What is a “leaf miner”?

Have you ever looked at a leaf and noticed some squiggles or blotches on it? Well that could be the work of a leaf mining insect larvae – there might be an insect in there!

A mysterious squiggle on a leaf – made by a leaf mining insect larvae!

I’ve been describing the subject of my PhD research as “a group of leaf-mining micro-moths”, but what does that phrase actually mean?

Leaf mining larvae inside a Eucalyptus sp. leaf
Leaf mining – it’s a lifestyle!
A diagram depicting examples of types of leaf miners (drawn by Lapsap_Art)

The term leaf miner actually refers to many types of insects, so there are leaf mining moths, flies, sawflies etc. It is a description of a life style, similar to the term “burrower” or “cave dwelling”. In the same way that “cave dwellers” can refer to many groups of animals that live in caves, “leaf miners” refers to many groups of insects that make mines in leaves. It just so happens that insects are the only things that are small and diverse enough to have evolved this lifestyle. Leaf mining insects include sawflies (Hymenoptera), beetles (Coleoptera), flies (Diptera) and moths (Lepidoptera). Of the leaf mining insects, a majority of the species are moths.

What do leaf mines look like? And how are they made?
A leaf mine in an Acacia sp. leaf. The dark areas are frass, and you can see the larvae in the middle of the clear patch.

Leaf mines can look quite different, depending on the type of insect and the species of the insect that made it. In general, when I look for leaf mines I look for a blotch or squiggle on a leaf, and then I hold it up to the light. If it is a leaf mine you’ll see some pellets (aka. frass aka. insect poop) in the leaf and if you’re lucky you’ll even see the larvae inside too. But what are they doing in there?

Growing up inside a leaf
A cross section of a leaf, with a leaf mining larvae shown in the epidermis (drawn by Lapsap_Art)

Leaf mining insects live and feed internally in a leaf. The female adult lays an egg on the outside of a leaf, and the larvae burrow (or mine) into the leaf and the larvae then feeds on the internal tissue of the leaf, resulting in some interesting shapes and patterns which may be observed on the outside of a leaf. Sometimes they spend their entire larval (and perhaps pupal) stage inside a leaf, and never move to a new leaf.

A leaf mining larvae feeding in the epidermis (as in the above diagram).

All insects have a larval stage. In butterflies, the larvae are commonly known as caterpillars and in flies the larvae are commonly known as maggots. When referring to “leaf miners”, it is really the behavior of the larval stage that is being referred to. The adult leaf miner is often a nondescript fly or moth that can be hard to identify as the adult form of a leaf miner (even with a microscope!).

What about the adults?
Here one of my leaf miners (a moth) exited the leaf mine and spun a cocoon on the lid of the container.

Good question! In general, once the larvae has finishing feeding it will develop into a pupa (e.g. spin a cocoon) and then eventually moult to become an adult insect (e.g. a beetle, fly, or moth). Many adult leaf miners do not feed, they have reduced or non-existent mouth parts and their aim is to disperse and mate. The females will then lay an egg on a new leaf and the cycle continues…

So what are “leaf mining micro-moths”?!
A specimen of the invasive oak leaf miner Phyllonorycter messaniella (Family: Gracillariidae) in a vial. (Yes it’s really there!)

The leaf miners I study are part of the moth family Gracillariidae, where a majority of the species are leaf miners. The adult moths are smaller than grains of rice, hence the “micro-moth” designation. Leaf miners in Australia remain quite understudied, so I’ll be updating this blog with some of the interesting findings I’ve made. Stay tuned!