Effects of environmental parameters on the chestnut gall wasp and its complex of indigenous parasitoids

Abstract

The chestnut gall wasp (CGW), Dryocosmus kuriphilus, an invasive pest native to China, has caused severe yield and economic losses to chestnut production in Europe since its arrival in 2002. In Southern Italy, the complex of indigenous parasitoids colonizing CGW was monitored between 2013 and 2015, with the aim of estimating the composition of the indigenous parasitoid complex, its ability to control CGW populations, and the interactions of both factors with several measured environmental parameters. We compared results among three differently managed field types. Results showed an increase in the rate of parasitism both when the host population density was lower and in unmanaged chestnut stands with more natural conditions. The percentage of parasitism in galls was related to morphological traits of the galls and to higher seasonal temperatures, which reduced the parasitism intensity because CGW develops earlier under such conditions. The host-parasitoid mortality inside galls varied among sites and was associated mostly with rot fungi during wet spring and summer months. Parasitoid species richness was similar among the study sites, but the proportion of parasitoid species differed between orchards and unmanaged coppice stands. The timing of attack by parasitoids followed a species-specific successional sequence throughout the larva-to-adult life cycle of the CGW. These interactions should be considered in future research on trophic relationships and when modeling invasive scenarios for new pest species.

Seasonal flight activity and genetic relatedness ofTorymusspecies in Italy

“…geranii were recorded on chestnut galls. These data concerning the emerged native Torymus species confirm previous observations reported in the literature (de Vere Graham & Gijswijt, ; Askew et al ., ; Alma et al ., ; Ferracini et al ., ; Bonsignore & Bernardo, ; Panzavolta et al ., ). Regarding the emergence from chestnut galls, the three native Torymus species were found only during the first 2 years of investigation.…”

“…In countries where D. kuriphilus was accidentally introduced, the attack rate by indigenous parasitoids was generally low (Quacchia et al ., ; Alma et al ., ), although it varied according to environmental conditions (e.g. chestnut formation, oak occurrence) (Bonsignore & Bernardo, ; Ferracini et al ., ). Consequently, the exotic T. sinensis was released and proved to be a successful BCA (Moriya et al ., ; Borowiec et al ., ; Matošević et al ., ; Ferracini et al ., ).…”

Short-term Thermal Changes Affect Parasitism on Asian Chestnut Gall Wasp

“…Parasitism rate can be affected by the phenology of parasitoid species associated with oak gall wasps, which are often multivoltine, as parasitoids must parasitize their hosts during the appropriate "window of vulnerability" [13,15]. Therefore, variation in parasitism rates is an immediate response to environmental factors, including seasonal temperature variations, climate change, and variations in habitat [13,[16][17][18]. However, it is difficult to pinpoint what factors determine variations in the composition of parasitoid species and why some species are more successful after thermal stress.…”

“…The species also represents a perturbation to the natural trophic relationships between the community of native parasitoids and oak gall wasps [26][27][28]. ACGW was first detected in the Calabria region (southern Italy) in 2009 [29] about seven years after its first record in Northern Italy (Piedmont), and heavy infestations of chestnut fields in the Aspromonte National Park have since been reported [18]. ACGW is a univoltine species whose females lay their eggs into chestnut buds, which induces the formation of galls on the growing shoots that can inhibit shoot development and flowering [30,31].…”

“…In recent years, ACGW has become widespread in Europe [39], and in all invaded areas it is parasitized by native parasitoids [40][41][42][43] associated to oak galls, for which ACGW represents a nonsaturated adaptation space. A large community of generalist parasitoids related to oak galls rapidly shifted to ACGW [28,41], with local variations in the composition that depends on phenological overlap and habitat features [13,18,44,45]. The composition of this parasitoid community depends on morphological traits of galls (i.e., size, shape, hardness) [30,46], on the developmental stage of the gall (i.e., young or fully formed), cline, and on the number of gall chambers [47].…”