Hi Ho-Hi Ho-It’s Off to Mine we Go

by Jolene Adams, past ARS President

We’re talking about Miners ... Leaf Miners.

You’ve seen these squiggly lines on your rose leaves - - what is doing this and how can you get rid of the problem and keep it from coming back?

What is a leaf mine? It’s a translucent trail left by a tiny larva as it feeds in the parenchyma of a leaf. Mines may be linear in shape, or serpentine, trumpet, blotch, or just wriggly all over the leaf. The larvae grow as they feed, and so does the circumference of their mine. Kinda like “botanical doodling.” If you look closely at a mine you will see that it starts as a small dot and widens as the larva inside grows.

The Pest

Foliage miners are the larvae of several families of small moths and some small flies. They are the most common foliage-mining pests in the garden. Larvae of other insects, including sawflies and wasps, also mine some plants, feeding inside leaves, needles, shoots, or buds. Each kind of miner feeds on only one kind of plant or several closely related plants. The host species and characteristic form of the larva's damage help to identify the insect species.

Every plant family has its own miners - most of us here in California will have the kind that are in the family Liriomyza.

The Damage

Foliage-mining insects cause off-color patches, sinuous trails, or holes in leaves. Portions of a leaf or patches of foliage may gradually turn yellow or brown and die back. Severe infestations can slow plant growth; established woody plants tolerate extensive foliage mining but are rarely if ever killed.

The adult female flies over to a nice, juicy leaflet and deposits eggs under the leaf surface. When the larvae hatch they tunnel into the leaf and begin feeding..... and continue eating and eating. They are consuming the leaf tissues as they slowly grow larger. You will notice that the wiggly line starts as a very small spot on the leaf and slowly grows wider and wider as the larva inside gets bigger. Depending on the thickness of the cuticle on the leaf (the cuticle is the top layer of the leaf) you will either see a whitish trail or a black trail as the larva heads for the edge. Remember - what goes in has to come out - so the “mine” or “trail” fills up with excrement.

The larvae live and eat inside the leaflet for 2 to 3 weeks before they mature. They are steadily heading for the edge of the leaf where the tissue is thinner and it can exit. Its next big adventure starts when it falls to the ground under your rose bush. The larvae overwinter in the soil of your garden and emerge in the spring as young adults - ready to mate and start the damage all over again.

Solutions

Provide proper care, especially irrigation, to keep plants vigorous. Remove and dispose of foliage infested with larvae as soon as you see the evidence. This gets rid of the larvae before they can wriggle out of the leaf and begin their next step in their life-cycle to become adults and start a new generation of miners to infest your roses. Clipping off damaged leaves also restores the plant's aesthetic appearance.

While pesticide is the most common form of control methods for leaf miners, it is not the most effective. Naturally killing leaf miners with beneficial bugs is better for your garden and the beneficials work for free! They also will lay eggs and multiply - keeping miners more or less in check. You can purchase wasps called Diglyphus isaea from reputable nurseries. These leaf miner natural enemies will make a meal of the leaf miners in your garden.

Another way of naturally killing leaf miners is to use neem oil. This insecticidal oil affects the leaf miner’s natural life cycle and will reduce the number of larva that become adults and thus the number of eggs that the adults will lay. While neem oil is not an immediate way to kill leaf miners, it is a natural way to treat these pests.

If you decide to spray with a general pesticide such as acephate, carbaryl, neem, or pyrethrin, spray it only on the infected plants. The trick to this method of how to kill leaf miners is to spray at the right time. If you spray too early or too late, the pesticide will not reach the leaf miner larva and will not kill the leaf miner flies.

To effectively figure out when to rid plants of leaf miners with a pesticide, in the early spring place a few infected leaves in a ziplock bag and check the bag daily. When you see small black flies in the bag (which will be the leaf miner larvae becoming adults), spray the infected plants daily for a week.

Yes - you may be saying “ick” and “no-no-no” but ... this is also an IPM solution - it is the LAST thing you should choose, but it IS a choice. And you ONLY treat those roses that are infested.

Maintain plant health with natural fertilizers and proper watering to allow plants to outgrow and tolerate pest damage. Keep your soil alive by using compost and other soil amendments.

Photos submitted by Jolene Adams