Patterns of Herbivore Incidence on Experimental Arrays and Field Populations of Ragwort, Senecio jacobaea

Abstract

Despite predictions that herbivorous insects should selectively attack high-density host populations, research suggests that different herbivores may exhibit very different responses to host density. I surveyed natural and experimentally planted populations of common ragwort (Senecio jacobaea) for a variety of herbivorous insects. Early and late instar cinnabar moth (Tyria jacobaeae) larvae differed significantly in their incidence patterns on the experimental arrays, with young caterpillars most prevalent on closely spaced hosts, and older caterpillars on more widely spaced plants. The difference suggests opposite density biases may apply to maternal oviposition and larval dispersal behaviours. Two other herbivores (the leaf miner Chromatomyia syngenesiae and the flowerhead boring Botanophila spp.) showed significant biases towards low density hosts in the experimental plantings. In natural populations, another leaf miner, Liriomyza strigata, preferentially attacked low-density plants, whilst C. syngenesiae incidence was highest at intermediate host densities. The difference between natural and experimental populations suggests a difference between colonisation dynamics and longer-term population effects. I review and contrast six possible explanations for the diversity of density responses found amongst herbivores in this and other comparable studies. Most are at least qualitatively consistent with the patterns observed here, and thus require additional research to differentiate.