July 23, 2016

1 - Leaf miner insects are common pests in New Mexico gardens, but difficult to manage with insecticides.

Yard and Garden July 23, 2016

Q.

I have a problem about my beans and luffa. The leaves keep turning yellow and drying out. I have been planting bush beans and spaghetti beans for last three years. The same problem keeps coming back even when the bean are still small. Someone told me it is caused by leaf miner. But I do not see any worms or eggs on the leaves. I believe it is the problem of the soil. What do you think? I attached a picture for you to identify.

** - Yulin**

A.

The person who diagnosed leaf miner insects is correct. In this case the problem exhibited in the photograph is neither nutritional (related to the soil), nor is it due to a disease.

There are several unrelated insects that 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 of the leaf. 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 to be yellowish or brown line or larger blotch in the leaf. 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.

While the insect is inside the leaf, most insecticides are ineffective. Many people consider the problem to be only cosmetic and not significant enough to warrant treatment. In the case of leafy vegetables, the damage may be significant, but in beans, squash, luffa, beets, onions, and many other vegetables it may not warrant treatment. While there are some insecticides that may kill leaf miner larvae before they enter the leaf, once they are inside the leaf, most insecticides available to home gardeners are not effective. There are natural predators insects that feed on leaf miner larvae. These natural predators often keep the leaf miner population under control unless insecticides that kill the predators are used. In that case, severe leaf miner infestations may develop.

I suggest that you take a sample of infested leaves to your local NMSU County Cooperative Extension Service office to have the insects positively identified. The Extension agent may need to send the sample to the NMSU College of Agriculture's Plant Disease Diagnostic Laboratory in Las Cruces, so take the sample to your Extension agent early in the week to allow time for the sample to reach the Diagnostic Lab before the weekend. (They will also identify insect problems.) The County Agent can also advise you with regard to the need for insecticides and the appropriate insecticide or other appropriate control measures if control is needed based on the insect identified and the level of infestation in your garden. Do your best to maintain the beneficial predator insect population. That means there must be some of the leaf miner insects to allow survival and reproduction of the beneficial insects. That also means that you must be very cautious when choosing chemical controls for the insect pests so that you do not harm the beneficial insects. Sanitation, cleaning up affected leaves when they fall from the plant, will help reduce the number of adult insects to reinfest your garden.

Thank you for sending the excellent photograph that made it possible for me to confirm that leaf miner insects were present.

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.

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