A leafminer is the larvae (baby) of the adult moth who lays eggs underneath young leaves. When they hatch the larvae burrow into the leaf and start feeding between the lower and upper leaf surfaces. Their feeding habits make trailing marks on the leaves and causes leaves to curl inward and warp.
There are many different types of leafminers and they all tend to be host specific. For example: there is the introduced Asian Citrus leafminer, the lantana leafminer, Azalea leafminer, Callistemon leafminer, Cineraria leafminer and so on.
Generally, the life cycle of leaf miners is between 2 and 4 weeks but that can vary with the species. Young plants can be affected by leafminers more so than older plants. This is largely due to the amount of leaves the plant has.
Most experts agree that plants generally don’t die from leafminers.
Prevention largely depends upon the type of plant and the associated leafminer. Let’s take the citrus leafminer as an example. If we fertilise our citrus plants in summer we’d expect to get some new growth. That new growth will be attractive to leafminers.
If we use a high nitrogen fertiliser, it’ll be much more attractive to leafminers. A synthetic high nitrogen fertiliser will be even more attractive.
Therefore, we should fertilise our citrus in late winter to early spring. This is because the citrus leafminer does most of her damage in autumn and is relatively low in numbers in winter to spring.
Managing leafminers on our plants
There are a number of ways to manage leafminers once they are on our plants.
The number one way to reduce leafminer numbers is by encouraging their natural predators. One key predator of leaf miners are parasitic wasps. There are a number of species of native and introduced wasps that prey on leafminers.
Lacewings are also good leafminer predators. We attract predators to our gardens by having a healthy, pesticide free garden and by providing predators with food. So a few leafminers on a couple of our plants will help feed the predators we need in our gardens.
Another way is to squish the leafminer whilst it is in the leaf. If we have good eyes and we can see the leafminer in the leaf we can simply squash them. If we put the leaf up to the light we are better able to see them.
Another remedy is to prune off the individual leaves that are affected. We should also bag the leaf prunings to prevent them from hatching and flying away and back onto our plants.
If we take care, we can spray or paint some horticultural oil underneath the new leaves. Moths don’t tend to like laying eggs on oily surfaces. However, horticultural oil will kill any other insect that gets sprayed or painted. This includes our predatory insects such as predatory mites, spiders, lady beetles, wasps, lace wings. It works by suffocation.
There are other insecticides we can use. They’re harmful and I wouldn’t advocate using anything other than what I’ve described here. Leafminers cause ugliness more than they cause death and disease.