When you've mastered basic vegetable gardening and planted a few herbs, you might consider planting a few citrus trees (Citrus spp.) -- not only for their fruit but also for their spring blooms and attractive foliage. You're in luck if you live in U.S. Department of Agriculture plant hardiness zones 9 through 11, where most citrus thrive. Unfortunately, so do a number of fruit tree pests, including the citrus leafminer.
The citrus leafminer, Phyllocnistis citrella, is native to Asia and began appearing in citrus-growing areas of the United States in the 1990s. Tiny, light-colored adult moths lay eggs on the undersides of leaves, which hatch into larvae -- juveniles that resemble little worms -- that eat through leaves. Larvae leave light-colored trails of feces, or frass, just under the surface of the leaf. As leafminers mature, they leave serpentine "mines" through the leaves' interiors until they emerge as moths.
The larva interrupts the respiratory system of the leaf as it eats through the leaf and rolls the leaf edges as it pupates before flying away. Moths seek green, new growth to deposit their eggs, so leafminers choose young trees and typically avoid trees older than four bearing large crowns of mature growth. Because most homeowners don't have the option of locating young lemon (Citrus limon) or Clementine (Citrus reticulata) trees among mature groves, pheremone traps hung in trees can identify the presence of moths during the early spring when young trees have their first burst, or flush of new growth. Larger populations emerge during successive flushes in early summer and fall.
Although the University of Florida documented attacks on mature lime (Citrus aurantifolia) foliage in a 2001 study, leafminers typically present a nuisance rather than a threat. Young trees leaf out quickly, limiting the pest's window of opportunity.
Predatory insects, called parasitoids, are available from nurseries and mail-order companies to provide natural control.
- Predatory Ageniaspis citricola are native to both Florida and California.
- Cirrospilus and Pnigalio wasps are effective parasitoids when released during larval activity -- look for miner moths in pheromone traps and beginning tracks in new leaves.
- Check with the agricultural extension department for information on other local predators. Add shallow-flowered herbs and annuals to attract these native helpers.
Modification of fertilization and pruning practices can also help minimize leafminer infestation.
- Limit the use of nitrogen fertilizer, which encourages excessive new growth, to periods when rapid growth, or flushing, occurs in early spring and fall. Never overfertilize.
- Wait to prune citrus until foliage darkens in maturity. Take suckers from the base of the tree and water shoots -- the upright sprays of shoots that form along branches in dry periods -- down to their bases to remove tempting tender foliage. Prune citrus only once a year to minimize new growth.
- Avoid pruning leafminer-damaged branches until harvest so remaining leaves can contribute to the tree's food production rather than waste energy producing new leaves.
- Horticultural oils, including Neem oil, discourage infestation but must be applied using tree sprayers weekly during each flush period as new growth occurs.