Rarotonga, part of the Cook Islands, is a small island in the Pacific Ocean, only about 8km in diameter. Its isolated evolutionary history has resulted in high levels of endemism but has also made the island prone to invasion by alien plants. LandCare Research New Zealand has been working to control a number of the alien invasive plants that have invaded Rarotonga using biological control. The first step commenced in 2014 and it was a prioritisation exercise to select the most appropriate target plant species for the use of biological control. Targets were selected based on how damaging the plants were in Rarotonga, as well as the chances of achieving successful biological control. This resulted in a list of species, some of which have been successfully controlled using biological control elsewhere in the world, and others that are novel targets. One of the novel targets was the African tulip tree (Spathodea campanulata), which is native to Central and West Africa.
The Rhodes University Biological Control Research Group (RU BCRG) has been involved with the development of biological control for African Tulip Tree since 2006 (funded by the Secretariat of the Pacific Community) and has been working in collaboration with LandCare Research New Zealand (Quentin Paynter from LandCare Research NZ above) on this project since 2014. Two potential biological control agents were imported into quarantine at Rhodes University in early 2014, one is an eriophyid mite (Colomerus spathodea), which forms hairy galls on the leaves and shoots of the plant resulting in deformations. The other is a flea beetle (Paradibolia coerulea) which mines the leaves as larvae and feeds on the leaves externally as an adult. Host specificity testing for the mite has been completed and indicated that C. spathodeae is monophagous and cannot survive on any host besides African tulip tree. Paradibolia coerulea host specificity testing is almost complete and thus far, the only plant that the beetle can survive on is African tulip tree.
The Spathodea mite was cleared for release in the Cook Islands by both the Cook Island and New Zealand authorities. In January 2017, the mite was imported to Rarotonga via quarantine in New Zealand where it underwent a phytosanitory inspection to ensure that no other organisms besides C. spathodea were released inadvertently. Releases were made at 10 sites on the island of Rarotonga and initial establishment of the agent has already been confirmed. A small nursery of African tulip trees that have been inoculated with the mite is being kept at the Department of Agriculture of the Cook Islands and will be used for future releases.
Hopefully the release of C. spathodeae will successfully control African tulip tree in Rarotonga. There are many other islands in the Pacific that are seriously affected by this weed that would benefit from the agent and it is expected that if the agent is successful, it will be transported all around the Pacific. The beetle Paradibolia coerulea will also be released in the next few years, further reducing the weed density. African tulip tree is one of the worst weeds in the world, so the release of the first ever biological control agent against this weed is an important achievement and the culmination of many years of work by students, technical and research staff of the RU BCRG.
Further reading:
G.F. Sutton, I.D. Paterson, S.G. Compton and Q. Paynter. 2017. Predicting the risk of non-target damage to a close relative of a target weed using sequential no-choice tests, paired-choice tests and olfactory discrimination experiments. Biocontrol Science and Technology. In Press.
Paterson, I.D., Paynter, Q., Neser, S., Akpabey, F.J., Orapa, W., Compton, S. 2017. West African arthropods hold promise as biological control agents for a Pacific Island invader. African Entomology. In Press.