While citrus trees (Citrus spp.) are easy to grow, new leaves are vulnerable to citrus leafminers (Phyllocnistis citrella). Plants in the citrus genus vary in hardiness, with most growing in U.S. Department of Agriculture plant hardiness zones 9 through 11. While leafminers can affect a citrus tree’s appearance, infestations rarely kill the tree and damage can usually be prevented without chemicals.
1. Identification and Damage
Leafminer larvae create serpentine patterns as they mine their way through citrus leaves. As the larvae develop, they leave a frass or feces trail inside these mines that looks like a thin dark line just under the leaf’s surface. The last larval stage emerges from the mines and moves to the edge of the leaf where it rolls the leaf around itself and pupates. Larval mining causes new leaves to curl and distort, but citrus leafminer larvae can only survive in the new leaf flushes of citrus and related species. Older, hardened-off leaves are not typically susceptible to damage unless leafminer populations are extremely high.
2. Monitoring Populations
To determine if leafminers are present, monitor your citrus plants using a trap baited with a citrus leafminer pheromone, which is an insect sex hormone that attracts male moths. You can buy traps at most garden centers and nurseries. Traps do not control leafminer populations or prevent infestations -- they are tools that help determine when leafminer moths are present and laying eggs, which is important to effective organic control. Hang a pheromone trap inside the canopy of a citrus tree from March through November. As long as your yard is less than 5 acres, you only need one trap. Check the trap weekly for trapped moths. When moths are present, you'll need to step up your prevention methods.
Citrus leafminer moths infest new leaf growth. Limiting pruning to no more than once each year can prevent infestations, because this shortens the duration of new growth flushes that attract leafminer moths. Leafminer larvae cannot mine old, hard leaves, so you don't need to worry if your tree's leaves are old. Leafminer populations tend to be highest during the summer and fall, so you should also avoid nitrogen fertilizer applications during this time to prevent rapid new growth that will be most affected by leafminers.
4. Biological Control
Several natural predators kill leafminers, including parasitic wasp species (Cirrospilus spp., Pnigalio spp.). These non-stinging wasps lay eggs on mined leaves and the larvae hatch and consume the leafminer larvae. Natural predators are usually already present, and by avoiding chemical insecticides, you ensure their numbers remain high. You can also buy some of these beneficial insects from garden centers or online sources.
5. Natural Insecticides
Spinosad is a natural insecticide derived from the fermentation juices of a soil bacterium called Saccharopolyspora spinosa. Insecticides that contain spinosad can effectively control leafminer larvae without harming natural predators and beneficial insects. You may have to reapply spinosad to citrus every seven to 14 days for long-term control. Spray a ready-to-use spinosad product to cover the leaves of your citrus. Ready-to-use neem oil applied to new leaf growth is another natural insecticide option that may prevent leafminer moths from laying eggs on the citrus leaves. During each cycle of new growth flush, apply neem oil weekly for the most effective control.