Post-release evaluation of a combination of biocontrol agents on Crofton weed: testing extrapolation of greenhouse results to field conditions

Two biocontrol agents, a leaf-spot pathogen, Passalora ageratinae, and a stem gall fly, Procecidochares utilis, have been released against Crofton weed, Ageratina adenophora (syn. Eupatorium adenophorum) (Asteraceae), in South Africa. This work reports the first post-release evaluation of the effect of both agents acting together in the field. A greenhouse trial using both agents had predicted an additive (beneficial) interaction between the agents. This study investigated if the additive interaction was present in the field. Four month old stems were exposed to one of the following three treatments (n = 20 plants per treatment): pathogen-only, pathogen plus single fly-galled, and pathogen plus double fly-galled, for 11 months. The interaction between the agents was equivalent to both agents acting independently (i.e. there was no additive effect on the weed’s growth). The greenhouse trails were therefore not predictive of field conditions.