When is a bug a feature?

In the wake of Pollinator Week and the inaugural Bug Hunt, organised by Invertebrates Australia and the Invasive Species Council, many people have been talking about the wonderful world of insects! It’s been great to hear about the diversity of flies, beetles, bugs and other insects people are seeing and identifying.

It’s also fun to watch the records roll in via the iNaturalist app that is such a great resource for citizen and professional scientists alike; if you haven’t tried it yet, give it a go next time you see an interesting or unfamiliar animal or plant. I’m far from an entomologist myself, but have found out a bit about some of our local six-legged friends on my plant learning journey.

For example, one technique I’ve learned to help identify plants has been examining the impacts on them of the assorted insects that spend time with them. The patterns made in leaves and trunks by different insects are often quite recognisable. Sometimes I’m not aware of the species or even the genus of the insect, but the pattern is enough to discern the species of the plant! For this article, I sought the assistance of Dr Chris Reid of Beetling About fame to identify the insects.

The Red Ash tree (Alphitonia excelsa) is one where characteristic leaf damage helps with recognition. As the summer advances, leaves can be almost skeletonised by the caterpillars of various butterfly and moth species, giving the canopy a light and ‘lacy’ appearance. When the foliage is high up, leaf shape and colour can be difficult to discern, meaning that this lacy look is a useful feature to watch out for.

Another plant that often has characteristic leaf damage from insects is the aptly-named shrub Devil’s Needles (Solanum stelligerum). This spiky-leaved shrub is fairly common in a range of ecologies including recovering rainforest, and the plant can look similar to a few other species.

But none gets the leaf-munching insects so excited, making it easy to tell from other plants such as young Native Peach (Trema tomentosa var. aspera) or the weedy Jerusalem Cherry (Solanum pseudocapsicum), which is widespread in the region. In this case, the munching is likely to be down to flea beetles (genus Psylliodes) which has metallic green or blue adults, as pictured below.

Perhaps surprisingly, the Giant Stinging Tree (Dendrocnide excelsa), which is renowned for the painful sting caused by its leaves (and many other parts of the plant) is also vulnerable to insect attack, as the below image shows. Somehow, certain moth and butterfly caterpillars are able to eat the soft part of the leaf without being damaged by the stinging hairs that are so painful to us humans. 

While the Giant Stinging Tree is a pretty unique tree, being large and with pale green round leaves, it is useful to be able to tell its leaves from those of other rainforest species, in order to avoid being stung. The presence of insect damage helps distinguish this species from the Round-leaf Vine (Legnephora moorei), which has a similar shape and size of leaf but rarely has much insect damage evident.

Many other features are important to tell apart the leaves of the innocuous Round-leaf Vine and the dangerous Giant Stinging Tree, but this is a useful one to add into the identification mix. And it is important to be able to tell these two apart, because they often occur in the same areas, in remnant or recovering Illawarra Subtropical Rainforest. On guided walks and bush regeneration activities up at the Illawarra Rhododendron and Rainforest Garden in Mount Pleasant, we often encounter both species.

Insect impacts on plants aren’t limited to the leaves, of course. Wasps and other insects can ‘hijack’ plants’ natural growth function to create swollen structures known as galls that protect the developing insect larvae. Borers can kill large stems or even whole trees. Termites can hollow out massive eucalyptus trunks, creating nesting places for native fauna. And curl grubs (which include the larvae of the much-admired Christmas Beetles) can also damage or even kill plants.

It’s important though to recognise that these impacts are part of the incredible web of life, with flora, fauna and fungi all interacting in complex ways that have evolved over millions of years. We are fortunate to be part of this dynamic and resilient web.

Special thanks to Dr Chris Reid for identifying the insects included in this post.