Phenological alignment of a galling insect used as biocontrol agent for an invasive tree following hemisphere translocation

Abstract

Acacia longifolia is one of the most widespread invasive alien plant species along coastal areas in Portugal. In South Africa, this species is also invasive and has been controlled for over 40 years by the Australian gall wasp Trichilogaster acaciaelongifoliae. This biological control agent was introduced in Portugal in 2015, being released immediately after import, i.e., without a laboratory rearing period to adjust the life cycle, and is now established in many locations along the coast. When released in the northern hemisphere, the agent came from the austral spring, with the host-plants having tiny buds, and faced boreal winter with host-plants having large buds. As such it had to synchronize its life cycle with the seasons and phenology of the host-plant in this new location. Both locations have mild summer Mediterranean climates. The present work aims to better understand the life cycle of T. acaciaelongifoliae in the northern hemisphere and how it has aligned its phenology to these new conditions, namely different host-plant phenology and seasons. Two sites were selected (Pataias and São Jacinto), and monitored monthly, from November 2021 to October 2022, to: i) describe the early stages of the life cycle, since oviposition, by collecting and dissecting A. longifolia buds of 5 branches/five trees/site; ii) characterize the stages of the life cycle of T. acaciaelongifoliae inside the developing galls, by collecting and analysing 50 galls/site; and iii) monitoring the development of galls on 10 A. longifolia marked branches/ five trees, in the field. The results suggest that T. acaciaelongifoliae in the northern hemisphere has a univoltine life cycle, but the generations are not completely synchronized at the two sites and even within each site. It is not excluded that two generations may occur throughout the year, i.e., a bivoltine cycle, in some individuals. The alignment of the life cycle may not yet be stabilized. The life cycle seems to be more advanced in the site located further south, Pataias, where adults emerged earlier. Reproductive galls dominated in almost all months, as in South Africa. The present work advances the knowledge on the life cycle of T. acaciaelongifoliae, especially in the northern hemisphere, providing details about the timings and duration of each life cycle stage in this new location. These results are relevant for the management of A. longifolia, stressing the plasticity of the agent life cycle which can facilitate the planning of the biocontrol agent releases in new locations.