May 2019

Caterpillar Cluster

Are these ‘itchy grubs’ and why are hundreds of them gathering on our back patio?


Processionary Caterpillars nest. Photo courtesy of Susan WightleySilk threads, laid down by caterpillars on the move. Photo courtesy of Susan WightleyTangle of Processionary Caterpillars. Photo courtesy of Ken Griffits
Tangle of Processionary Caterpillars. Photo courtesy of Ken GriffitsSometimes commonly called itchy grubs because of their capacity to cause skin irritations upon contact, this cluster of caterpillars is the larva of the Bag-shelter Moth (Ochrogaster lunifer). At this stage in their development they are commonly called ‘processionary caterpillars’ due to their conspicuous, almost comical, behaviour of forming long ‘trains’. These trains form because of the caterpillar’s instinct to follow the silk thread layed down by each caterpillar as it moves about. At certain times of the year, hundreds can be seen crossing roads, footpaths, walking tracks and parking lots. One such train observed in Toohey Forest on Brisbane’s southside contained 312 individuals nose to tail.

Around November their life cycle begins as an egg mass laid at the foot of a food tree, usually a species of Acacia (wattle) or Casuarina (She-oak). Tiny caterpillars emerge from the egg mass around a month later and scale the tree trunk to begin a feeding frenzy. The group shelter during the day in a silken nest built at the base of the tree or amongst the branches and scale the trunk at night to feed, always producing silk threads to follow. The food tree becomes lined with dozens of threads of silk as it is decimated by the hungry horde. As the tree becomes defoliated a procession will start as the caterpillars move off en masse to find more food. They follow each other because of the silk thread that is produced by each caterpillar in the chain. Any caterpillars of this species that stumble across the silk threads will automatically follow it … leading to more and more joining the caterpillar conga line. They spend the next several months moving from tree to tree growing bigger and bigger until the weather begins to cool. Usually by May the caterpillars stop growing and smaller groups break away and begin to look for a safe place to wait out the cooler months. This is done in a silken ‘bag’ in the soil under leaf litter.

We cannot be absolutely certain as to why this group ended up amassing on your patio, but given the time of year a possible scenario is that small splinter groups moving off to find a final feast before hunkering down for the winter have crossed paths with other groups. They may have become distracted by false signals or momentarily lost while negotiating obstructions in their trail ending up clumping on your patio. As the caterpillars following behind are driven to follow the silken thread of the one in front they can easily end up folding back on themselves resulting in a writhing mass of hairy caterpillars and silken threads. They would stay like this until a few ‘escape’ the cluster to continue on their way leading to another mass exodus, nose to tail. Whatever the reason for this patio visit, it is a fantastic opportunity to witness this amazing natural phenomenon.

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