Environmental thermal levels affect the phenological relationships between the chestnut gall wasp and its parasitoids

Abstract

Studies of thermal level‐related asynchrony in a host–parasitoid relationship are necessary to understand the effects of climate change on new host–parasitoid interactions. In the Asian chestnut gall wasp Dryocosmus kuriphilus (Hymenoptera: Cynipidae) and its Chalcidoidea parasitoids, phenological synchrony is assumed to be weather‐dependent in a new area of expansion. To evaluate the effects of environmental thermal regimes on the host, a phenology model for different cynipid stages (larvae, pupae, adults, and adult emergence) and a host–parasitoid phenological estimator are developed in three chestnut fields during two successive growth seasons and subsequently validated in areas with chestnut fields at two different altitudes. Comparisons of the timings of the juvenile and adult stages with those of the parasitoid complex demonstrate that the shortest period of occurrence for cynipids within galls has negative effects on the host–parasitoid relationships at higher temperature levels, thereby increasing phenological asynchrony for some parasitoids species. Reducing the development time of pupae and adults decreases the likelihood of success for some parasitoid species at higher temperature levels. We also record the extension of the gall wasp development time (approximately 15 days) at higher altitudes (linked to a lower mean temperature of approximately 1.5 °C). These results highlight how parasitization on the new hosts is dependent on the host phenology and, in the present study, is limited by the short duration of the presence of the host in galls, which could explain the considerable differences in cynipid gall wasp parasitization recorded at different altimeters.

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

“…Each shoot was wrapped in plastic film and stored at a controlled temperature (8°C) in a laboratory fridge for 7 days. This temperature is lower than the theoretical value at which ACGW stops their development but does not die; hence it is a stress factor [13]. These shoots were then repositioned on trees in the field where they were collected to evaluate the response of ACGW and parasitoids to cold stress (lab treatment).…”

“…Anomalous and unpredictable cold spells can be observed more and more frequently in a short period of the year [11], thus are among the rapid thermal fluctuations worth investigating to assess possible effects on the abundance, distribution, and function of species in a food web. Temperatures can affect the phenology of both hosts and parasitoids [7,12,13] and asymmetric changes in the seasonal activities of species between closely interacting species are likely responsible for desynchronization in their life cycles [8].…”

“…Insects at higher trophic levels are expected to be more strongly affected by environmental changes than organisms from low trophic levels due to cascading effects in the food chain [14]. 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].…”

“…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.…”

“…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].…”