Impact of defoliation by the biocontrol agentZygogramma bicolorataon the weed Partheniumhysterophorus in Australia

The leaf-feeding beetle Zygogrammabicolorata Pallister was introduced from Mexico intoAustralia in 1980 as a biocontrol agent for the weedParthenium hysterophorus L. (Asteraceae). Z. bicolorata became abundant in 1990, and since 1992there has been regular outbreaks resulting in thedefoliation of the weed in central Queensland. In thisstudy we evaluated the impact of defoliation by Z. bicolorata on P. hysterophorus from 1996 to1998. Z. bicolorata caused 91–100% defoliationresulting in reductions in weed density by 32–93%,plant height by 18–65%, plant biomass by 55–89%,flower production by 75–100%, soil seed-bank by13–86% and seedling emergence in the following seasonby 73–90%. At sites with continued outbreaks ofZ. bicolorata, it is expected that the existing soilseed-bank will be minimised, resulting in reduceddensity of parthenium in 6 to 7 years.