Annals of Botany

Aspen attracts amicable arthropods

My enemy’s enemy is my friend, so an old saying goes. In nature this strategy is exemplified by the attraction to plants of predatory or parasitic arthropods, including spiders, ants, wasps and flies, that target herbivores, as discussed by Wooley et al. (University of Wisconsin, USA, pp. 1337–1346) in relation to the development of extra-floral nectaries (EFNs) in Populus tremuloides (aspen). In aspen, EFNs develop on the leaves and attract flies and wasps that attack major herbivores. EFN production varies between genotypes and the variability is heritable. Young plants, in which leaves are a more valuable resource, produce more EFNs than older trees: there is a clear negative ‘gradient’ of EFN density from 1-year-old to 10-year-old trees. In a given tree, the leaves in the upper canopy develop more EFNs than those in the lower canopy. In evolutionary terms this may be because the upper canopy leaves are more photosynthetically efficient and therefore more worth protecting. Further, the production of EFNs is inducible. Young trees, initially 1 year old, were extensively defoliated over 2 years by exposure to tent caterpillars and/or cutting off 75 % of each leaf with scissors. This resulted in the formation, on average, of 23 % more EFNs in the third year (the range was 0–100 % in different genotypes and the capacity for induction was heritable). The authors determined the concentrations of normal defence chemicals, such as tannins and phenolic glycosides; there was no correlation, either positive or negative, between the concentration of these compounds and EFN density and thus no evidence of trade-off between EFN production and other defences. Neither was there a correlation between EFN density and the number of visiting predaceous or parasitic arthropods. However, it is possible firstly that EFNs affect the length of a visit and secondly that other cues also affect arthropod behaviour. Overall, therefore, the data are consistent with Optimal Defence Theory.