Interactions between a stem gall fly and a leaf-spot pathogen in the biological control of Ageratina adenophora

[Display omitted] ► Evaluated leaf-spot pathogen and stem gall fly as biocontrol agents of crofton weed. ► Additive effect of using two biocontrol agents. ► Repeated stem galling by fly provided greatest impact on plant growth. ► Weed compensated for galling with increased sideshoot growth. ► Pathogen inhibited sideshoot growth, therefore important addition to control of plant. Many biological control projects involve the release of multiple agents. Ageratina adenophora (crofton weed) has two agents, Procecidochares utilis, a stem gall fly, and Passalora ageratinae, a leaf-spot fungal pathogen, released against it in South Africa. This study investigated whether both agents, individually or jointly, increased or decreased the impact on crofton weed under greenhouse conditions. Six-month-old plants were exposed to one of six treatments (n=15 plants/treatment): control (no agents), pathogen-only, single-galled only, double-galled only, pathogen-single-galled, and pathogen-double-galled, all for a period of 6months. Individually, both of the agents reduced stem height and percentage of live leaves, but there was no synergistic effect of the two agents together. Pathogen-double-galled stems had significantly fewer pathogen-infected leaves relative to the other pathogen-infected treatment stems, suggesting a negative interaction between the two agents on pathogen establishment. Pathogen infection did not affect the size of the galls. Double galling by the fly inhibited stem growth above the gall position on the stem. Crofton weed compensated for galling by increasing the number of sideshoots. The pathogen inhibited sideshoot growth, thereby curbing the plant’s ability to offset galling. Overall, there was an additive interaction between the two agents which enhances their usefulness as biocontrol agents of crofton weed.