antixenosis has been established as a key mode of resistance, because reduced feeding frequency is associated with reduced oviposition as well as increased mortality and delayed development (Bell, 1991; Bell and Puterka, 2004; Butt et al., 1988, 1989; Puterka, 1997; Robert et al., 1999).
Observation of breeders’ selections in the orchard suggested that some genotypes were at least moderately resistant to pear psylla (unpublished data). The purpose of this research was to assess the degree of resistance in breeders’ selections in a controlled experiment. The overall goal was to select the most promising parents for further breeding and cultivar development.
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