Leaf Miners Cause Minor Damage

Leaf miner patterns in a luffa leaf. Photo credit Forest and Kim Starr, Wikimedia Commons

By Dr. Curtis Smith! with an added note by Dr. Marisa Thompson - Partial reprint from July 23, 2016

Question:

My bean and luffa plants have been diagnosed with leaf miners. What should I do now?

Answer:

Several unrelated insects are called leaf miners. The one infesting your vegetable leaves is probably the larvae of a very small fly. Like all leaf miner insects, the eggs are laid by the adult female insect on the outside of the leaf, usually on the underside. When the egg hatches, the larva immediately enters through the lower surface of the leaf into the layers of cells between the top and bottom surface. The larva then feeds on the layers of cells inside the leaf that contain chlorophyll. Because the green cells containing chlorophyll have been consumed, the path of the larva's feeding appears white or yellowish, and the affected area can turn brown as the cells die, creating a larger blotch in the leaf.

Minor leaf miner damage tomato plant leaflets. Photo credit Scot Nelson, flickr.com (public domain)

If you hold an affected leaf up to a bright light (do not look toward the sun), you may see a dark spot in one of the tunnels or in one of the larger blotch-like areas. This dark spot may be the larva, or it may be the pupa formed as the larva prepares to metamorphose into the adult insect, which will then emerge from the leaf to lay eggs to infest additional leaves. Some other types of leaf miner insects may emerge from the leaf and pupate in the soil under the plant to emerge and continue the infestation when they mature, but the leaf miner you have probably pupates in the layers of the leaf.

Many people consider the problem to be only cosmetic and not significant enough to warrant treatment. While the leaf miners are inside the leaf, most insecticides are ineffective. Several natural predators (i.e., beneficial insects) feed on leaf miner larvae and keep the leaf miner population under control. Severe leaf miner infestations may develop if insecticides that were intended to control the pests kill more of the beneficial insects instead.

Do your best to maintain beneficial predator insect populations. That means there must be some of the leaf miner insects to allow survival and reproduction of the beneficial insects. That also means you must be very cautious when choosing chemical controls for insect pests so that you do not harm the beneficial insects.

Leaf miner damage on aspen in Carson National Forest. Photo credit Marisa Thompson

Additional notes from Dr. Marisa Thompson: I received a similar question in a live Q&A session this week and was happy to find Dr. Smith’s response in the Southwest Yard & Garden Archives. When I’m lucky enough to encounter a leaf with excavation lines, my first instinct is to hold it up to the light, too, but I hadn’t thought to catch the culprit in the act.

When diagnosing plant problems, pay attention to which leaves are affected. Are symptoms showing up on the new or old growth? Whether damaged by insects, sunburn, or tattered by the wind, older leaves may look shabby, and that’s okay. No leaf lasts forever. I think of leaves like strands of hair. They get damaged over time, develop split ends, and shed. It’s more concerning when new leaves are looking harried.

While I was digging around for more on leaf miners, I also found the NMSU Extension Guide H-174: Integrated Pest Management (IPM) Strategies for Common Insect Pests of Trees in New Mexico and H-243: Economic Insects of Chile. Both resources affirm that leaf miner damage is usually minor and warn against harming beneficial insects that are helping by devouring pests. One more bit of good news: leaf miners only feed on leaves, not on fruit.

Marisa Y. Thompson, PhD, is the Extension Horticulture Specialist, in the Department of Extension Plant Sciences at the New Mexico State University Los Lunas Agricultural Science Center, email: desertblooms@nmsu.edu, office: 505-865-7340, ext. 113.

Links:

For more gardening information, visit the NMSU Extension Horticulture page at Desert Blooms and the NMSU Horticulture Publications page.

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