Evaluation of biogeographical factors in the native range to improve the success of biological control agents in the introduced range

Biogeographical factors associated with the invasive weed Arundo donax were evaluated at 22 different locations in four countries in its native range in reference to its key herbivore, an armoured scale insect, Rhizaspidiotus donacis. Data on climate, plant health and quality, soil conditions and anthropogenic influences were analysed for each site and compared to populations of R. donacis. Climate modelling from location data in Spain and France accurately predicted the native range of the scale in the warmer, drier parts of Italy and Greece and was used to predict its distribution in North America. Presence of the scale was not associated with specific soil type or characteristics. However, it was positively associated with a higher percentage of dead stems and significantly lower biomass of A. donax. Micronutrient sampling using leaf material found that sulphur was negatively correlated with aluminium and positively correlated with boron for scale density. Disturbance of field sites by cutting and/or addition of supplemental irrigation during summer appeared to disrupt the synchronised seasonal phenology of A. donax and R. donacis, leading to more robust stands. These biogeographical factors from the native range indicate that R. donacis should have the greatest impact in warm, dry climates in the introduced range where A. donax is undisturbed.