Author
Terence Olckers
olckerst@ukzn.ac.za
University of KwaZulu-Natal
Durban
Coauthors
Sphelele Bhengu, University of KwaZulu-Natal, Durban, South Africa
Abstract
The stem-galling fly Procecidochares utilis (Tephritidae) was released in the KwaZulu-Natal midlands of South Africa in 1984 for the biological control of Ageratina adenophora (Crofton weed; Asteraceae), but its impact is deemed to be moderate. The fly is attacked by native parasitoids in the field, which could explain its limited impact. We assessed the extent of galling by P. utilis on A. adenophora populations at four sites in the KwaZulu-Natal midlands over four seasons. We also sampled galls at each site to determine the extent to which the fly has recruited native parasitoids. Preliminary results indicate that although most plants exhibit galling, gall loads are too low (ca. 2 galls/plant and only 12% of available shoots galled) to influence plant growth. Parasitoids emerged from many occupied galls, with five species recorded so far. The total numbers of P. utilis and parasitic wasps that emerged from the galls was similar across the duration of the study. Despite establishment in the field for four decades, population densities of P. utilis are constrained by recruited parasitoids. Mass-rearing and inundative releases of P. utilis at key stages in the weed’s phenology may facilitate higher population densities and thus improved biocontrol success.
Keywords
Crofton weed
gall-forming agents
parasitism
post-release evaluation
Procecidochares utilis
Highlights
Procecidochares utilis galls are present on most sampled plants, but in low numbers
Adult flies and parasitoid wasps emerged from galls in similar numbers
Recruited parasitoids play a role in constraining agent efficacy