Invasive oaks escape pre-dispersal insect seed predation and trap enemies in their seeds

Species introduced to habitats outside their native range often escape control by their natural enemies.Besides competing with native species,an alien species might also affect the native herbivores by introducing a new source of different quality food.Here,we describe the case of northern red oak(Quercus rubra)invasion in Europe.We collected data on insect(moth Cydia spp.and weevil Curculio spp.)seed predation of northern red oak in its native(USA,North America)and invasive(Poland,Europe)range,as well as for sessile oaks(Quercus petrea)in Europe.We also evaluated the quality of acorns as hosts for weevil larvae by collecting infested acorns and measuring weevil developmental success,and quantifying acorn traits such as seed mass,tannins,lipids and protein concentration.We used DNA barcoding to identify insects to the species level.The predation by moths was similar and very low in both species and in both ranges.However,red oaks escape pre-dispersal seed predation by weevils in Europe.Weevil infestation rates of northern red oak acorns in their invasive range were 10 times lower than that of sessile oaks,and also 10 times lower than that of red oaks in North America.Furthermore,even when weevils oviposited into northern red oaks,the larvae failed to develop,suggesting that the exotic host created a trap for the insect.This phenomenon might gradually decrease the local abundance of the seed predator,and further aid the invasion.

Seed loss before seed predation: experimental evidence of the negative effects of leaf feeding insects on acorn production

Insect herbivory decreases plant fitness by constraining plant growth,survival and reproductive output.Most studies on the effects of herbivory in trees rely on correlational inter-individual comparisons and could,thus,be affected by confounding factors linked to both herbivory and plant performance.Using the Mediterranean Holm oak(Quercus ilex)as a study model,we followed an experimental approach in which leaf-feeding insects(mainly Lepidoptera caterpillars)were excluded from some shoots in all study trees.Shoots subjected to herbivore exclusion exhibited lower defoliation rates and produced more acorns than control shoots.Defoliation constrained shoot growth throughout the study period,but had no effect on the number of female flowers produced per shoot.Acorn production was,however,lower in control shoots due to their higher abortion rates,and also to their greater mortality risk during summer drought,as shoots with fewer leaves were less likely to survive.Plant reaction to herbivory inhibits certain physiological pathways involved in plant growth,which,together with the effects of physical damage,reduces the amount and efficiency of the photosynthetic tissue.This increases their vulnerability to environmental stresses,such as water deficit,which limit resource assimilation.Defoliation is likely a key factor affecting oak regeneration,as it may be a significant source of seed loss prior to pre-dispersal acorn predation.Further experimental studies could help to elucidate its effects in contrasting environments.In Mediterranean regions,the harsher droughts predicted by climate change models could worsen the effects of insect herbivory on oak reproductive output.