Asian chestnut gall wasp: biological control and cultivar susceptibility.
Alien species are non-native or exotic organisms that may become invasive when they are introduced deliberately or unintentionally outside their natural habitats into new areas where they establish, invade and outcompete native species. One of the most recent examples of pests accidentally introduced in the European forestry environment is the Asian chestnut gall wasp, Dryocosmus kuriphilus Yasumatsu (Hymenoptera: Cynipidae). Native of China, it established as a pest in Japan, Korea, USA, Nepal, Canada, and since 2002 in many European countries, affecting chestnut orchards and coppices (Castanea spp.). The chestnut gall wasp is a univoltine and thelytokous species, the larvae induce the formation of galls that develop at the time of budburst suppressing shoot elongation and causing twig dieback. Severe reduction of fruiting with yield losses due to insect attacks have been estimated to reach between 65% and 85% in northern Italy. Since chemical control was not feasible and due to the successful experience carried out in Japan and USA, a classical biological control program was carried out in Italy with the int roduct ion of Torymus sinensis Kami jo (Hymenoptera: Torymidae). This exotic parasitoid, native to China as its host, is phenologically well synchronized with D. kuriphilus proving to settle successfully in the chestnut-growing areas in north-western Italy, and significantly reducing the gall wasp outbreaks after 10 years from its first release. In its native distribution, D. kuriphilus populations are controlled by natural enemies. A rich parasitoid community consisting of 67 hymenopteran chalcid species belonging to seven different families (Cynipencyrtidae, Eulophidae, Eupelmidae, Eurytomidae, Ormyridae, Pteromalidae, Torymidae) has been reported, but a rich parasi toid communi ty consist ing of 11 hymenopteran chalcid species belonging to five different families (Eulophidae, Eupelmidae, Eurytomidae Ormyridae, Torymidae) has been reported, but the at tack rates of these parasi toid species have remained low (typically less than 2%). The gall wasp lays eggs in all cultivars although with different degrees of preference, probably due to their different levels of attractiveness. The factors involved in susceptibility are still unknown, but it can be assumed that they include the bud size and bud texture and the presence of volatile substances in the cortex. Histological examinations revealed the presence of eggs and larvae at first instar even in resistant cultivars, such as 'Bouche de Bétizac', but larvae failed to reach the second instar due to a hypersensitive reaction by the plant, very likely controlled by a gene with simple Mendelian inheritance. To study the plant response and understand which factors can lead the plant to develop or not the gall, the plant transcriptomes of buds during the early stages of the interaction at budburst were sequenced. Sequences of genes putatively involved in the interaction are being studied to investigate the mechanism of response in the resistant cultivar and in Castanea sativa genotypes recently selected for the absence of infestation symptoms under controlled conditions.