Citrus Leafminer
By Norman Smith
The Citrus leaf miner does most of its damage to the flush of new growth on citrus that occurs during the warm summer months, although it does not damage the fruit. Feeding damage occurs on the leaves.
The citrus leaf miner is a microlepidoptera (a small moth) that lays eggs on the new growth. These eggs hatch into tiny larvae that will feed inside the leaf beneath the epidermis, with the mines getting bigger and bigger as larvae grow. When this mining happens to young developing leaves, the leaves deform as they grow and look unsightly. But that is pretty much where the bad news ends. If left alone, that branch with the unsightly, deformed leaves will develop normal flowers and leaves the following year.
The citrus leaf miner is not active until after the spring flush of flowers and leaves have developed so those are not in jeopardy. Basically, the damage is cosmetic and does not affect the trees ability to set, size and ripen normal fruit. Therefore, we do not recommend any type of chemical control for this pest. However, pheromone treatments are commercially available to home gardeners. The pheromone treatment won't eliminate all ovipositing females, but it may lower their numbers so that damage is not as extensive.
One additional note, if you do see mining on the peel of a citrus, that would be interesting and you should inform San Luis Obispo County Ag Commissioner's office. The citrus peelminer will mine the peel of citrus as well as the leaves. But it is not known to occur in San Luis Obispo county. It is prevalent in the Coachella Valley and San Joaquin Valley and finding it here would be problematic for commercial growers in this region. Growers would likely have to treat for citrus peel miner to prevent a downgrade of marketable fruit. Read about the differences between citrus peelminer and citrus leafminer here - https://anrcatalog.ucanr.edu/pdf/8321.pdf
About the author: Norman Smith was the Fresno County Entomologist for 30 years. I now enjoy working in my garden, traveling with my wife, golfing and bowling, taking insect collecting trips in the US and overseas in the tropics, and working on some personal research of some small wasps. I also enjoy working with and for the UC Master Gardener program in SLO.