Climatic influences on post-release populations of the biological control agent Cecidochares connexa in northern Queensland

Abstract

Cecidochares connexa, stem-galling fly, is Australia’s only mass-reared and released biological control agent for the invasive weed species, Chromolaena odorata (Siam weed). Releases of C. connexa commenced in late 2019 in northern Queensland and have now covered eight Local Government Areas. In this paper we will assess the gall infection rate of C. connexa, at two trial sites within two different climatic zones over several years. One site is within the wet tropics (Nyleta Hill), with a mean annual rainfall of 3,294 mm and the other site is situated in the dry tropics (Black River), with mean annual rainfall of 1,135 mm. All climatic parameters were analysed, with strong positive correlations existing between an increase in rainfall and gall infection rate (galls versus growing tips). Black River experienced an increase of C. connexa galls by 2,428% in early 2023, compared to the previous years. This was due to the site receiving double the amount of rainfall than the previous year (1,844 mm) in the same period. These boom-and-bust rainfall events in the dry tropics compared to a constant supply of moisture in the wet tropics creates different plant and agent responses.