BioControl 45: 501–512, 2000. © 2000 Kluwer Academic Publishers. Printed in the Netherlands. Impact of defoliation by the biocontrol agent Zygogramma bicolorata on the weed Parthenium hysterophorus in Australia K. DHILEEPAN 1∗ , S.D. SETTER 1,3 and R.E. MCFADYEN 2 1 Tropical Weeds Research Centre, Queensland Department of Natural Resources, Charters Towers, Qld, Australia; 2 Alan Fletcher Research Station, Queensland Department of Natural Resources, Sherwood, Qld, Australia; 3 Current address: Centre for Wet Tropics Agriculture, Queensland Department of Natural Resources, South Johnstone, Qld, Australia ∗ author for correspondence; e-mail: dhileepank@dnr.qld.gov.au Received 13 October 1999; accepted in revised form 19 April 2000 Abstract. The leaf-feeding beetle Zygogramma bicolorata Pallister was introduced from Mexico into Australia in 1980 as a biocontrol agent for the weed Parthenium hysterophorus L. (Asteraceae). Z. bicolorata became abundant in 1990, and since 1992 there has been regular outbreaks resulting in the defoliation of the weed in central Queensland. In this study we evaluated the impact of defoliation by Z. bicolorata on P. hysterophorus from 1996 to 1998. Z. bicolorata caused 91–100% defoliation resulting 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 by 13–86% and seedling emergence in the following season by 73–90%. At sites with continued outbreaks of Z. bicolorata, it is expected that the existing soil seed-bank will be minimised, resulting in reduced density of parthenium in 6 to 7 years. Key words: biocontrol, defoliation, herbivory, impact assessment, weed, Asteraceae, Cole- optera, Parthenium hysterophorus, Zygogramma bicolorata Introduction The parthenium weed, Parthenium hysterophorus L. (Asteraceae: Heli- antheae) is native to the Gulf of Mexico and central South America, and has become naturalised in North America, South America, the Caribbean and many parts of Africa, Asia and Australia (Navie et al., 1996). In Australia, parthenium mainly occurs in Queensland, affecting 170,000 km 2 of prime grazing country and causes Au$16.5 million/year loss to the pasture industry (Chippendale and Panetta, 1994). Parthenium has the potential to spread through out Australia and to cause Au$109–129 million/year loss to the beef industry (Adamson, 1996). Parthenium also causes human health problems