Evaluation of susceptibility to Dryocosmus kuriphilus Yasumatsu (Hymenoptera: Cynipidae) in Castanea sativa Miller and in hybrid cultivars

The presence of the gall wasp Dryocosmus kuriphilus Yasumatsu in North western Italy (Cuneo province) has effected both growth and yield of the tree. Although the biological control, that started in Italy by the introduction of the specific parasitoid Torymus sinensis Kamijo from Japan, appears to be the most promising and efficient method for reducing the pressure of the pest in chestnut forests, it is likely that this will not be not sufficiently effective to guarantee high yield and good nut quality in orchards. A study has been carried out for defining the different levels of susceptibility to D. kuriphilus on Castanea sativa Miller and hybrid cultivars. With this aim, in July 2004, 2005, and 2006 young plants (3 replicates) of 41 cultivars, including 7 inter-specific hybrids, were infested inside insect-proof screenhouses using a controlled number of cynipids. In the following spring the number, size and position of galls were observed and recorded. Results obtained so far indicate that all tested C. sativa cultivars are susceptible to the gall wasp. Among euro-japanese hybrids, Bouche de Bétizac and Marsol showed opposite reaction to the insect: no gall development was observed in Bouche de Bétizac while the highest levels of infestation was observed in Marsol. In 3 years of observations complete resistance to D. kuriphilus was thus found only in Bouche de Bètizac and the genetic bases regulating this resistance is currently under investigation.