Bungee-jumping caterpillars

Seychelles is an archipelago of tiny islands scattered across the tropical western Indian Ocean. Their isolation protected them from permanent human settlement until about 250 years ago. Since that time the islands have seen huge changes at man’s hand. This has included the introduction, deliberate and accidental, of many animal and plant species from other parts of the world. The last thirty or so years have seen huge development, much of it associated with tourism, that has involved vastly increased maritime and aviation contact with the outside world, its people and trade. This has provided opportunities for many other organisms to reach and establish a foothold on the islands and spread among them. Thankfully, this has not so far included malaria!

In early 2015, a small, white innocuous-looking moth, arrived by unknown agency. Its voracious caterpillars are not innocuous, however. They defoliate bushes and trees, especially introduced castor oil plants (Ricinus communis) and much larger indigenous Indian Almond (Terminalia catappa) trees among others. Important fruiting trees, like Mango (Mangifera indica)and Zamalak (Eugenia javanica), also apparently meet their requirements and this can bring the caterpillars close to humans.

Semir plim - the attractive but irritating caterpillars that have arrived in Seychelles
Senir plim – the attractive but irritating caterpillars that have arrived in Seychelles

The identity of the moth and caterpillar is still not known for certain, but it appears to be a member of the Euproctis group in the family Lymantriidae. In Creole it is called “senir plim”. These moths are somewhat unusual in that their caterpillars can feed on a wide variety of vegetation and are not restricted, like many moths, to specialisation on just one of two plant species. The caterpillars are characterised by being hairy, probably a defence against predators, but for humans (and some domestic pets) the problem is that the hairs can stimulate intense itching, and sometimes blistering, of the skin with which they come into contact. In extreme cases they can cause headaches and breathing difficulties. The hairs break off the caterpillar easily, so a human can be affected by hairs blowing in the wind, without contact with the caterpillar.

After feeding on leaves, the caterpillars produce long strands of silk to enable them to descend from the tree canopy to the ground. As they do so, the stands reflect sunlight and the hanging caterpillars bob about in the breeze, appearing to be in the process of a slow bungee-jump.

Following their arrival in Seychelles the moths spread rapidly among the larger islands and had even reached the remoter Bird and Denis Islands by June 2015. Attempts to control them have centred on fogging with insecticides (with unknown effects on populations of indigenous and endemic insects and predators, which might include parasites and predators of the moths and caterpillars). During my visit in December 2015 there appeared to have been a resurgence, possibly associated with the wetter north-west monsoon season. Having reached Seychelles, this suggests that the invasive insect is here to stay and highlights the importance of enhanced biosecurity for this once-remote island nation.


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