Differential susceptibility and suitability of domestic and wild apple species for a florivorous weevil and its parasitoids

Abstract

Crop plant domestication can change plant resistance to herbivores leading to differences in pest pressure experienced by crop plants and their wild relatives. To compare resistance to herbivores between domesticated and wild fruit trees, we quantified direct resistance and indirect resistance to a pest insect, the florivorous apple blossom weevil Anthonomus pomorum (Coleoptera: Curculionidae), for the cultivated apple Malus domestica and two wild apple species, the European crab apple M. sylvestris and the exotic M. kirghisorum. We measured weevil infestation and performance (weight, sex ratio), and weevil parasitism by parasitoid wasps for different cultivars of M. domestica and for the two wild apple species. To explain weevil and parasitoid responses to different apple species, we quantified tree characteristics including nitrogen content, size of flower buds, bark roughness, tree size, tree phenology and tree position. We found significant differences in susceptibility to weevil infestation between apple species, with lowest infestation (highest apple resistance) in M. domestica and highest infestation in M. kirghisorum. The suitability of apple species also varied significantly: weevils emerging from M. sylvestris were significantly lighter than those from M. kirghisorum. Parasitism of A. pomorum by different parasitoid species was significantly higher in M. sylvestris than in M. domestica. Infestation, weevil weight and parasitism were positively related to tree characteristics: infestation to bud nitrogen content and bark roughness, weevil size to nitrogen content and bud size, and parasitism to tree height and bud density. Our study revealed marked differences between apple species in susceptibility and suitability for the pest herbivore, but also for antagonistic parasitoids. Whereas direct resistance appeared to be higher in cultivated apple, indirect resistance via parasitoids was apparently higher in wild apple trees. Our findings suggest that wild and cultivated apple trees possess different resistance traits that may be combined to optimize resistance in commercial apple cultivars.

Unravelling pest infestation and biological control in low-input orchards: the case of apple blossom weevil

“…Overall we found seven species of parasitoids, with up to five species being found per site. Thus the assemblage of larval parasitoids detected in our study was richer than that described in previous studies (see for example Knuff et al 2017;Mody et al 2017). However, equally high levels of richness have been found for other animal groups in the low-intensity agroecosystem of Asturian cider apple orchards (e.g.…”

“…500 trees/ha in all but two cases, where trees were grown on seedling rootstock with 250 trees/ha. All orchards comprised several cultivars, but all sampling was done on the local cultivar 'Regona' to homogenize sampling (given that weevil infestation and parasitism rates depend on cultivar; Mody et al 2015;Knuff et al 2017).…”

“…One local factor not explicitly considered in our study but with large potential influence on weevil populations is the apple genotype (e.g. Mody et al 2015;Knuff et al 2017). We conducted our study on a single cultivar to control for potential cultivar effects, but, anyway, there is still a possibility that weevil incidence on this target cultivar could be affected by the presence of other cultivars.…”

“…These adults could be attacked by predators like birds or crawling arthropods. (3) When the flower is in D-E (56-57), females oviposit inside the flower buds (Toepfer et al, 2002;Knuff et al, 2017) and (1)…”

“…1). Weevils are attacked by several parasitoid species, mainly during their larval stage inside the blossom (Miles 1923;Cross et al 1999;Mody et al 2011;Knuff et al 2017). In their turn, parasitoids have several resource requirements (floral resources for adults, shelter habitat, other hosts) at both the orchard and the landscape scale (Gillespie et al 2016).…”