Patchy herbivore and pathogen damage throughout the introduced Australian range of groundsel bush, Baccharis halimifolia, is influenced by rainfall, elevation, temperature, plant density and size

Abstract

The intensity and type of damage exerted by a biological control agent on an invasive plant is often influenced by plant architecture or size, density, distribution and phenology as well as microhabitat. Here, herbivore and pathogen damage thought to be caused by seven biological control agents introduced up to 40 years ago for control of groundsel bush (Baccharis halimifolia) is examined. Sampling was conducted throughout the known introduced range of groundsel bush in Australia to determine how damage type and intensity were influenced by rainfall, temperature, elevation, population density, average plant size and variability in plant size. Sori abundance score (characteristic of the introduced rust fungus) was higher on large plants at high rainfall. Leaf skeletonization was negatively correlated with elevation and leaf miner damage was lower in high density plant populations. Leaf hole damage increased with rainfall particularly on large plants in homogeneous sized populations. Stem-borer damage increased with plant size, but this relationship was influenced by rainfall and plant population density. There was less stem-boring in large populations but the strength of the relationship was influenced by temperature and variation in plant size. Overall considerable variation in intensity of herbivore and pathogen damage was found throughout the range of groundsel bush. If herbivore and pathogen damage impact on plant vital rates and demography it would be expected that biological control success for this species would be patchy and depend on plant size, population density and site location.