Abstract
Two related issues in studies of biological invasions are how quickly the enemy complexes of invasive species become as species-rich and efficient as those of native species and how important enemy release is for the establishment and spread of invaders. We addressed these issues for the geometrid moths Operophtera brumata and Agriopis aurantiaria, who invaded the coastal mountain birch forest of northern Norway by range expansion approximately a century and 15 years ago, respectively. This was done by comparing larval parasitoid species richness and prevalence among the invaders and the native geometrid Epirrita autumnata. We found that E. autumnata and O. brumata both hosted seven parasitoid species groups, whereas A. aurantiaria hosted only one. Several parasitoid groups were shared between two or more of the geometrids. Total larval parasitism rates were similar in all three geometrid species, and comparison with published studies on larval parasitism in Western Europe suggested that O. brumata and A. aurantiaria do not suffer lower parasitism rates in our study region than in their native ranges. Our results indicate that accumulation of larval parasitoids on invasive geometrids in coastal mountain birch forest may reach completion within a few decades to at least a century after the invasion, and that establishment and spread of such invaders is unlikely to be facilitated by release from larval parasitism. Our investigations also uncovered a high degree of spatiotemporal synchrony between the total larval parasitism rates of O. brumata and A. aurantiaria, suggesting that larval parasitism of different geometrid species in the study system is governed by some common external factor.
Highlights
Invasions by alien insect species, facilitated by climate change or human introduction, are reported from a growing number of ecosystems worldwide
Our results indicate that accumulation of larval parasitoids on invasive geometrids in coastal mountain birch forest may reach completion within a few decades to at least a century after the invasion, and that establishment and spread of such invaders is unlikely to be facilitated by release from larval parasitism
Both O. brumata and E. autumnata hosted two groups that were not reared from any other host species, while no such groups were found in A. aurantiaria
Summary
Invasions by alien insect species, facilitated by climate change or human introduction, are reported from a growing number of ecosystems worldwide. It is typically found that invasive insects (i.e., hosts) initially suffer lower parasitism rates and are attacked by fewer parasitoid species in their invaded than native ranges (Cornell and Hawkins 1993; Hawkins and Cornell 1994; Schonrogge et al 2006; Menendez et al 2008; Grabenweger et al 2010). The speed with which an invasive host accumulates parasitoids can sometimes be assessed by comparing its parasitoid complex between areas where it has different residence times (e.g., Schonrogge et al 1995, 2006). The results of such biogeographical comparisons can be difficult to interpret due to geographical gradients in parasitoid species richness and/or efficiency, which may affect the parasitoid complex of an invasive host independently of its residence time (Grobler and Lewis 2008)
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