Prioritisation of damaging weed biological control agents for prickly acacia (Vachellia nilotica subsp. indica) based on insect exclusion studies in the native range

Abstract

Prickly acacia, Vachellia nilotica subsp. indica (Benth.) Kyal. & Boatwr. (Fabaceae), native to India, is a target for biological control in Australia. Native range insecticide exclusion studies were conducted at two sites (Coimbatore and Thoppur) in India to evaluate the impact of insect herbivores on prickly acacia, to prioritise agents. At each site, prickly acacia plants were either exposed to or protected from insect herbivores and the incidence of insects were recorded at quarterly intervals. More insect species occurred at Thoppur (12) than at Coimbatore (9) and among them the scale insect Anomalococcus indicus Ayyar (Hemiptera: Lecanodiaspididae) was the predominant species at both sites. Insect herbivory significantly reduced plant height, basal stem diameter, number of branches, the number of leaves, and stem biomass at Coimbatore, while the impact was evident only for plant height at Thoppur. However, there were significant reductions in plant height (38–55%), basal stem diameter (20–31%), number of branches (21–45%), number of leaves (79–99%), leaf biomass (81–96%), stem biomass (67–84%) and root biomass (61–84%) in plants infested with A. indicus than in plants without A. indicus in insects excluded plots. Infestation by A. indicus resulted in 18 and 27% plant mortality in Coimbatore and Thoppur, respectively. Based on potential impact, A. indicus was prioritised for importation and detailed host-specificity testing in Australia.