By George Giltner, Adv. Master Gardener, MS Biology
If you have planted Satsuma’s or other citrus within the past 3 years, chances are that the leaves have been mined by the citrus leaf miner, Phyllocnistis citrella. Close observation of the distorted wrinkled leaves will show the twisting larval tunnels, larval frass trail, and possibly the live larvae itself.
When the larvae are near the leaf marginal edge, use a fingernail to extract one. Then you have successfully controlled one leaf miner, needless to mention that several hundred that may be ready to hatch from recently laid eggs by the 2 mm long adult female moths. The singly laid eggs hatch after 4 to 5 days, then begin their mining operations for over a month, usually from midsummer to early winter. The leaf distortions and curls are caused by the destruction of upper and lower cellular tissue.
As you review the entire citrus plant, notice the pattern of infestation. New fast growing and tender leaves are infected, but the hardened thicker leaves are resistant to leaf miners. Therefore activities that stimulate rapid plant growth like pruning, vertical water sprout growth, and heavy fertilization actually make your citrus more favorable to citrus leaf miners. Also notice that older (4+ years) trees are not impacted like younger trees. The aged and hardened older leaves cannot be penetrated and mined like fresh leaves.
Commercial products containing imidacloprid, a systemic insecticide, have been used during periods of flushing (rapid growth) of young citrus. These soil or irrigation treatments can protect the plants for up to 3 months. However, there is concern over the use of this insecticide and its effect on bees. The systemic insecticide will be inside plant tissues, pollen, and nectar. Experts report that use of this chemical is one of the many possible causes of bee decline and colony collapse disorder (CCD). The European Food Safety Authority (Jan. 2013) stated that neonicotinoids (imidachoprid) pose an unacceptably high risk to bees.
If you are an internet user, some suggestions for leaf miner control are absolutely hilarious. One suggests using moth balls. The response was to take good aim at the moths to be effective. Another was to use Sevin, but the larvae is tunneled and protected by the waxy leaf cuticle.
Biological control can be the most effective means of control. Worldwide, 39 parasites of CLM have been identified. 14+ natural enemies (many wasps) have been identified within the U.S. Since the initial identification of CLM in Homestead, Florida (1993), it has spread throughout the South into Louisiana and Texas. It is now a common pest. However recent research has shown up to 90% mortality due to endemic beneficial insects. The message from researchers is to not use wide range killing insecticides (malathion, carbaryl, and pyrethroids) that destroy beneficial predators of citrus leaf miner. Better control with less environmental impact can be obtained by using spinosad formulated products like Conserve, Green Light Spinosad, Success, Leafminer & Tent Caterpillar Spray, etc.
Gardeners and homeowners of citrus may choose to not treat citrus for citrus leaf miners. That is correct – do nothing and let nature take its course (Dr. Malcolm Manner, UFC) . Young citrus will be infected and look ugly for several years. Nature will begin to supply CLM predators without the use of harmful insecticides. The mature hardy thick green leaves develop. Then the citrus tree matures into its round dark green form.