For several years now, the CIRAD has undertaken twice-yearly surveys in Réunion to estimate populations of sawfly Cibdela janthina and re-colonization rates after they succeeded in eliminating the noxious weed, Rubus alceifolius, known as giant bramble. This study is co-funded by the Ministry of Ecology, Sustainable Development and Energy.
Monitoring Cibdela janthina and the defoliation caused by Rubus alceifolius in Reunion
Surveys show that by 2012, the sawfly had spread around the island since it was first observed in 2008. It is now present in all lower-altitude areas of the island, but densities are lower than in previous years. In the east of the island, Cibdelajanthina is still present despite the scarcity of Rubus alceifolius, which has been virtually eliminated in the sectore xtending from St. Benoit to St. Joseph. Any remaining shoots are rapidly consumed by sawfly larvae. The summer and winter surveys have shown that lower temperatures at higher altitudes strongly influence populations of C.janthina and their impact on R. alceifolius. A thesis was began in late 2011 to identify and model the long-term balance between the sawfly and the giant bramble, at different altitudes.
Biology and temporal dynamics of C. janthina of R.alceifolius at different altitudes
The goal of this study is to build a model of sawfly population dynamics which reflects its impact on Rubus alceifolius at different altitudes and in different seasons. The model will be able to predict the long-term effectiveness of the sawfly control Rubus alceifolius. Thus, larval survival rates and leaf consumption, as well as fecundity, longevity and spawning behavior of females were studied in the laboratory and in large outdoor cages. Meanwhile, study sites at various altitudes along two vertical transects were monitored providing a better understanding of seasonal population dynamics of Cibdela janthina. These data were used in the construction of the model.
Up until the end of 2011, small populations of sawfly had been observed at higher altitudes, but they had only negligible impact on Rubus alceifolius. Since 2012, the upper altitude effectiveness limit for sawfly against Rubus alceifolius is around 1100 - 1200 m (e.g. La Plaine des Palmistes). Laboratory studies have also shown that the insect is very sensitive to low temperatures. The cold slows the development of larvae, significantly increasing their mortality and rendering the sawfly less effective against Rubus alceifolius. At 15°C, for example ,there is a very high mortality rate, with only two adults surviving from every 100 eggs laid.
Re-colonization after biological control measures
In any biological control measure against an invasive plant, re-colonization should be monitoring over several years to assess its impact. Once the target plant is checked or eliminated by the control species,the space freed can be occupied by native and/or exotic plants.
Biological control of R. alceifolius by C. janthinaled to large-scale defoliation which created canopy openings, allowing local, native or exotic species to return. Canopy gap re-colonization in Mare Longue in the Parc National des Hauts has been assessed annually since the summer of 2009-2010. Overall, there has been an increase in species diversity (increase in number of plant species) of both native and exotic species, with a slightly higher recovery rate for native species. Areas free of exotic species presentan ideal opportunity for the regeneration of native species – there are up to 45 native species in the forest gaps of Takamaka alone.
The elimination of R.alceifolius will cause a long-term environmental impact which will not beknown for several years, even decades. Also, at this stage in the study, it is still too early to tell if the canopy gaps created by the removal of R. alceifolius will be fully re-colonized by native species. So far, however, in artificial and manmade environments (e.g. field borders and roadsides) or in preservation areas (e.g. primary forests), species replacing R.alceifolius are invariably species already present on the island.