LAUSR

Abstract Reducing the reproductive output of an invasive weed using biocontrol can result in a decrease in population density and help control the spread of the weed. Two biocontrol agents, a stem gall fly, Procecidochares utilis (Tephritidae), and a leaf-spot fungal pathogen, Passalora ageratinae (Mycosphaerellaceae), have been released against Crofton weed, Ageratina adenophora, in South Africa. In this study we investigated the impact of the agents, individually and together, on Crofton weed’s reproductive output, under both greenhouse and field conditions. Galling by P. utilis, with or without P. ageratinae, resulted in 32% fewer flowering structures per stem, compared to ungalled treatments in both the field and the greenhouse; however, without P. utilis, P. ageratinae did not reduce the number of flowering structures per stem. In the greenhouse, galled stems had 24% (single-galled)-55% (double-galled) fewer filled achenes than ungalled stems, while pathogen-infected stems had 27% fewer filled achenes per stem in comparison to uninfected stems. Although the biocontrol agents reduced the number of filled achenes, the percentage germination remained high and similar for all treatments (69.3–74.4%) in the field and the greenhouse. The combination of the two agents together did not provide more control of sexual reproduction than P. utilis alone. However, Crofton weed compensates for galling with increased sideshoot growth (vegetative reproduction), which in time also flower. But P. ageratinae inhibits growth of sideshoots. Therefore, the combination of the two agents best enhances the level of control rather than either agent in isolation.