We receive many calls and plant samples each year from home citrus trees infested with the citrus leaf miner (Phyllocnistis citrella Stainton). This insect is a tiny moth that lays an egg under the leaf from which a larvae hatches that burrows into the leaf. The larvae then tunnels around in a serpentine pattern between the leaf surfaces (hence its name “leaf miner”) leaving a whitish, silvery winding pathway through the leaf. Damage from this pest causes citrus foliage to cup and twist, creating a malformed leaf. After one to three weeks the larvae enter their pupal stage in the leaf which can last for another one to three weeks depending on temperatures. From the pupal stage the adult emerges to start the cycle over again.
Leaf miners attack succulent new growth and generally avoid fully developed, tougher older leaves. On established citrus trees these pests are generally a nuisance, and do not usually affect the overall health of the tree enough to warrant control efforts. New trees however are in a very active process of growth and extensive damage from this pest can significantly reduce their active leaf area setting them back considerably. Therefore efforts to protect young trees (first three years) are recommended.
Citrus leaf miners have a number of natural enemies including several species of parasitic wasps that lay their eggs in the leaf miner’s tunnels or into the larvae. These natural enemies can build up to the point which they maintain leaf miner populations at acceptably low levels. Therefore use of broad spectrum products such as malathion, carbaryl, and pyrethroids are not recommended as they will kill beneficial insects and can result in a buildup of whiteflies, scale insects, and other citrus pests.
When spraying is deemed necessary, applications of horticultural oil applied in spring when a new flush of growth appears and repeated every 7-10 days until the leaves reach full size and become more leathery are an option for control. It is critical to thoroughly cover the lower and upper leaf surfaces with the oil spray to achieve acceptable results. Stop oil applications when temperatures reach the mid 80’s to avoid damage to the plants.
Products labeled for use on citrus that contain Spinosad are a low toxicity option for control of this pest and can be used at any time in the season. Start spraying when a new flush of growth emerges and the first signs of leaf miner damage are found and repeat sprays every 7-14 days. Good spray coverage of the foliage is important for effective control. Once foliage reaches its mature size and starts to become dark green and more leathery stop spraying. The goal of spraying is to control the pests before they are able to enter the leaf since topical applications of insecticides to the foliage will generally not affect larvae feeding inside the leaf.
Another option is to apply a systemic product containing imidacloprid and labeled for use on citrus trees (ex. Bayer Advanced Fruit, Citrus, and Vegetable Insect Control) to the ground around the base of citrus trees once per year in the spring. This product takes 1 to 2 weeks to move from the roots to the leaves, so it should be applied as soon as a new flush begins to appear. It lasts for 1 to 3 months and since it is translocated in the interior of the plant won’t harm beneficial insects on the surface of the foliage. However to protect bees, avoid applying imidacloprid during the period 1 month prior to or during bloom.
A cultural technique to avoid increasing citrus leaf miner problems is to avoid pruning during the growing season and to minimize fertilization to avoid promoting more flushes of growth.
The following web page has more information on citrus leaf miner: