Effectiveness of introduced biocontrol insects on the weed Parthenium hysterophorus (Asteraceae) in Australia

Six species of insects and a rust fungus have been successfully established for biocontrol of the weed Parthenicum hysterophorus L. in Queensland, Aus tralia. Effectiveness of biocontrol insects was evaluated at two properties in Queensland during 1996-97 based on an exclusion experiment using insect icides. Parthenium-infested plots with and without biocontrol insects were sampled at monthly intervals and the impact of biocontrol insects on parthe nium at individual plant and whole population levels monitored. Biocontrol insects were more effective at Mt Panorama (central Queensland) than at Pla in Creek (north Queensland). At Mt Panorama, the leaf-feeding beetle Zygogr amma bicolorata Pallister caused 96% defoliation and the stem-galling moth Epiblema strenuana Walker affected 100% of the plants, resulting in reducti ons of 90% in weed density, 40% in plant height, and 82% in flower producti on. Exclusion of biocontrol insects resulted in a 52% increase in seedling emergence and a seven-fold increase in the soil seed bank in the following season. At Plain Creek, E. strenuana was the only prominent agent. It affec ted 92% of the plants and prevented 32% of plants from producing any flower s, reduced plant height by 40% and flower production by 49%, but did not re duce the plant biomass, weed density or soil seed bank. However, exclusion of biocontrol insects resulted in an eight-fold increase in the soil seed b ank in the following season.