Host range expansion is density dependent.

Area of Science:

• Ecology
• Entomology
• Evolutionary Biology

Background:

• Herbivore host range is expected to increase with population density.
• Phylogenetic relatedness and trait similarity between native and exotic plants may increase susceptibility to native herbivores.
• The density dependence of native herbivore host range expansion to introduced species remains largely unknown.

Purpose of the Study:

• To investigate whether the host range expansion of native herbivores to introduced plant species is density-dependent.
• To examine the influence of herbivore population density on host range and phylogenetic clustering of attacked pine species.
• To determine if phylogenetic proximity to native hosts predicts attacks on introduced species.

Main Methods:

• Monitoring pine processionary moth (PPM, Thaumetopoea pityocampa) attacks over nine years on 41 pine species (8 native, 33 introduced) in an arboretum.
• Comparing PPM attacks during latent and outbreak periods to assess density effects.
• Analyzing host range expansion and phylogenetic clustering of attacked species.

Main Results:

• Pine processionary moth attacked 28 out of 41 pine species, with no difference in attack probability between native and introduced species.
• Host range increased and became more phylogenetically clustered during outbreak periods compared to latent periods.
• During high population density (outbreak), PPM expanded its diet to include introduced pine species closely related to its native hosts.

Conclusions:

• This study demonstrates the density-dependent nature of host range expansion in the pine processionary moth.
• Phylogenetic proximity of host species is a significant predictor of herbivore attacks, potentially more so than the introduction status of the plant.
• Findings aid in predicting new plant-herbivore interactions and understanding ecological responses to invasive species.