Many site managers are faced with the uneasy task to control Buddleja. However, it may be that in places where this flowering bush has been present for a number of years, fauna may have adapted to its abundance and profited from it. So, before removing Buddleja, we thought it would be a good idea to assess its ecological role as a food resource.
This is continuation of our earlier post from 30.07.2014 and is kindly provided by Dr. Arnold van den Burg, leading a research project at the ENCI quarry in Maastricht.
This preliminary communication of results concerns leaf-eating insects only, but we note Buddleja for its nectar-rich flowers as well. Here, we collected data to measure the importance of Buddleja in relaying energy trapped by photosynthesis into the food chain through insect herbivory. We compared the data with 5 native species of bush.
We scored leaf herbivory as percentage leaf eaten in July 2014. We estimated herbivory on 20 leaves per bush (starting from the youngest leaves), and 20 individual bushes per species. Of each bush we sampled 3-10 intact representative leaves to determine leaf dry weight (DW). Because leaves are of different shape and size we wanted leaf herbivory to be expressed as surface-percentage eaten, biomass-percentage eaten (DW) and the number of leaves per bush with scores >1.
Results & Discussion
If species are important for herbivores and major contributors to fuelling the ecosystem, we expect high percentage leaf damage both in surface area and biomass (i.e. the upper right corner of fig. 1). If leaves are small, high percentages of eaten leaf area ca easily occur, but expressed as biomass, leaf damage will be low. So, in graph 1, the leave-size effects runs from the lower-right corner to the upper-left corner with increasing leaf size.
Taking leaf size into account, Buddleja is only eaten a little by insect herbivores. Bramble and field maple are the most preferred species by leaf herbivores. Dog rose and hawthorn are intermediate; in blackthorn the large variation between bushes is noteworthy. Perhaps, the importance of small-leaved bush species is somewhat underrepresented as low biomass percentages per leaf may well be compensated by eating from more leaves, which is, however, partly taken into account by sampling 20 leaves.
Only very few herbivores were observed alive, by far insufficient for further analysis. However, in most bush species leaf herbivory did not differ between the youngest versus older leaves (fig.2). Only in bramble, leaf herbivory increased with leaf age, indicating that herbivory on bramble has been stronger in June than July. Also in this analysis, Buddleja appeared to be a poor food plant for insect herbivores.
Native bushes show considerably more leaf-damage by insect herbivores than Buddleja which does not seem to contribute a lot to fuelling the ecosystem through leaf herbivory. From this perspective only, little harm will be done to the ecosystem by removing Buddleja from the site. In our final report we will include a list of species known as leaf herbivores on Buddleja and their native food plants, which any of such species may switch back to. In the Netherlands, bramble is often selected against in nature management as it is a nitrogen indicator (in relationship with high nitrogen deposition levels in the Netherlands). Our data indicate the importance of bramble for insect herbivores (besides other benefits brambles bring to an ecosystem).