Our trees are under attack! But could these hungry marauders actually be helping our forest along?
An army of slug moth caterpillars was spotted on one of our Cyclobalanopsis edithiae trees at our forest restoration site this summer. This was both a blessing and a curse. The return of insects is an unmistakable sign of increasing tree diversity and a direct result of the regenerating forest providing an attractive variety of food options to a growing range of wildlife. However, the downside was a fear that our trees might not survive the attack.
Spotting something we have never seen before on our restoration site is always a delight. That is how we know we are doing it right. However, encountering these Limacodidae caterpillars also made us wonder whether our young forest is ready to deal with an onslaught of herbivores.
This troop of intruders has a mighty defensive mechanism. You can probably deduce their tactics just by looking at them: they are covered in an outrageous coat of stinging hairs!
Just a glimpse of their alien appearance is enough to fill your mind with horror and trepidation. This is a surely an indication of an evolutionary strategy that has shaped the response of would-be predators. So well armoured is this caterpillar that it is able to defend itself against even the most ardent invertebrate predators, including paper wasps and assassin bugs. Indeed, a study has shown that wasps perform ‘aversion learning’, by which they learn to avoid such spiny caterpillars as compared to non-spiny ones, after previous experience of being stung by them (Murphy et al, 2009). But the question remained, will our young trees be able to defend themselves in the face of such marauding herbivores?
It turned out we really just needed to put our faith in nature to do its job, for the forest had launched its own response long before we could say “Limacodidae”: once the caterpillars had defoliated about 20–30% of the crown of our beloved Cyclobalanopsis edithiae tree, they decided to move on! This was in line with the findings of a study that revealed that the herbivores switch from one host tree to the next once they have munched their way through 30–40% of its leaves, thereby avoiding killing their food source altogether (Duke, 2002). This was the miraculous lesson in ecological engineering that these amazing creatures taught us.
Pictures and videos showing a troop of Limacodidae making their way from one tree to the next:
Such a phenomenon not only serves our aspiring moths and butterflies, it also helps the surrounding plant community. Cyclobalanopsis edithiae has a dense canopy that does not normally allow much sunlight to shine through. But if the canopy is reduced by 30%, more light can penetrate to the forest floor, benefitting the seedlings that grow there. On top of that, the caterpillars feeding in the canopy are so voracious that you can actually hear a constant patter as their droppings fall down onto the plants in the understorey. By consuming all those nitrogen-rich leaves above our heads, they are producing a steady flow of fertiliser that nourishes the forest from the bottom up. Effectively, they are working as natural foresters, thinning tree crowns to improve the performance of the ecosystem as a whole.
The growing diversity of trees in our nature reserve is a call for native animals to return, too. We expect to see a chain reaction that involves the arrival of herbivores first, then an array of animals such as birds, wasps and beetles that feed on these herbivores, followed by other predatory animals higher up the food chain later on.
Although it is tempting to simply think of herbivores as detrimental, it is important to bear in mind that they are invaluable building blocks of a healthy ecosystem. Every species has its part to play in the big picture, for in a self-sustaining ecosystem, every role serves a purpose. Slowly but steadily, animals are creeping and crawling back to find a footing in our emerging young forest.
By Kit Lee
Duke, N. C. (2002) Sustained high levels of foliar herbivory of the mangrove Rhizophora stylosa by a moth larva Doratifera stenosa (Limacodidae) in north-eastern Australia. Wetlands Ecology and Management 10: 403–419.
Murphy, S.M., Leahy S. M., Williams, L. S. and Lill, J. T (2009). Stinging spines protect slug caterpillars (Limacodidea) from multiple generalist predators. Behavioural Ecology 21: 153–160.