Intraspecific interaction of host plants leads to concentrated distribution of a specialist herbivore through metabolic alterations in the leaves

1. Recent studies suggest that changes in leaf traits due to interactions
between plants affect resource utilisation by and the distribution of
herbivores. However, this has not yet been confirmed experimentally. Here,
we investigated the effects of phenotypic plasticity in leaf traits of
Rumex obtusifolius (host plant) in response to intra- and interspecific
interaction on the distribution of two leaf beetles, Gastrophysa
atrocyanea (specialist herbivore) and Galerucella grisescens (generalist
herbivore). 2. We investigated the local population density of R.
obtusifolius plants and the presence of leaf beetles on the plants at five
study sites. Leaf chemicals (condensed tannins and total phenolics) were
compared between aggregated and solitary R. obtusifolius plants. To
clarify the effects of the interaction environment of R. obtusifolius
plants on their leaf traits and on resource utilisation by the leaf
beetles, we compared leaf chemicals and preferences of adult leaf beetles
among treatments where R. obtusifolius experienced intraspecific
interaction, interspecific interaction, or no interaction in cultivation
experiments. Finally, we evaluated the independent and combined effects of
patch size and intraspecific interaction on leaf beetle distribution in
mesocosm experiments. 3. In the field, the presence of the specialist leaf
beetle G. atrocyanea was positively correlated with the local population
density (rosette overlap ratio) of R. obtusifolius plants; however, there
was no correlation in the case of the generalist leaf beetle G.
grisescens. In the cultivation experiments, plants in the intraspecific
interaction treatment increased their leaf contents of condensed tannins
and total phenolics, and G. atrocyanea consumed more of these leaves than
leaves in other treatments. Similar results were observed in the field. In
the mesocosm experiments, larger numbers of G. atrocyanea were distributed
on R. obtusifolius plants exposed to below-ground intraspecific
interaction than on plants not exposed to intraspecific interaction. 4.
Our results provide experimental evidence that leaf-trait changes in
response to intraspecific interaction between host plants influence
specialist herbivore distribution. This highlights the need to integrate
plant–plant interactions into our understanding of plant–animal
interactions.