How to prevent leaf miners

Q: Several of my perennials and tree’s leaves in my backyard get leaf miners every year. I’ve tried many insecticides (the latest was Sevin powder) already, but they don’t work. Is there something that prevents or controls leaf miners?

A: Leaf miners are really incredible insects. They are the larvae of certain kinds of beetles, flies, moths or sawflies.

Imagine a plant leaf. It is only a few cell layers thick — about the thickness of a few sheets of paper. The female insect lays an egg on the leaf or inserts an egg in the leaf (usually only one per leaf).

The resulting larva chews its way around the leaf like a coal miner in a seam of coal. It leaves a layer of leaf above and below for protection. The tunnel is visible because the inside layers of leaf are missing. Look at the tunnel and you will see that it enlarges as the larva grows. It often spirals around the leaf, but it rarely crosses itself. If there is more than one larva in a leaf, the tunnels may cross or just become a large, blisterlike area.

If you hold the leaf up to the sun, you may see the larva is still inside the leaf. If there is a hole at the wide end of the tunnel, it will have hatched and gone on to lay eggs of its own.

Since the leaf and its waxy coating protect the larvae from predators and surface insecticides, you need to use other methods of control. Read the label of the insecticide Sevin and you will see that it says it is used for chewing and sucking insects. Leaf miners are not listed on the label.

You will need to use a systemic insecticide. These insecticides soak into the plant and then move throughout the plant. Many systemics last for several weeks inside the plant. If you are using the insecticide on food crops, be sure to read the label for the correct timing of how many days you must wait before harvesting the crop.

Because many of these insects have several generations per season, you may have to reapply the insecticide. If you know the life cycle of your particular pest, you could apply Sevin to the leaves so that the female does not lay her eggs. Sevin is very safe for food crops and can be washed off easily. It is so safe it is used in some formulations of flea and tick controls for cats and dogs.

If there are only a few leaves infected with miners and they are still in the leaf, you could just pluck the leaf and step on it.

Q: I have several rhododendrons that have outgrown their space. They are five feet tall with leaves just near the top, and I want to prune them back to three feet tall. Can I do the pruning this fall?

A: You can, but pruning on rhododendron and azaleas should be done in the spring right after they finish flowering. By midsummer, next year’s flower buds are already on the plant. It is best to prune them back to an existing green bud or leaf, but old, spindly plants may not have live buds on old wood. Try it and see what happens, but it may be better to just move the tall plants to a better location and replant with suitable plants in the original location.

The best time to move the plants is in the fall or winter when they are dormant. Rhododendrons have shallow, small roots that spread out widely. You will not have to dig deep, but it would help to get as large of an area as you can manage. Prepare the new hole first, and add lots of peat moss in the backfill and in the mulch.

Email questions to Jeff Rugg at info@greenerview.com.