Effects of host and nonhost substrates on probing and preoviposition activity were monitored for adult summerform pear psylla,Cacopsylla pyricolaFoerster. Tendency to initiate probing activity during a 15-min observation period was similar on a susceptible pear (Pyrus communisL. ‘Bartlett’), a presumed nonsusceptible pear (P. calleryanaDecne.), and apple, but was reduced on quince. Depriving pear psylla access to pear for 4–6 h or 14–16 h before the observation period increased the percentage of psylla that probed during the observation period; effects of host deprivation on tendency to probe were independent of substrate. Tendency to initiate probing onP. communiswas greater on lower leaf surfaces than upper leaf surfaces, which may suggest that probing is initiated in response to cues received at the leaf surface. The percentage of pear psylla that probed during the observation period was not affected by age ofP. communisfoliage. Mean probe duration and mean percentage of the 15-min observation period spent probing were unaffected by substrate; both measures increased linearly with amount of time that pear psylla were deprived of pear before the observation period. Mean percentage of the observation period spent in contact with the leaf surface was higher forP. communisandP. calleryanathan non-Pyrusspecies. Despite extensive probing activity on apple, very little preoviposition behavior was noted; similarly, age ofP. communisfoliage did not affect tendency to initiate probing behavior, but did affect preoviposition activity. These two results suggest that probing and preoviposition activities were released by different cues.