The Art of Seeing

It is a good idea to keep an eye on the plants in your garden, but it’s just as important to understand what you are seeing.

Regular visits to your garden will help you detect any change that may occur, such as an increase of pest insects, or evidence of their damage. On the plus side regular monitoring will also allow you to see the buildup in numbers of beneficial insects such as ladybird beetles or lacewings.

Some insects like leaf miners, borers, and gall-inducing insects spend most of their life cycle hidden from view. You don’t necessarily have to be able to see the pest insect itself to be able to identify it – if you know what the damage they cause looks like. That’s knowledge worth having!

Leaf miner larvae burrow inside leaves where they feed on the leaf tissue between the upper and lower surfaces. The damage you see may be squiggly trails, papery blisters or see-through windows on a leaf, sometimes with a grub visible inside. A number of insects are leaf miners, including the larvae of some moths, flies and sawflies. In many cases you rarely see the adult insects – only the minor damage caused by their larvae. For example, the squiggles caused by the larvae of the cabbage leaf miner fly Liriomyza brassicae pictured below.

There are many kinds of wood-boring insects, but the main culprits in gardens are the larvae of longicorn beetles (Cerambycidae) and the larvae of jewel beetles (Buprestidae). Telltale signs that a plant has beetle larvae tunneling away inside include: sawdust on the ground or around the junction of branches, holes with droppings and sawdust in and around them, and flowing sap. Wattles look particularly miserable when infested with wood-boring beetle larvae. They exude an enormous amount of sap (see below) in an attempt to defend themselves, giving the appearance the tree is bleeding to death.

The damage caused by gall-inducing insects is even more obvious – check out my blog post Gall-inducing insects – look for woody lumps and bumps on leaves, twigs and branches. Galls may occur on leaves, stems, buds and roots, and may be caused by certain species of wasps, flies, beetles, psyllids, coccids, thrips, moths or aphids. With most gall-inducing insects, the insect itself is fully enclosed by the gall during most of its life stages.

Some leaf chewing insects damage leaves in characteristic ways, so you can identify the culprit even if they are hiding or have flown away. For example, Christmas beetles leave a characteristic zigzag pattern on gumtree leaves initially, but they may consume leaves down to the midrib if the population of beetles is high. Adult weevils often chew leaf margins resulting in leaves with a typical ‘notched’ appearance (see below), unlike caterpillars which typically cause smooth scalloping of the leaf margins.

Did you know that some beneficial insects, particularly parasitic wasps, also leave tell tale signs of their activities. Have you noticed swollen bronze coloured aphids on your plants? These are aphid ‘mummies’ caused by the parasitic wasp Aphidius (see my blog post Revenge of the Mummy).

I met a gardener once who had pulled all the kale plants out of her vegetable garden and destroyed them, because they were covered in ‘pests’. The ‘pests’ were aphid mummies (see below)! Not only were the aphids dead but there were beneficial wasps developing inside those aphid mummies. They are a good thing to have in the garden!

Some beneficial predatory insects may also be confused with pests, if you don’t know what you are seeing. I know of instances where lacewing larvae and ladybird larvae have been sprayed with chemicals because they were mistaken for pests! Do you know what the larvae of lacewings and ladybird beetle look like?