Oh no, here comes an annoying problem for those of us who take pride in our beautiful garden citrus trees. An insect native to Asia is invading California’s citrus groves.
It showed up in Southern California in 2000, making its way from Florida, where it was first spotted in 1993. The little pest is creeping northward and arrived in Marin in 2012.
The citrus leafminer (Phyllocnistis citrella) begins its assault on the new leaves of lemons, limes, oranges and other citrus varieties and related species. Larvae feed on the interior of the tender leaves, creating little tunnels (mines) as they go. There are other mining-type pests, including the citrus peelminer, but as its name suggests, it goes after the fruit (and stems). The citrus leafminer is the only mining insect that attacks the leaves.
Citrus leafminer damage is easy to spot. As the yellowish larva moves through the leaf, it leaves a noticeable meandering trail of frass (feces) just under the surface. Emerging from the mine in its last stage of development, it moves to the edge of the leaf, rolling the leaf around itself and pupating, distorting the shape of the leaf.
The adult that eventually emerges from the pupal stage is a tiny moth, less than ¼ of an inch in length. The adult does not cause damage, but a newly emerged female quickly mates, laying eggs on the underside of the leaf that hatch within a week. The larvae then molt four times over the next two to three weeks. The entire life cycle of the citrus leafminer is just about seven weeks. Knowing this cycle can help you control the spread of this pest.
Fortunately, there is more good news than bad with this new invader. Not only is damage easy to spot (look for it if shopping for a new tree), but citrus leafminer infestation does not necessarily have a major impact on the growth and fruiting of mature trees, since these have many hardened leaves to sustain them that the pest usually leaves alone. Young trees may not grow as well or as quickly if infested, but they will seldom die.
The heat in the interior of our state seems to suppress leafminer numbers. Our cooler coastal areas can have populations that last throughout the summer and well into fall, but the problems created are mainly cosmetic.
So, what if you discover that the citrus leafminer has taken up residence in the leaves of your prized Meyer lemon? What do you do? The University of California encourages the use of biological/cultural control before the use of pesticides, although there are a few that, applied correctly, can be effective without hurting the natural predators that can eventually control the citrus leafminer. Luckily, unlike some other foreign invaders, there are natural enemies of this bug in our state, such as tiny, parasitic wasps whose larvae feed on the citrus leafminer larvae (Cirrospilus and Pnigalio).
Leaving well enough alone to allow the balance of nature to do its thing may be enough, although it might take a couple of years, and you may be rightly concerned now about a very young tree. Timely pruning and fertilizing to assure limited new growth during the citrus leafminer season, and removing suckers and water sprouts (both actions limiting mining sources), can be helpful. Pheromone traps are available that attract male citrus leafminers — not to control them, but to help correctly identify the moth and choose the appropriate course of action.
More good news comes from Eric Richardson, an inspector for the County of Marin Department of Agriculture, Weights and Measures, who says our natural predators must be doing their job because the incidents of citrus leafminer infestation and damage are down significantly from a high in 2013. But if you suspect that your citrus is affected by the pest, please don’t hesitate to call the Marin Master Gardener help desk or go to the UC Integrated Pest Management site at ipm.ucdavis.edu for helpful tips on management and care.