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PUTTING AN END TO A DEADLY MUTUALISM

The tropical island of Cousine in the Seychelles has been intensively restored over the last 15 years, the whole story of which is related in the new book Tropical Island Recovery, by C·I·B core team member Michael Samways, and co-authors Peter Hitchins, Orty Bourquin and Jock Henwood.

Pheidole megacephala tending to scale insects

Pheidole megacephala tending to scale insects, Icerya seychellarum. (Photo: M.J. Samways)

However, one of the recent challenges has been the rise in numbers of the invasive alien ant Pheidole megacephala. The reason for the sudden increase is unknown, but was likely that ant populations exceeded a certain level of abundance, after which numerical and behavioural dominance, coupled with mutualism, fuelled the rapid population growth. This ant enters into a mutualism with the scale insects Pulvinaria urbicola and Icerya seychellarum, and the mealybug Dysmicoccus sp. The ant protects the scale insects from parasitoids and predators and in return the ant receives nutritious honeydew from the scale insects and mealybug. The end result is masses of ants, scale insects and mealybugs, which then kill trees such as Pisonia grandis. This tree is an important tree ecologically, as it is used by certain nesting sea birds and the fallen leaves as food for the threatened Seychelles giant millipede. Something had to be done, and fast. In response, René Gaigher, C·I·B PhD student, was deployed on the island from May to July 2010 to find a way of controlling the ant and break the mutualism to save the trees. This ant control had to be done without incurring any harm to the wide range of threatened endemic invertebrate and vertebrate species on the island.

Bait station example

Example of the bait stations used to deliver toxic ant bait in the field. Each station is filled with 10 g of bait and placed on its side at the base of scale infested trees. Ants gain access to the bait through the 7 mm holes indicated. This design prolongs bait efficacy and excludes larger taxa from the bait. (Photo: R. Gaigher)

This challenge was tackled by deploying the selective ant bait, Siege, in bait stations where only the ant could reach the toxic bait. Bait stations were placed at the bases of scale infested trees where ant nests were concentrated around tree bases. Plots of 1-2 ha were treated at a time and bait stations were left out for 7-10 days, after which they were collected, refilled and deployed in adjacent areas. The control was immensely successful, with a crash in the scale insects and the mealybugs as the ant disappeared. Pitfalls and ant foraging activity estimates were used to determine the effect of the treatment. To determine the effect of the treatment on the populations of scale insects, percentage cover estimates of scale insects on P. grandis leaves were used.

As soon as the ant was controlled, the natural enemies of tree-damaging scale insects moved in surprisingly quickly and literally flattened the scale insect and mealybug populations. Already the trees have responded by a new flush of leaves. The end result is that the restoration of the island is back on track as soon as this mutualism involving the interaction between four alien invasive insects had been brought under control.

For further information, please contact Prof Michael J Samways, email: samways@sun.ac.za or René Gaigher: reneg@sun.ac.za.

Rhodolia chermesina (ladybird)

The ladybird Rhodolia chermesina, which instantly appeared in large numbers from pupae as soon as the ant Pheidole megacephala had been controlled using highly selective bait stations. The ladybird instantly started devouring the tree-damaging scale insect Icerya seychellarum. (Photo: M.J. Samways)

René assessing the Pisonia grandis trees

René Gaigher performing scale insect abundance assessments on Pisonia grandis trees. (Photo: R. Gaigher)