Invasive Spiraea tomentosa: a new host for monophagous Earias clorana?
The introduction of alien species can have a significant impact on the food preferences of native phytophagous insects. The moth Earias clorana L. has previously been considered to be monophagous, ingesting only plants in the genus Salix. In recent years, we have observed larval E. clorana feeding on Spiraea tomentosa L., an invasive shrub species in Central Europe that is native to North America. We hypothesised that this insect can feed on Spiraea tomentosa leaves with no negative effects on its growth and development, and that the leaves of Spiraea tomentosa as a source of food for E. clorana are equally as good as leaves of Salix viminalis L. Our results showed that despite significant differences in the chemical composition of the studied species' leaves, including a much higher concentration of defence compounds (total soluble phenols and condensed tannins) in Spiraea tomentosa leaves than in those of Salix viminalis, feeding on a new host plant did not significantly affect the survival of larvae. The change in host plant had an unfavourable effect, however, on several parameters of growth and development for the larvae (masses of larvae and pupae, relative growth rates, and efficiency of conversion of ingested food). We conclude that, in comparison to Salix viminalis, Spiraea tomentosa is not a particularly favourable food for larval development. Perhaps, even without direct improvements in adult foraging efficiency, however, the costs of switching hosts may be minimised in larvae that develop on very abundant, invasive species, such as Spiraea tomentosa in Central Europe.